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WEDGE-TAILED EAGLE
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There be three things which are too wonderful for me, yea, four which I know not: the way of an eagle in the air; the way of a serpent upon a rock; the way of a ship in the midst of the sea; and the way of a man with a maid. Hebrew Bible. Agur, son of Jakeh, in Proverbs 30:18–19.
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WEDGE-TAILED EAGLE
PENNY OLSEN Illustrations by Humphrey Price-Jones Photographs by Peter Merritt
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© Penny Olsen 2005 All rights reserved. Except under the conditions described in the Australian Copyright Act 1968 and subsequent amendments, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, duplicating or otherwise, without the prior permission of the copyright owner. Contact CSIRO PUBLISHING for all permission requests. National Library of Australia Cataloguing-in-Publication entry Olsen, Penny. Wedge-tailed Eagle. Bibliography. Includes index. ISBN 0 643 09165 3. 1. Wedge-tailed Eagle – Australia. 2. Endangered species – Australia. I. Price-Jones, Humphrey. II. Merritt, Peter. III. Title. (Series : Australian natural history series).
598.9420994 Available from CSIRO PUBLISHING 150 Oxford Street (PO Box 1139) Collingwood VIC 3066 Australia Telephone: Local call: Fax: Email: Web site:
+61 3 9662 7666 1300 788 000 (Australia only) +61 3 9662 7555
[email protected] www.publish.csiro.au
All photographs by Peter Merritt unless otherwise specified; illustrations by Humphrey Price-Jones Front cover photo: Greg Holland, www.birdphotos.com.au Back cover photo: David Whelan Set in 10.5/14 Sabon Cover and text design by James Kelly Printed in Australia by BPA Print Group Please read this important information Readers are warned that this book may contain images and other references to deceased Indigenous people, which may cause sadness or distress, particularly to the relatives of these people.
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Contents Acknowledgments
vi
Chapter
1
Musings
1
Chapter
2
Eagles and Aborigines
7
Chapter
3
Early records and names
11
Chapter
4
Eagles and their relatives
15
Chapter
5
The eagle’s country
19
Chapter
6
Eagle specifics
23
Chapter
7
Flight and sight
35
Chapter
8
Reproduction
41
Chapter
9
From egg to adult
69
Chapter 10
Hunting and prey
79
Chapter 11
Threats
87
List of scientific names
94
Bibliography
96
Index
112
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Acknowledgments The Wedge-tailed Eagle is one of the most impressive and better-studied of Australia’s birds so it somewhat surprising that there has been no previous attempt to summarise what is known in a dedicated book, although the eagle has not been without its scribes, particularly for a young audience. This book draws on the hard work of several people including Michael Brooker, Nick Mooney, Stephen Debus and Bill Brown, and discussions with eagle enthusiasts such as Nicky Birks, Tony Ross and Robert Bartos. To them and others over the years who have shared their ideas about eagles, I owe much. I am especially grateful to Michael, Nick, Michael, Stephen and Bill for reviewing the draft and generously contributing some of their own observations. Terry Dennis provided the estimate of eagle numbers on Kangaroo Island. Thanks also to others, evident in the reference list, with the energy to observe, study and communicate their understanding of eagles. Volume 2 of the Handbook of Australian New Zealand and Antarctic Birds and the Raptor Information Centre’s bibliographic database— http://ris.wr.usgs.gov/—made compiling the bibliography relatively painless. Stephen also sent me his bibliography of Australasian eagle species, which yielded some overlooked references, and his careful editorial eye picked up several silly errors. This would be a very dull book indeed without the marvellous images that bring the mighty eagle to life. Australia is fortunate to have so many talented artists and photographers of the natural world, and I am fortunate that two in particular were willing to contribute to this modest book. Peter Merritt’s luminous photographs go beyond mere technical excellence, and Humphrey PriceJones’ beautifully drafted illustrations show his depth of knowledge of the bird. There are other photographers to thank. Greg Holland provided the evocative cover photo and, with Leon Keasey at www.birdphotos.com.au, has always responded with great generosity to requests for photos for Wingspan, the membership magazine of Birds Australia, which I edit. David Whelan supplied the silhouettes on the back page, taken as part of his quest to alert authorities that Yaloak Vale—then, but happily no longer, mooted for development as a windfarm—was heavily populated with eagles at great risk of collision with the turbines. The Australian National Wildlife Collection, CSIRO Sustainable Ecosystems, kindly allowed use of a slide of the Port Jackson Painter painting, taken by Ederic Slater many years ago at the British
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Museum of Natural History, Tring. As publication date loomed, Robert Gosford alerted me to the work of Woodrow Denham, who was particularly helpful in providing images of Aborigines and eagles. At CSIRO Publishing, Nick Alexander contributed more than is usually required of a publisher, overseeing design and editing the manuscript, and Briana Elwood managed the project. Thanks to Nick for taking on the book, and to CSIRO Publishing for continuing to offer natural history writers one of the few options for publication in Australia today, and for doing it so well. I wrote this book while I was a Visiting Fellow at the Division of Botany and Zoology, The Australian National University, and thank Andrew Cockburn, Mike Double, Janet Gardner, Peter Marsack, Sarah Legge, Steve Murphy and Libby Robin, among others, for their companionship and encouragement. Not least, I am grateful to my parents, for the opportunities they have given me, and to my children Anna and Peter, for their special love and support.
‘I rode up to one sitting on a small tree, on a plain west of Duck River, which allowed me to approach within fifteen yards, and would perhaps not have flown away at all if I had not stopped on my horse.’ Lonsdale Holden in A.J. North (1911–1912).
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1 Musings
‘The story of the great Wedge-tailed Eagle of Australia—the second largest eagle in the world—is one to thrill the imagination and enlighten the mind of young and old alike.’ Will Lawson, poet, balladist, in foreword to Monarch of the Western Skies: the Story of a Wedge-tailed Eagle, C.K. Thompson (1946).
Every nation has its eagle, the biggest, boldest and most beautiful in the world. Eagles fire imaginations and fuel obsessions. Even their parts promise as much as their whole: mighty wings for freedom, eyes for insight, talons for power. We have them lead battalions to war, yet some would have them blamed and tamed for the loss of a few sheep. To me, quintessential Australian landscape is one in which two great dark shapes soar above in dignified solitude. Hence, my perceptions of eagles stem from this view of them as one of the more majestic players in nature’s game. Clearly early explorers and surveyors felt similarly, reflected in the number of landmarks named for eagles or eaglehawks. As a teenager I crested a hill on horseback and we, horse and I, came suddenly upon a huge black bird perched on a fence post. The eagle looked back over its broad shoulders deep into my eyes. After some minutes’ contemplation it turned away and launched gracefully off the hillside. We were within three metres but not a scrap of fear was apparent all round. I couldn’t say that there was a meeting of souls, but the mutual calm acceptance between the three of us had a certain unforgettable spiritual quality.
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Much of my adult life has been spent studying falcons but I have often come across Wedge-tailed Eagles, if only at the end of a frenzied attack by one or another of the Peregrine Falcon pairs I was studying. On one memorable occasion, Tony Ross and I were standing beside our boat only a few metres from a flock of Wood Ducks resting prone along the riverbank, unfazed by our recent arrival. The scream of wind rushing over taught wings reached us seconds before a huge dark form sped over from behind, its draught ruffling the hair on our heads. The backs of our necks bristled. Almost in sequence, the line of ducks tipped their bodies and fell the half metre or so to the water, and the eagle swept on between the trees and up through the canopy, empty-handed. My first hands-on encounter with a Wedge-tailed Eagle was with a scrawny brown bird with a twisted leg that had spent several years in Melbourne Zoo before being transferred to the Australian National University to be used in research on vision. The project was at its end and euthanasia and a visit to the taxidermist were on the agenda. We offered a home and a name—Boris. Over the years we came to realise that Boris was in fact a very sweet female, and imprinted on humans. The latter we discovered when Tidbinbilla Nature Reserve was closed to the public for the trial of a new strain of myxomatosis on the reserve’s numerous rabbits. This presented an opportunity to ease Boris back into the wild while there were plenty of sick or dead bunnies for her to eat. The rangers agreed to leave out the odd dead laboratory rat to help her adjust to fending for herself. The problem was that she adjusted too well. After a month or so of independence, presumably feeling at home among wired walls, she appropriated the duck enclosure and surrounds, and began attacking the rangers, swooping so close that she caused one to fall from his bike. With the park due to reopen in the near future, we were asked to remove her. Such behaviour is typical of raptors that have been raised from young chicks by humans. It also explained her rather undersized physique and twisted leg—she was probably raised on inadequate diet and, possibly, without exposure to sunlight. Bone, for calcium, and sunlight, for the production of Vitamin D and harnessing of calcium, are essential for eagles, especially for the development of a strong skeleton. These basics have commonly been overlooked by home raisers, particularly in the years when Boris would have been a nestling. She was probably hatched in the late 1960s, when the conservation movement was just beginning to be widely embraced, and stolen from a nest. It wasn’t until 1979 that all native wildlife was given the full protection of the law and it became illegal to remove eaglets from the wild and to keep them. One of the movement’s developments, a widespread concern for wildlife reha-
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bilitation, became ‘fashionable’ and since then a store of knowledge on appropriate techniques, including those for raising chicks, has been built up in Australia, where rehabilitation took the place of falconry as a way for people to gain a particular close understanding of raptors. In 1977, a couple of years after Boris came to us, a male with the last digits of his wing shot away was brought to us for return to the wild, which, of course, was not possible. The pair has been together since then and have produced young that were released to the wild. They are by no means pets. Even when raised by humans or trained for falconry, raptors retain a certain wildness, which quickly becomes more evident if contact isn’t sustained. After decades during which most contact was to feed them, they both clearly recognise me, but I am not their friend. He is dangerously aggressive—with a knack for driving a well-placed talon through the quick of a thumbnail, causing grown men to faint from the piercing of bone—and she retains a slightly nervous but devoted distance. In recent years, as a form of birth control, I chose not to give them sticks for their nest. There is no need to supplement the wild population with captive-bred youngsters, nor would I wish to sentence ablebodied birds a life in captivity. They taught me how affectionate with each other the eagles are when left to their own devices, and how playful they can be. When I moved to the city late in 2001, the pair went to live with Humphrey Price-Jones and began their new life as an artist’s models. They have a vast cage overlooking miles of paddocks occasionally broken by wooded hilltops, the sort of place they might choose to live if they still had that choice. This book grew from a much grander vision, Humphrey wished a fullblown coffee table production, which would do justice to his art: Humphrey’s painterly interpretation of the bird, accompanied by this text, my understanding of the bird. But, alas, although we have tried since 2002, in these straightened times for the publishing industry, no publisher has been willing to risk such a large format book. I am grateful to CSIRO Publishing for offering another option. I have attempted to gather and synthesise what has been learned by dedicated researchers such as Michael Brooker and Michael Ridpath, in Western Australia, and Nick Mooney in Tasmania, and the gems among a multitude of anecdotal observations. Where the facts are unknown but there is strong basis for assumption, I have taken a few liberties, but I have generally stuck to the known. The text is discursive rather than scientific, but readers in search of greater detail can hopefully find it in the documents listed in the extensive bibliography.
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In the 1960s and 1970s, when Brooker and Ridpath carried out their studies, wool prices were high and their profits funded research on the eagles, which were widely regarded as pariahs. By the 1990s the pendulum of public sentiment had swung such that Mooney’s work arose from a concern that certain forestry practices, guaranteed to raise controversy, were impacting on the endangered Tasmanian subspecies. Yet, looking back through the written history it is clear that those in the know, from early egg collectors and naturalists to latter-day researchers, have long had a clear-headed view of the eagle, its capabilities and needs, regardless of the prevailing tensions in human-eagle relations. Certainly, David Fleay, a champion of the eagle and other wildlife throughout the 1900s, had intimate knowledge of the bird. I have attempted to illustrate this understanding by liberal use of quotes. Throughout history, eagles have fuelled obsessions. Take the Lebanese taxi-driver, who, excited by the eagle billboard marking Birds Australia’s national office, showed me his eagle medallion. Eagles were his bird, his talisman, and had been so since he was a boy, even though he had never had any contact with live eagles. He bought every eagle image he could find, from lampshades to books. The idea transfixed him. Like Humphrey, who paints commissioned landscapes only grudgingly, because they leave him less time to paint eagles. The eagle soars in highest air’ Farther than the eye can gaze For his strong sight, unharmed, can bear The sun’s meridian blaze; If I had wings, I’d soar like him, Amid the clouds on high; Or reach yon mountain-top, so dim, That seems to touch the sky. From ‘The Eagle Soars’, National School Song No. 5, Toowoomba Chronicle and Queensland Advertiser 13 March 1862, p. 1.
Human nature demands that we revere or revile the most arresting of our fellow creatures. We endow eagles with the nobility to lead armies into battle and the courage to overcome the mightiest foe, at the same time damning them as a bloodthirsty menace to wildlife and farmer alike. Eagles are indeed strong, faithful and ever vigilant, and possessed of exceptional powers of flight and sight. But, while their natural qualities are both extraordinary and ordinary, to us earthbound creatures, they need neither exaggeration nor glorification. With the Dingo, introduced from the north
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thousands of years ago, and Tasmanian Devil, they are Australia’s most powerful predators. They may kill to survive, just as humans do, and soar majestically, but they also sleep, scratch, enjoy a bath, make tender parents, and form lasting relationships. Whether squabbling over a fly-encrusted carcass or spiralling up to be swallowed by the sun, eagles seldom fail to impress. At once despised and exalted, of earth and ether, long may eagles refuse to be contained within a single perception.
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2 Eagles and Aborigines
‘At the beginning of winter, the iguanas hide themselves in their homes in the sand; the black eagle hawks go into their nests; the garbarlee or shingle-backs hide themselves in little logs, just big enough to hold them; the iguanas dig a long way into the sand and cover up the passage behind them, as they go along. They all stay in their winter homes until Mayrah blows the winter away.’ From ‘Mayrah, the Wind that Blows the Winter Away’, Australian Legendary Tales, collected by K. Langloh Parker (1897).
The earliest human records of Wedge-tailed Eagles are found in the rock paintings that were part of the religious and practical life of many Australian Aboriginal tribes. In Western Australia, the people of the Kimberley painted some particularly beautiful and easily recognisable eagles onto a rock shelter, at least 5000 years ago. Wedge-tailed Eagles are so identifiable, powerful and omnipresent that it is hardly surprising that they have a strong presence in Aboriginal custom and mythology. Many south-eastern moieties, a division of society that governs rituals and marriages, are named after Eagle and Crow. The ancient story of eagle and crow is at the core of Aboriginal culture. Although based mostly around the Murray-Darling River systems, versions of this ancient myth have been transmitted as far as the northwest of Western Australia. One interpretation is that the relationship between this pair of similar birds with contrasting qualities is a metaphor for moiety (clan) systems, which are often
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Photographs by Woodrow W. Denham.
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Three dead eagles brought in by a member of the Alyawarra language group of Central Australia, in 1971, are plucked of their down: one eagle yields enough to fill a Golden Syrup can. The white down is later used for ceremonies—stuck in lines to faces, bodies, shields and poles with a mixture of human blood and water, and sometimes rubbed in powdered red rock to colour it. (The bodies are not eaten, in part because the eagles were killed using a poisoned kangaroo carcass, but also because they are believed to contain ‘devils’.)
seen as opposites, for example, those we can marry versus those we cannot marry. One of the shorter versions of the Eagle-Crow legend tells of Wildu, the eagle, who was cross because his nephews and nieces, the crows, had not invited him to their dance. The crows were all white in those days. Eagle watched them from a hill, and after a late night they all went into a big cave to sleep. Then Wildu came and burnt them. That is how crows came to be black. The myth has metaphysical meaning as well as psychological, social and cultural lessons. As with more detailed versions, the seemingly simple story expresses basic human desires and their consequences in the absence of social controls. Typically, the Eagle-Crow myth is built around the dichotomy of strength-cunning, father-son, and rivalry over their mother or wives. Thus, it is often sexual in nature, reinforcing taboos over incest and other socially undesirable sexual unions. Fire represents sexuality or life itself and when crow plays with fire, as punishment the more powerful eagle ensures that crow gets burnt. Eagles also played a role in sorcery, which was a way of enforcing social and religious custom and exacting revenge. In some tribes the medicine men
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were believed to assume the form of an eagle so that they could travel far at night to the camps of rival tribes, where they caused pain and suffering. By extracting the eagle’s talons, the local medicine man could effect recovery. To certain tribes raptors were seasonally important for subsistence, although members of the eagle moiety or totem could neither eat nor use the plumage of eagles. The eagles were caught with boomerang, throwing stick or spear, especially after they had gorged on a meal and had difficulty getting airborne. Some tribes used the attraction of smoke to various raptors, including eagles, hopeful of catching animals flushed or injured by the fire. The eagles were caught by hand as they perched on top of the hide or swept by, lured in closer by the trapper twirling feathers. As the quote above illustrates, Aborigines understood the eagle’s habit of nesting in winter (on the mainland). The birds and their plump chicks were plucked, cooked and eaten and their much prized feathers and down used in hair belts, headbands and ceremonies. It also seems likely that the Aborigines used the sight of soaring eagles to locate fresh carrion, and, when presented with the opportunity, relieved eagles of their kill. The eagles must also have caused some concern; it would be surprising if they did not try to capture small, unattended children, just as large African eagles do on rare occasions. Certainly, cautionary tales were told to children to keep them safe and obedient, so that they wouldn’t wander off and be captured by the eagle or its giant, mythical equivalent. Just as the term eaglehawk lives on in many place names, including Eaglehawk Island, Neck, Bore, Mine, Dam, Lakes, Park, Bay, Rock, Street, Gully and Mountain, several eagle-based Aboriginal names have been adopted as the official name. Warringal means eagle’s nest and Mullum Mullum Creek is eaglehawk creek. Cootapatamba, ‘the lake where the eagles drink’ on the side of Mt Kosciuszko, the highest of the country’s five glacial lakes, is presumably of metaphysical significance as eagles seldom drink.
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3 Early records and names
‘Many colonists call the eagle an “eagle-hawk”, which is about as sensible and significant as if we said “goose-duck” when naming a goose.’ Louisa Anne Meredith (1880).
It has been claimed that the first written record of the Wedge-tailed Eagle was made by William Dampier in his account of his second voyage to the Great South Land, in 1699. Of his landing at Shark Bay, the westernmost point in Australia, he wrote ‘… there were but few Land-Fowls; we saw none but Eagles, of the larger Sorts of Birds …’ Nonetheless, it cannot be certain that Dampier distinguished between eagles and other large raptors. Wedge-tailed Eagles were obviously known to the colonists in the early days of settlement at Port Philip. The names ‘eaglehawk’ and ‘mountain eagle’ appear in writings from the closing decade of the 1700s. Presumably, the first name derived from uncertainty about a new species (was it a hawk or an eagle?) and the second because the eagles would be more likely to have been encountered in the ranges than on the coastal plains. Sometimes eagles were mistakenly thought to be vultures. George Vancouver, in his account of British expedition that paused at King George’s Sound, Western Australia, in late 1791, noted ‘… of the birds that live in or resort to the woods, the vultures may be said to be most common, as we saw several of this species, or at least birds that were so considered.’ And John Latham, when he published the first scientific description of the Wedge-tailed
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Photo by Ederic Slater of the original in the British Museum of Natural History.
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A watercolour of a young Wedge-tailed Eagle painted by one of several artists, known collectively as the ‘Port Jackson Painter’, who recorded the natural history of the young colony of New South Wales in 1788–1792. It is possibly the first depiction by a European. Although many new species were formally described from the series, the eagle wasn’t recognised by science until a decade later.
Eagle in 1801, along with a swag of other Australian species in Supplement II to the General Synopsis of Birds, named it Vultur audax, the bold vulture. Latham described the species on the basis of a drawing of a live eagle made the previous year. A hand-coloured engraving of the drawing, titled ‘The Mountain Eagle of New South Wales’, was published by Collins in his 1802 book on life in the new English colony of New South Wales. A Captain Waterhouse had wounded the eagle at Broken Bay, New South Wales, and was planning to take it back to England. The eagle still had plenty of fight for, although lying trussed in the bottom of the boat, it drove its talons through a man’s foot. Collins reports that it rejected food from all except one person, the natives refused to go near it, and the settlers claimed never to have seen the species before. After ten days it shredded the rope around its leg and escaped. Evidently the collector knew something of the eagle, noting in his journal that it carried off middle-sized kangaroos. Though, by the time it went to press, the caption to Collins’ illustration had them eating large kangaroos. The convention of precedence in scientific naming led to the change in generic name from Vultur to Uroaetus and finally, by the 1950s, to Aquila, as the true affinities of the species became apparent. Aquila had been in use for
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some centuries, for the Golden Eagle Aquila chrysaetos, indisputably one of the Wedge-tailed Eagle’s closest relatives. The word ‘eagle’ is derived from the French ‘aigle’, which in turn derived from the Latin ‘aquila’, meaning ‘eagle.’ The Latin root is ‘aquil’ or ‘dark.’ Etymologists, people who study the origins and meanings of words, suggest that the name ‘aquila’ was first used to describe the largest raptors, because their expansive silhouettes appeared to darken the sky. Whether such birds were actually eagles or large vultures is open to question. ‘[Wedge-tailed eagles are] … arrant cowards’ D.W. Gaukrodger (1924).
‘The specific name (‘bold’) is a misnomer for this normally shy and wary bird’ Stephen Debus (1998).
Unless it is cornered, the eagle is generally quite wary and timid, even deserting its young if its nest is breached or at best flying anxiously around at a safe distance yelping. In that sense it doesn’t deserve its dashing moniker, bold eagle. Yet, regardless of the wariness of its nature, the eagle has survived human slings and arrows for centuries. Surely then, there is something truly audacious about this great bird.
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4 Eagles and their relatives
‘All that has been said by previous writers respecting the courage, power, and rapacity of the Golden Eagle applies with equal force to the Aquila audax: in size they are also nearly alike; but the lengthened and wedge-shaped form of its tail gives to the Australian bird a far more pleasing and elegant contour.’ John Gould (1865).
Eagles belong to the family Accipitridae, one of the most species-rich bird families, which includes the kites, harriers and hawks but not the falcons— they are in their own family, the Falconidae. Wedge-tailed Eagles are members of a group of 33 or so true or booted eagles, considered to be the most highly evolved species in the family Accipitridae. All are large, generally dark, eagles with feathered tarsi, that is, their legs are fully feathered right down to the top of their feet, or ‘booted’. In Australia, the only other true eagle is the Little Eagle, which is just over half the dimensions of the Wedge-tailed Eagle and more compact. It is also much lighter in weight and the difference between the sexes is greater than in its larger cousin: males weigh about 630 g and females 1000 g, compared with 3 and 4 kg, respectively. The other Australian ‘eagles’ include the White-bellied Sea-Eagle and Whistling Eagle, more correctly known as the Whistling Kite, both of which are kites. Among the kites’ shared features are bare legs, scale-covered rather than feathered. They throw their heads back to make powerful far-carrying
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calls, and rely heavily on carrion and theft rather than self-caught prey, and tend to skin their meal. The two booted eagles, Little and Wedge-tailed, make surprisingly feeble warbles and yelps, are more active predators, and often consume the skin of their prey. Although the Sea-Eagle is highly aggressive and about the same size and weight as a male Wedge-tailed Eagle, the largest live prey it tackles is around the size of a small macropod, up to 5 kg, a mere snack compared with the 60 kg adult kangaroo that Wedge-tailed Eagles can overpower, in part because it will hunt in teams whereas the Sea-Eagle is a loner. The Wedge-tailed Eagle’s closest relative, Gurney’s Eagle, is an incidental visitor to Australia, occasionally reaching Boigu Island, just inside Australia’s northernmost border with New Guinea. Wedge-tailed Eagles live and breed in the TransFly of New Guinea; elsewhere, in all but the highlands, Gurney’s Eagles replace them. Gurney’s Eagle is smaller, about the size of the very smallest of Wedge-tailed Eagles, lacks the rufous nape and its tail has only a slight wedge shape. It hunts medium-sized prey, such as arboreal mammals in lowland forest and wallabies on the coastal plains, from New Guinea to the Moluccas. ‘… with their patches of white across the scapulars, they are now most reminiscent of a pair of African Black Eagles.’ David Hollands (1984).
The Wedge-tailed Eagle and Gurney’s Eagle (Australasia), Golden Eagle (Northern Hemisphere) and Verreaux’s Eagle (Africa) together form a superspecies, each replacing the other geographically in the open forests, woodlands and savannahs that ring the globe. A further five to eight species complete the genus Aquila: Spotted Eagle; Lesser Spotted Eagle; a newly proposed Indian Spotted Eagle; Tawny Eagle; Steppe Eagle; Imperial Eagle; Spanish Eagle, which is sometimes considered to be a subspecific form of the Imperial Eagle; and Wahlberg’s Eagle, which is sometimes placed in Hieraaetus. Hieraaetus has always been regarded as the closest group to Aquila but recent DNA evidence suggests that the two genera are paraphyletic and should be combined under Aquila, bringing the Little Eagle and Wedge-tailed Eagle into the same genus. The study also identified Verreaux’s Eagle as the closest relative of the Wedge-tailed Eagle among those tested (which didn’t include Gurney’s Eagle), pointing to southern hemisphere origins for the genus (perhaps even Gondwanan, although the continents supposedly separated well before the radiation of open-country raptors). In the 1950s the Wedge-tailed Eagle was split into two subspecies, one confined to Tasmania and the other elsewhere in Australia and in New
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Guinea. The Tasmanian subspecies was named Aquila audax fleayi in honour of David Fleay, a prominent naturalist and the first to propose in writing that it was bigger. Tasmanian eagles are said to differ from those on mainland in three main ways: they are larger, especially larger-taloned; blonde-naped as juveniles; and largely forest-dependent. This split has never been substantiated by a thorough investigation and there are problems with all three of the claimed differences. For Tasmania, the few measurements available of both birds and eggs indicate that they lie towards the larger end of the range for the mainland. This could simply be because there is a temperature-related cline—a gradual overall change from smaller individuals in the north to larger in the south—not unusual among vertebrates. Only if Tasmanian birds were bigger than expected after allowing for any clinal effects, would larger size provide some support for a subspecific difference. As for the other characters, although both may be in the minority on the mainland, forest dwelling Wedge-tailed Eagles and blonde-naped juveniles are not unusual. It seems unlikely that there is absolutely no flow of eagles from the mainland to Tasmania. Like all eagles it seems that Wedge-tailed Eagles do not like to cross large bodies of water, so any interchange between residents on the mainland, larger Bass Strait Islands and Tasmania is likely to be small and hard to detect. An unpublished molecular study indicates that the Tasmanian birds may be somewhat inbred, perhaps a step towards subspeciation, but the definitive research is yet to be done. Nevertheless, the amount of interchange between populations necessary to maintain genetic similarity is remarkably small, such that one migrant per generation is thought to be sufficient. It is therefore open to question that the Tasmanian population is distinct, though it may be a subspecies in the making. That said, at the level of subspecies the determination of distinctiveness is rather arbitrary, hence, unless a thorough assessment is made of the Tasmanian eagle’s taxonomic standing, for conservation purposes it is best to err on the side of caution and retain its subspecific status. At the other extreme of the eagle’s north–south distribution, no movement of eagles has been recorded along the islands of Torres Strait, linking Australia to New Guinea. If there is indeed no movement between the two countries, the Wedge-tailed Eagles in New Guinea must be also considered candidates for subspecific status. But again that seems unlikely (see ‘Movements: dispersal and migration’ on page 21), and they appear to be indistinguishable from mainland Australian birds.
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5 The eagle’s country
This noble bird is so universally spread over the southern portion of Australia … it is also as numerous in Tasmania and on all the islands in Bass’s Straits, being of course more plentiful in such districts as are suited to its habits, and where the character of the country is congenial to the animals on which it subsists … I believe that it becomes more and more scarce as we advance northward from the south coast.’ John Gould (1865).
Distribution Most of the aquilas are widespread and the Wedge-tailed Eagle is no exception. It is found right across Australia and in the Trans-Fly region—the far south-west of New Guinea and south-east of Irian Jaya, the small underbelly of Papua-New Guinea. It also inhabits many of the larger continental islands but apparently not the islands of Torres Strait. It breeds across its entire Australian range and in New Guinea. Of course, there are parts of the country that do not particularly suit the Eagle, for example, it visits but does not stay to breed at the highest altitudes, and it is uncommon (but breeds) in the far north (Kimberley, Top End and Cape York) and on the Nullarbor. As Gould surmised, it seems that temperate and semi-arid parts of all but the far north of Australia support relatively large, stable numbers and the far north and New Guinea have relatively small, stable populations. He also
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noted that they were on all the Islands in Bass Strait; today they are seen only rarely on King Island, which was forested in Gould’s time. In the arid zone, although there is a core breeding population in many areas, particularly around inland ranges and watercourses, numbers fluctuate more widely. Coastal plains tend not to be the eagle’s preserve except where they meet ranges or other favoured eagle habitat. Actual numbers are difficult to gauge, although the eagle is numerous for a large predator. In Tasmania there are about 425 territorial pairs, seven of these in the Furneaux group of islands. The estimate for Kangaroo Island, South Australia, is 30–35 pairs, mostly in mallee. Across Australia there is likely to be tens of thousands of pairs. As well as these territorial pairs, there are likely to be just as many, if not more, immature birds and non-breeders.
Habitat In my mind’s eye I see Wedge-tailed Eagles circling in twos or threes above the gentle, bare hills of Burrinjuck Dam, near Yass, New South Wales. Others may picture them coursing along a rocky escarpment or squabbling over an outback roadside carcass. There are few habitats that do not support at least a smattering of eagles. Quite possibly the only major habitat that they typically avoid is urban sprawl, which they occasionally deign to traverse at a great height. Still, where urban design leaves space for wildlife, such as in Canberra, Sydney’s fringe and Hobart, eagles can nest only a few hundreds metres from houses. In the past, when it was heavily persecuted, the eagle gave cities and larger towns a wider berth, but in recent years a few bold pairs have begun to nest in bushland on the suburban fringe, where it is reasonably undisturbed, perhaps a sign of increasing tolerance by both humans and eagles. The eagle’s natural habitat is tropical, temperate and arid woodlands, forests and open country from sea level to alpine country, up to about 2000 m on the mainland, 1100 m in Tasmania. It frequents coastal plains, highlands and inland plains, from shore to shore, from rainforest to grassland, wetland to waterless desert, and remote mountains to pastoral land. For nesting and roosting, treed country is favoured, but the eagle very occasionally nests on cliffs and roosts on the ground. It seems most at home in rough, structurally diverse country: mosaics of woodland and scrubland or forested slopes, rocky outcrops and river valleys or where undulating farmland meets woodland and wetland. Hence, structural features of the habitat are thought to be more important to the eagle than vegetation type or land use. Prey is also a key element. Before the decline of Hare and Nailtail Wallabies in Western Australia the eagle was common in sandy deserts, but is now virtually absent. Conversely, where rabbits are common, particularly in pastoral land, so too are eagles.
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Movements: dispersal and migration Adult Wedge-tailed Eagles that hold territory tend to remain there all year round. By contrast, immatures and unattached adults tend to wander. As with all raptors that are said to be nomadic, their movements are unpredictable in timing, place or direction. In reality there probably are patterns which we do not yet understand. Quite likely ‘wandering’ eagles move, if necessary, to areas away from resident birds, to where the weather is mild and prey or carrion is available; perhaps preferring the plains, where flight, on thermals, is energetically cheap, to rugged, more wooded country, where experienced birds cope better. Typically, juveniles disperse from their natal area as the new breeding season begins (autumn-winter on the mainland, later in Tasmania), but the period of fledgling dependence is quite variable. Presumably, the dispersers travel only as far as they must to escape aggression from adults and find food, then move on if these change. Recoveries of birds banded as nestlings indicate that while some move little, others travel far. The longest recorded distance was a bird shot 868 km and seven months from where it fledged. Another travelled 856 km from Canberra to south-east Queensland within eight months of banding. As is typical of many birds, those that survive, particularly the males, eventually tend to make their way back towards the place where they fledged, to breed. In the intervening years, on the mainland many seem to head inland and northwards, or to coastal areas where there are no resident breeders, following ranges, chains of hills and watercourses, sometimes associating with other itinerant individuals. There may well be some seasonal movement of high country eagles to lower altitudes in the winter months, but this has not been studied. Although movement has not been observed across either Torres or Bass Strait, the distances involved in island-hopping are achievable. In Bass Strait, the eagle nests in the Furneaux Group and Three Hummock Island, and visits King and Curtis Islands, 100 and 57 km from the Tasmanian and Australian mainland, respectively. The over-sea distances involved would not be beyond its capabilities, particularly if it could soar up high above one island and glide across to the next. There is less than 50 km between major islands along the chain of islands from Tasmania through Flinders Island to the mainland. The eagles have not been recorded on any of the Torres Strait Islands. However, it is possible that the odd individual traverses the sea at altitudes beyond human vision, for example, when the summer wet blows down from New Guinea, carrying with it many better-known migrants returning to Australia to breed. Certainly the odd Gurney’s Eagle, with similar limitations to crossing large expanses of water, reaches the Australian mainland.
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6 Eagle specifics
‘[Wedge-tailed Eagles are] … often the only moving objects above a wilderness of trees and stones, and undoubtedly add to the grandeur and beauty of the landscape.’ A.J. North (1911–1912).
Identification Typically, the Wedge-tailed Eagle is observed when it soars. From its size and shape, few would have difficulty recognising it as an eagle. Indeed, it is like no other Australian raptor: large, lanky and dark, with a narrow protruding head and an unmistakable, long, diamond-shaped tail. It soars in high, lazy circles on long, broad wings, spanning two metres, often held rigidly angled up in a shallow V, upswept and fingered at the tips. Its generally dark undersurface is broken by its pale feet and undertail coverts, and a pale line made by the white bases of the primaries visually splits the wing down the long axis. The blackest eagles are definitely adults, whereas youngsters are more golden or blonde. However not all adults are black, and the age at which the change from lighter juvenile plumage to dark adult plumage occurs varies. In Tasmania as a rule, and on the mainland occasionally, the oldest birds attain a deep chocolate brown and first-year birds are light brown with a striking blonde nape. Languid and masterly in the air, the eagle seldom has need for flapping flight, but when it does its wing-strokes are deep and powerful. By contrast, it can be awkward and apparently ill at ease on the ground. As Nick Mooney says, the eagle walks like a sumo wrestler. Unless on a hilltop, it struggles to
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take off, particularly when full-cropped or in still conditions. With a lolloping canter, one foot ahead of the other, and wings held outstretched, it ‘runs’ into any wind that might help lift it airborne. Perched on a sturdy limb, or some other commanding position, the eagle stands about one metre from the top of its head to the tip of its tail, its wing tips reaching to well short of its tail tip. Heavy-shouldered, it has diagnostically feathered legs, and a pale, archetypal bill, so familiar in profile: long, deep, arched, and laterally flattened. The type of beak which evokes the description of a particular human nose as aquiline. The eagle’s prominent brow tops a deep-brown eye, which does not change noticeably with age. By nature wary, perhaps reinforced by years, even centuries, of conflict with humans, the eagle is seldom seen close to human activity. Occasionally it soars high over the city or above a kangaroo shooter’s vehicle, or is come upon suddenly, perched on a fence post or roadside carcass, but such close views are fleeting. Even at the nest, most adults depart at the first sign of disturbance. Of course, there are exceptions to this apparent timidity. Lindsay Cupper photographed one particularly bold female that stayed on the nest when he climbed to it. Vehicles are a different matter and most eagles become accustomed to any regular movement of vehicles near their nests or look-outs. Large, brown eagles encountered near the coast, in particular, or large inland waterbodies, are likely to be young White-bellied Sea-Eagles. Unlike the Wedge-tailed Eagle, they are mottled brown and have bare legs. A very short, pale tail and sharply uplifted wings make their flight silhouette butterfly-like and, when perched, the wingtips hang lower than the tail-tip. In the arid and semi-arid zone, another potentially confusing species is the short-tailed, barelegged, Black-breasted Buzzard, which is as dark as a Wedge-tailed Eagle but has a distinctive white-bullseye in each wing and a characteristic rocking flight.
Interactions ‘In March, 1887, I saw a pair of Black-cheeked Falcons [Peregrine Falcons] that are resident about the cliffs at Circular Head, mobbing a Wedge-tailed Eagle that had been frequenting the Bluff for some weeks, making dashes at it as crows and other birds do to the Raptores in general.’ L. Holden in North (1911–1912).
For much its day the eagle is dogged by an endless procession of annoyances, which can also signal its presence to humans. Especially in spring, a flurry of
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corvids, Australian Magpies, Magpie-larks, Masked Lapwings, excited Brown Falcons and other raptors, mob the eagle, swooping and sometimes calling shrilly, repeatedly attacking, like so many angry wasps, until it passes by. Even when the eagle perches, Willy-Wagtails, Restless Flycatchers and other bolder passerines continue the kerfuffle. For the most part the eagle suffers with dignity even when its feathers are tweaked, only occasionally emitting a plaintive whistle. But every now and then it will roll in the air, presenting its mighty talons to the puny attackers, which may even become a meal. Peregrine Falcons have no tolerance for the much larger eagle, and the two species usually nest well apart or at least out of sight of each other, but nesting falcons attack passing eagles with a vengeance and on rare occasions dispatch them with a blow that breaks a wing or injures some other vital part. Such interactions are more often risky to the mobbers, but they obviously benefit by moving the eagle on, away from vulnerable offspring. The eagle also has hangers-on to contend with at a large kill or carrion. Ravens, Black Kites, Whistling Kites and foxes watch the sky for signs and arrive in the hope of some leftovers—indeed they must wait for the stronger eagle to tear open the carcass. Sometimes a great many eagles (20, 50 or more) may gather at a kangaroo carcass or hunters’ camp. The newcomers bound toward the carcass and are often deferred to if the incumbent is sated. If resistance is met the eagles may threaten with hackles raised, neck forward-bowed
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and arched, wings outstretched. So challenged, their movements are stiff and jerky. As in the air, their feet are their main defence. If pressed, the contestants keep their head well back and fling both feet towards each other. Often they lock talons and the two lie on the ground for a time before they loosen their grip and one withdraws. Such scuffles look serious but are mostly quite ritualised, and injuries are rare. The great birds appear to be surprisingly playful, especially in courtship. One may carry a stick or old bit of prey aloft and drop it for its partner to swoop in and catch. Bill Brown reports that a pair has been seen shadowing a bull making his way to join some cows; three times the female lightly touched the bull on the back causing him to toss his head and bellow but not increase his pace. Other eagles have been seen ‘playing’ with dogs, floating a few metres about the dog until it responded by barking and leaping, at which the eagle would float up until the dog settled, then repeating the ‘game’.
Plumage ‘There is a great variation in colour of adult birds, some of them being much paler in breeding plumage on the nape, hind-neck and upper wing-coverts, being of a light creamy-buff. Usually this is put down to youth but such is not always the case, as may be seen by the quills and tail-feathers, many being found breeding before they assume the blackish-brown plumage, and which is only the livery of very old birds.’ A.J. North (1911–1912).
The eagles go through three main plumages: from white, downy chick, to brown juvenile and subadult (second, third and fourth year) and, finally, darkbrown to black adult. When they fledge, young eagles are almost full-sized and fully feathered—only their wing and tail feathers are yet to gain a few more centimetres. They appear slightly streaky: their feathers are edged either redbrown or blonde and they have a red-brown or blonde head, nape and broad upperwing band. At close quarters their flight feathers are barred. As the birds age, the upperwing band increasingly darkens and appears narrower (the individual feathers become dark-centred, leaving only a fine margin of blonde or tan), the crown and nape gain more black reaching the extreme in the darkest birds, which have more or less fully black head and body and buffy or deep red edges only to upperwing coverts. The adult flight feathers are dark and unbarred. The darkest of birds are old males—females retain more rufous or brown in their plumage and seem slower to darken. In both sexes the change is grad-
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ual, between the ages of about two and nine years. Thus, whereas black birds are certainly older birds, the presence of a red-brown head and nape and upperwing band does not necessarily indicate a young bird. Early last century, an alleged albino was reported, and a white bird was video-taped in Tasmania in the 1980s, but such odd variations in plumage are rare. The eagle’s brows and face are relatively bare. Its face is sparsely-bristled and less densely feathered than in the Golden Eagle, presumably because of the warmer Australian climate rather than an adaptation to carrion eating, because those of Verreaux’s Eagle, which is not a carrion-eater, are similarly sparsely feathered and Golden Eagles also eat carrion. The elongated feathers of the nape form a mane that can be raised into faceframing hackles during conflict and in curiosity. To the human eye, the hind neck and nape contrast with general plumage and are most obvious in adult females (deep red brown or tan) and younger birds (red brown to buff), as are the upperwing coverts. In flight, from above, the white scapulars of the blackest adults form two giant white ‘eyes’. These are particularly obvious when the eagles are performing pot-hooking displays (see illustration on page 45), and when brooding or incubating. Presumably, the ‘eyes’ help to deter would-be attackers, deceived into believing they are being watched and may be used as flashing signals in aerial displays. Like most if not all birds, the eagle can probably see in the ultraviolet range, and might be expected to use these ‘colours’ as cues to age and status. The buffbrown of young birds tends to be poorly reflective in the ultraviolet range, whereas white, as in the scapulars of adults, and upperwing coverts of old males, is often highly reflective. All of which would serve to enhance the ‘eyes’.
Moult and feather care ‘She lowered herself into the water… and then began to wallow and splash,… got out and shook herself like a dog. She flew to her high perch looking a sorry, bedraggled object, fluffing her feathers in the sun, as they dried she preened each pinion with great care.’ Mary Patchett (1960) p. 210.
Adult eagles always have feathers of various ages, some worn and some fresh. A full moult is not completed for several years, by which time a new cycle has already begun, so that the sequence is difficult to decipher. The primaries are replaced descendently, and it takes about three years or more for the centre of moult to progress from the first, inner, to the tenth, outer, primary. The secondaries and rectrices (tail feathers) are also replaced sequentially, from the
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inner decks to the outer, sixth retrix. A new flight feather takes several months to grow and adjacent feathers are rarely missing at the same time, so there is always support for the growing feather. The eagle has an oil gland on its rump, at the base of its tail. When preening, which it does several times a day, it reaches around to gently squeeze oil from the gland. It nibbles a pulls the feathers through its bill to maintain their structure and condition. The only place it cannot reach is its head, which it scratches with its talons and rubs under its wing. As in all birds, the main fabric of the feathers is the interlocking barbs that give strength and flexibility, the eagle presses the barbs together to refasten the hooks (barbules), which must be kept linked and aligned for flight. The eagle does this with ease but it is all but impossible for human hands. The eagle also enjoys a bath, wading into water and then shaking water through its body and ducking in its head, or sometimes opening its wings to have a shower in rain. Afterwards it vigorously shakes the water from its feathers—rather like a dog—and dries with wings outstretched.
Size ‘Mr A.G. Bolan … states that he has found a number of eagles on the Nullarbor to measure nine feet from tip to tip of wings, and that one specimen which was caught in a dingo trap actually had an expanse of ten fee three inches. Further it has been claimed by a pastoralist in New South Wales that an eagle he shot measured eleven feet [3.4 m] across the wings.’ A.H. Chisholm (1934).
Eagles were obviously bigger in the early days! Indeed, most people are disappointed, when the tape measure comes out, to learn that the eagle is somewhat smaller than an F1-11 and weighs less than the average house cat. The wingspan ranges from 185 to 230 cm and total length from 85 to 105 cm. Males average 3.2 kg and females 4.3 kg, with top recorded weights of 4 kg and 5.8 kg, respectively. Among the least size dimorphic of the aquilas, the male Wedge-tailed Eagle averages about 89 per cent of the female’s winglength (although there is overlap between the sexes), and 75 per cent of her weight. Lower levels of dimorphism are typical of less predatory eagles, particularly those that have greater amounts of carrion in their diet. The Wedge-tailed Eagle is the largest raptor in Australia, rivalled only by the White-bellied Sea-Eagle, at about 90 per cent of the Wedge-tailed Eagle’s wingspan and 75 per cent of its weight. It is the fourth largest booted eagle,
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exceed by the most northern subspecies of Golden Eagle and the African Martial Eagle, and, except for the tail, just a touch smaller than Verreaux’s Eagle. Even the largest Aquilas, with females up to 6 kg, are positively diminutive beside those of the powerful Harpy Eagle and massive-beaked Steller’s Sea-Eagle, both of which reach 9 kg, the largest of all flying predators. The largest and heaviest Wedge-tailed Eagles are those in Tasmania and the smallest those in the northern Australia and southern New Guinea; although patterns of geographical variation are poorly known.
Wings and tail The eagle has long, broad wings, with 10 main flight feathers, the primaries: long, stiff and more or less emarginated and asymmetrical in shape, giving a slotted appearance to the spread wing. They vary in shape and length, the seventh primary from the inside usually being the longest. Closer to the body are about 13 broad secondaries, then four or five tertials, all quite symmetrical. Including the slightly concave warp, from the carpal joint (‘shoulder’) to the tip, the wings measure about 53–63 cm in males and 60–70 cm in females. The famous, ample tail is 85–105 cm long from root to tip and is attenuated: that is, the innermost of the six pairs of feathers are the longest and the others graduate sharply down in length to the shortest, outer feathers. The two inner decks lengthen with age to eventually produce a distinct, stepped point, especially in females. To give them the advantage of a lower wing-loading (body weight in relation to wing surface area) while they build up muscle and hone their flying skills, juvenile eagles have longer, slightly broader wings than the adults; this particularly assists with take-off and lessens the risk of stalling.
Feet and bill The eagle is among the most powerful of avian predators. Its short, stout legs, robust but rather short toes, and massive, deeply curved talons are typical of raptors that prey on large mammals. It has three forward-directed toes, the inner, sturdiest, toe opposing the hind, or ‘killer’, talon. This arrangement allows easy perching when roosting at night and, often, for long periods of loafing during the day—as well as powerful grasping of prey. The two main piercing toes, inner and hind, are the shortest but have the longest talons. Measuring the chord, the shortest distance from ‘quick’ to tip, not accounting for the curve, the hind talon is about 41 mm long in males, another 5 mm in females, and roughly the same length as its toe. On average, the middle toe is 66 mm in male eagles, 74 mm in females, about the length of a woman’s index finger.
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All three forward-facing talons are roughly half oval in cross section, but the ‘straight’ side is grooved (concave) to form a sharp edge down either side of the talon. The edges of the talons slice as the eagle uses its bill to pull prey through its grip. Special leg muscles, a ratchet-like system in the toes (under tension tendons lock against the inside of their sheaths), and pads under the toes all help to automatically lock the eagle to perch or prey. Compared with the other aquilas the Wedge-tailed Eagle, especially the female, has a relatively massive beak, presumably related to its large prey. Yet, the impressive-looking bill is a mediocre weapon. Although robust, it is used for hooking into prey gripped in the feet and for tearing by pulling, rather than for biting and killing. It is long and quite deep, but not broad, and terminates with a right-angled hook. In a straight line, from one end to the other, it averages 57 mm long in males 60 mm in females. The bill of south-west Australian eagles is smaller, by about 2–3 per cent, than in the south-east, perhaps a reflection of smaller average prey size. Straddling the base of the bill, the cere is a tough keratinous membrane in which the oval nostrils are situated. The cere is the same colour as the bill, enhancing the impression of size of the bill, which may be important in display to other eagles. If prey is not killed on impact, the eagle may grab the stunned animal’s head or thorax and squeeze spasmodically, a reflexive action that constricts the lungs, while the hind talons puncture vital organs. The eagle’s grip is bone piercing, and said to be almost 200 pounds per square inch (14 kg/cm2). By contrast the bill, when used in isolation, is far less capable of damage. Anyone who has handled an eagle knows that it is the feet to be feared.
Eagle song ‘The note of the Wedge-tailed Eagle is well represented by the syllables ‘Dirra lich, Dirra lich,’ quickly repeated several times in a shrill note.’ K.H. Bennett in North (1911–1912).
The Wedge-tailed Eagle is largely silent, with a limited vocabulary, and is seldom heard calling except in the breeding season. For such a large, fearsome bird, it has a surprisingly gentle, melodic voice, and most commonly makes a double-noted, slightly hoarse whistle ‘Dirra lich’. It does make the occasional excited hoarse barking yelp in sexual excitement, during territorial displays and when arguing over prey, but this hardly strikes fear into the heart. Less often heard is a loud, ear-piercing scream that lasts for several seconds and
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may be repeated. It is sometimes emitted when a bird is flushed from the nest, to deter intruders and warn the nestlings or summon its mate. At close quarters the male makes a high wavering continuous trill or yodel to the female, especially during courtship. The female, being larger, has a lower pitched voice, with less of a warble in it. Nestlings make a throaty, repeated trill to be fed and, once fledged, yelp excitedly in anticipation of a meal or when threatened.
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7 Flight and sight
‘Those who hope in the Lord will renew their strength They will soar on wings like eagles’ Isaiah 40: 31.
Eagles and soaring are one of those word associations that are inextricably linked in our minds and imaginations. Eagles soar with such visibility, ease and majesty that humans have long given the behaviour spiritual and metaphorical significance. The eagles themselves have much more earthly intentions, they are on the move, prospecting for food, claiming ownership of territory, or perhaps simply keeping cool. Once the air warms and thermals form the eagle can stay aloft almost effortlessly, reading the wind, soaring from thermal to thermal with nary a wingbeat. It also hangs on updrafts created by wind, moving only its tail to maintain balance. Nearer the ground, say, in pursuit of prey, it must actively flap to move forward in the air, but it is energetically expensive to keep such a large body airborne by flapping. Unless it leaps off a high branch or pinnacle, on take-off the eagle struggles to stay in the air, particularly when there is no wind to fly into for lift. In still conditions, it takes flight heavily, skipping clumsily across the ground to get some forward momentum, in a two-legged hop, one foot leading, then labouring with deep powerful flaps. Grounded within a forest, the eagle must clamber up from branch to branch to reach the canopy, which it does with considerable lack of grace. Engorged with food, an eagle can be run down and captured by hand.
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‘On the ground, Wedge-tail was a far different bird from the noble creature of the air. His big wings and thick feathered legs made his movements clumsy. If he were slow in his movements he could walk with some dignity, but any tendency to hurry turned him into a hopping, flopping ridiculous bird.’ C.K. Thompson (1946).
Typically the eagle takes to the air late in the morning to spiral skywards in the warm-air columns, or thermals, that rise from sun-heated ground. It becomes active later in the day than smaller soarers, such as Whistling Kites, when the columns are wider. The eagle also makes use of updraughts, where wind hits mountain or forest edge and is deflected upwards. Above about 2000 m, the eagle is not visible with the naked eye. But glider pilots (with altimeters) cruise with eagles up to 3500 m and have regular encounters at 3000 m, about the height that thermals dissipate above plains. Even on the hottest day the air is cool at such rarefied heights—from 30°C at sea level temperature drops to 7.5°C at 3000 m in still air; in rising air it can be 0°C at 3000 m and 10–15°C at 2000 m above sea level. A slight digression here: the eagles appear to view the gliders as potential competitors for territory, to be moved on. It is not unusual for a resident eagle to swoop from above and behind at high speed with wings folded back, stretching the talons forward to hit near the leading edge of the glider and leave a fair-sized tear in the wing. So hard did one Tasmanian eagle hit that it became firmly wedged in the frame of a wing. Experienced pilots estimate that eagles have maximum dive speeds of 120 km/hr and 60 km/hr in a glide, with a sink rate of 0.5 m/sec. The eagle glides across country from thermal to thermal but loses height in a glide and must occasionally flap to reach the next rising hot-air column and be carried effortlessly up again. Because there are no thermals over water, long sea crossings are avoided by aquilas; the maximum crossing attempted elsewhere in the world seems to be about 40–75 km. Instead, where islands offer ‘stepping stones’, the eagles fly up to 1000 m or so and glide between landmasses. Where high winds stream upwards the eagle can hang in the air, with wings raked back, tips narrowed and fingerless. If it opens its wings briefly, the wind shoots it up like a kite on a string. More typically, the eagle holds its wings fully open angled up in a slight dihedral, with tips fingered and upswept and its long, diamond-shaped tail fanned. The Wedge-tailed Eagle is sometimes said to be the only long-tailed Aquila, but the Golden, Verreaux’s and Gurney’s Eagles are almost as long-tailed, they
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just don’t appear to be. Like most soaring raptors the eagle has low-aspect wings, long and broad, to form a large surface area. To compensate for the lower aspect ratio, the eagle can open its ‘fingers’, formed by deep emarginations in the outer primaries, to make vertical slots that let air slip and increase stability and lessen drag on the distal edge of the wing. This prevents stalling in flight, and reduces turbulence at the wingtip, which is particularly useful when landing. It also forms several aerofoils, each of which can bend separately and together increase lift over that possible if the wing was one large unbroken flying surface. The eagle’s low wing loading (ratio of large wing area in relation to bodyweight) helps take-off from the ground and together with the emarginated and separated primary feathers, maximises turning and climbing performance in thermals. Such a wing is not only suited to gliding but allows slower speeds and greater manoeuvrability than a high aspect wing. Similarly, the tail is very flexible and the eagle can twist it or shape it into a deep V in cross section, to give stability in (low) active flight. Thus, the Wedge-tailed Eagle’s unusual combination of long, broad wings and long tail make it particularly dexterous for a large soaring bird, and presumably suit it to live in a variety of habitats from vast open plains to rugged bushland and forest. Certainly, even though most view it as a bird of open country, it is capable of hunting in quite dense forest. Do eagles swim? Very occasionally eagles, perhaps following territorial dispute or miscalculation over prey, end up in the water. With wings spread across the surface they are quite buoyant, but must row, overarm, to shore, which is an exhausting process. They emerge soaked and bedraggled and must dry out before they can take to the air again.
Senses ‘Eagle-eyed’ – keen of eyesight, vigilant or, by extension, perceptive’. Eagles are much admired for their powers of sight and vision is their primary sense. Their visual acuity, that is, the ability to resolve spatial detail, is variously claimed to be two to eight times that of humans. Resolution of detail is related to eye size, which is as big as humans’, so that eagles’ main advantage may be in a more acute ability to detect movement. An eagle is thought to be able to detect a rabbit a kilometre away, provided it is moving. Like many raptors, but unlike most vertebrates, eagles have two foveae in each eye, which can be used independently, one forward directed, the other lateral. The foveae have a high density of cones (cells receptive to bright light),
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which give an unusually keen perception of distance and movement. The central fovea is deeper and has the higher acuity and in effect is equivalent to having a modest pair of built in binoculars. Overlap in the field of vision of the two forward foveae gives a binocular field of perhaps 40 or more degrees (compared with 120 in humans), which enhances depth perception. An additional adaptation in raptors is that the cornea and lens are angled toward the beak, which increases overlap even more. Raptors have the largest and most elaborate pecten of all the birds. The pecten supplies nutrients and oxygen to the vitreous humour of the eye, thereby reducing the number of blood vessels in the retina. With fewer blood vessels to scatter light coming into the eye, raptor vision has evolved to be the sharpest vision known among all organisms. They also have exceptionally fast accommodation: well developed muscles control the curvature of the flexible lens, to give long focal length and make the rapid adjustment necessary to maintain focus as they swoop rapidly towards prey. Their flicker-fusion frequency, the time interval needed to separate two consecutive visual impressions, is 1/90–1/100 sec, about twice as fast as that of humans. Because their eyes are relatively large and semi-tubular, eagles are somewhat limited in their ability to move their eyes in their sockets. However, a flexible neck, with many vertebrae (14), compensates by allowing them to turn their head through a total arc of nearly 270 degrees (135 degrees each direction). Close objects are viewed straight on with both eyes (binocular vision), more distant objects of interest, such as birds flying high overhead, are investigated with head cocked sideways, through a lateral fovea (monocular vision) with especially fine acuity. When particularly intent on something, the eagle also sometimes makes a side-to-side movement of its head, this moves the image across the retina and is thought to aid in the perception of depth (range-finding). The eagle’s eyes are protected by the bony brow and three eyelids: the usual top and bottom lids and a nictitating membrane, an opaque, transparent lid that flicks from the inner corner of the eye to the outer, every few seconds, to screen the eye and clean and moisten it. The nictitating membrane closes on impact with prey, and more frequently in gusty weather than otherwise (safety goggles, in effect). The lower eyelid is frequently used during preening, head scratching with the talons, and bill-wiping after feeding. These activities could cause harm, either directly (damage to the eye) or indirectly during preening (dust, dead epidermal cells or down loosened and flying around). The upper eyelid is probably used only during sleep. Relatively few rods, receptors of dim light, mean that the eagle’s night vision is poorer than humans’ or, at least, takes much longer to adjust, and eagles seldom hunt at twilight. In addition to the human visible waveband of
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light (400–700 nm) and three main colours, it is likely that the eagle can see in the near ultraviolet part of the light spectrum (320–400 nm), which is unavailable to humans. This would allow it to see the scent-marked trails of some mammalian prey and may play a role in mate choice and display (intersexual signalling). For example, in the ultraviolet range the eagle’s nape area, signalling adulthood, may contrast strongly with the back. It is also likely that the eagles can see in the infrared range, which would allow them to detect thermals by sight. With regard to aural perception, the eagle has no external ear, simply a slight ridge of skin behind the ear opening, and its hearing is unexceptional, similar in sensitivity to humans’ in the 1–5 kHz range and reaching an upper limit of perhaps 10–12 kHz, lower than that achievable by mammals. Nor is eagle’s sense of smell thought to be unusual although they can certainly detect whether prey is putrid. The presence of taste buds on their muscular tongue indicates that they have a sense of taste, and they appear to select or reject food items partly by taste.
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‘It seems to me probable that an eagle has a breeding territory, which it is prepared to defend with some violence against other birds of its kind, but which is very loosely defined towards the edges… It is true enough that eagles of any particular species are spread out more or less evenly over a particular stretch of country …’. Leslie Brown (1955).
Territory and nest density Eagle territories are important and exclusive, and, where the landscape allows, tend to be relatively evenly spaced. Although most paired adults are resident year round, their territories are more easily located in the breeding season. The eagles nest as solitary pairs, typically several kilometres apart. Territories are pegged to the nest trees and defended against other eagles; their boundaries appear to remain relatively stable from year to year. As the breeding season approaches, in about March-April on much of the mainland, July in Tasmania, the owners step up their defence and territorial displays, aggressively evicting stragglers and would-be usurpers, and consolidating boundaries with neighbours. Pairs patrol ridges, gliding on outstretched wings, occasionally rocking from side to side. Boundaries with adjoining pairs are respected and when pairs meet they coolly perform undulating territorial displays either side of the boundary. Dangerous combat is usually avoided by ritual in the form of displays and observance of hierarchies; intruders tend to defer to incumbents,
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younger birds to older. Occasionally, however, one eagle kills another. If they are typical raptors, males are concerned about all intruding eagles, females mainly with female trespassers, and most conflict is between females. Although pairs stay together year round, they often move about independently. Activity is focused on the territory, but the eagles range much wider, over a home range which often overlaps with that of neighbours. In this shared area, which is not defended, there may occasionally be disputes with other eagles, in particular over food. Almost certainly, females range more widely than males, as is typical of raptors. Territories and home ranges are not necessarily regular in shape—much depends on the topography and habitat, and where nest sites and prey are located. For example, in hilly or mountainous country the eagles may nest mid-slope and hunt out over open country to the front of the slope, rather than in the opposite direction, up and over the crest. Indeed, ridges are often used as territory boundaries and there may be another pair on the other side. In contrast, where there is an open plateau topping a steep slope they may nest on the slope and hunt on the flatland above. Typically, where nests are very close they are out of sight of each other and the two pairs can go about most of their business unseen by their neighbours. The presence of other powerful raptors also influences nest spacing. Nesting Peregrine Falcons are generally avoided as are White-bellied SeaEagles, even though, ultimately, the Wedge-tailed Eagle is dominant. Whitebellied Sea-Eagles sometimes clash violently with Wedge-tailed Eagles over nest sites—a conflict perhaps exacerbated by loss of habitat and increasing competition for what is left. Where eagle nests are clumped, the distance between them gives a reasonable indication of territory size. Where they are placed linearly, such as along inland creeklines, territory size is harder to gauge. Provided that suitable habitat is extensive enough, nesting pairs are quite regularly spaced. All other things being equal, the distance between nests will vary with the availability of food—in good eagle country, nests will be more closely spaced than in poor eagle country. In an exceptionally good season a few more pairs may squeeze in among the usual long-term residents. Similarly, after a string of poor years, pairs may be lost and not replaced. Overall, although the eagles have relatively stable breeding populations, breeding density will be more dynamic in unpredictable environments, varying by over 25 per cent, than in stable environments, where annual variation of around 10 per cent might be expected. Measuring the density of nests is the only practical way to estimate the eagle population. It is also meaningful in biological terms because breeders are
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the individuals that contribute to future generations. Active nests have been found as close as 0.7 km apart, and distances of 2 km are quite common but, because nests are not necessarily in the centre of territories, this is unlikely to be an accurate reflection of territory size. Several attempts have been made to find all the eagle pairs in an area. In studies near Canberra, Armidale and in one year along the Strzelecki Creek, nests were spaced 5–6 km apart, while a study in Tasmania found nests werespaced 9–10 km apart. Densities are reported of from one pair per 10 km2 near Yellingbo, Victoria, to one pair per 53 km2 in semi-arid New South Wales, and up to one pair per 108 km2 in some parts of arid Western Australia and New South Wales. On the Nullarbor of Western Australia over several years, there were between zero and 21 occupied nests per 1000 km2 and near Carnarvon seven to 14 active nests per 1000 km2. In Tasmania, where Nick Mooney and, more recently, Bill Brown, have spent many years following up reports and surveying new areas, there are an estimated 450 territories, with an average one pair per 80 km2. This ranges from one pair per 12–15 km2 in the north-west, to perhaps one pair per 700–1000 km2 in the south-west (in places where fertility of the land is low). In the best eagle habitat on the island, a mosaic of dry sclerophyll forest and grassland at low to medium altitudes, nearest neighbour distances average 6–8 km; at the other end of the scale, particularly in regions where disturbance has removed pairs, nests may be on average 20–40 km apart. Home range has never been measured with marked or radio-tagged paired adults, which would be the only way to get an accurate estimate. Based on other similar eagles, they are likely to regularly travel tens of kilometres from the nest site hunting and prospecting. Younger, unpaired eagles have not been tracked but are thought to wander, occasionally taking up territories where, for a time, there is adequate prey. These temporary territories tend to be where the habitat is unsuitable for breeding and there are no residents.
The eagle’s sky dance: courtship ‘…the male began a plunging flight which is used particularly in the presence of the female. Still flying behind her, he dropped with closed wings but this time fell only ten metres before pulling out and into a vertical climb, until he stalled, appearing for a moment in heraldic pose as though suspended in the sky. It seemed that he must fall over backwards but the head dropped down and he moved into the next of a series of dives. I have seen this display
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many times and recovery from the peak of the climb seems almost always to be by the stall or half roll. Once however, on a warm and still morning, I watched an eagle go through a series of deep, swinging plunges in which, time and again, the impetus of the climb was continued up and over in a series of flowing backward loops of great beauty.’ David Hollands (1984).
As the breeding season approaches, beginning about May-June (July-August in Tasmania), pairs more frequently perform the exuberant flying displays that help define territories and cement the pair bond. Together they may soar together on thermals, circling up until invisible to the naked eye, then plummeting down. The male in particular puts great energy into spectacular displays of aerobatic skill. Mated pairs or lone males plunge, wings drawn back, pitching forward into a steep descent and then swooping up with the momentum to poise for a moment before tipping to fall again in a spectacular series of descending pot hooks (see page 45). The pair make playful mock attacks at each other and roll and parry, or the male pursues the female slightly above and behind, beautifully mirroring her movements. Sometimes, the lower individual rolls to present talons at the swooper, often the male. Very occasionally, they lock talons and cartwheel down, in a similar fashion to the genuine tussles of serious territorial disputes. The pair follow each other to the trees, where they land with a flourish, yelping excitedly, and may copulate. Quite often, the pair roost near to each other, often side by side. They are among the most affectionate of raptors and, unlike most, enjoy physical contact. They often allopreen each other, one nibbling gently at the bill or nape feathers of the other. As the courtship phase progresses the female becomes more inactive. She may perch and wait for the male to bring her food, either small prey or butchered parts of larger prey, plucked and otherwise prepared. These food exchanges are an important part of courtship, they serve to fatten the female so that she can lay eggs and survive the incubation period without hunting, and are thought to be a means by which females can gauge the quality of the male as a future provider of food. The female can then lay a clutch that will hatch the number of chicks that he might be expected to be able to support. The male’s gifts of food are sometimes, but not always, followed by copulation. Contrary to some fanciful accounts of mating on the wing, made as recently as the 1970s, the eagles prefer a steady branch. In the copulation ritual, both of the pair assume near horizontal positions, crouching slightly,
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and the male stretches out his neck, sometimes to rub bills with the female; both utter a loud, harsh, whining, ‘chew chew chew’ call like the food begging call of larger (hungry) nestlings. Often though, especially in established pairs, copulation proceeds without prior ceremony. The male lands on the female’s back, balancing with feet clenched to avoid harming her, and twists his tail under hers until their cloacas touch and she receives his sperm (he has no intromittent organ). Copulation is brief, lasting a few seconds, perhaps six to seven. After the male dismounts the pair may ruffle their feathers and rouse, or preen, or simply go about their business.
Mate choice and fidelity ‘…not every eagle keeps a partner for life. Some flighty birds tire of each other after one or two seasons, while others are content with their first choice and keep together for years.’ Thompson (1946).
Although it is often claimed that eagles mate for life such throwaway statements are a bit careless. Their monogamy has never been verified, as it could be with banded individuals or by molecular means. Nevertheless, there is no reason to suspect that infidelity or divorce is common. In this era of DNA fingerprinting, raptors are proving decidedly faithful when many other apparently monogamous species, especially passerines, are being caught out. Monogamous or not, they may still occasionally divorce, and changeover of individuals at some sites is likely to be high because the sites are suboptimal or highly disturbed. This can happen without any obvious sign and, unless the replacement bird is markedly different, human observers are none the wiser. Subadult pairs can form and an adult may bond with a subadult. If they are like other raptors (and humans!) in this respect, females prefer to pair with adult males, but male preference for adult females is not as strong. Overall, like-aged pairs probably tend to be most common.
The eagle’s nest ‘The nest was twenty metres high in a tall, white-trunked manna gum which grew out of the steep side of the gorge. The eagles built it years before. They had added to it again and again so that it now looked like a grey haystack in a tree.’ Graham Pizzey (2000).
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The eagle generally builds its nest in a sturdy fork or, less often, on a horizontal branch, but occasionally extends an old Whistling Kite’s nest or the nest of another large raptor such as a White-bellied Sea-Eagle. The nest site is mostly a live tree, but occasionally a dead tree (perhaps live when first occupied), rarely a cliff, rock pinnacle or breakaway. In arid areas with low vegetation the eagle may choose a bush, still the most commanding position in the stunted landscape. On islands it may even build on low ground, above the high tide mark. The main feature that nest sites share is a prominent position and, where possible, shelter from the prevailing winds and hot afternoon sun, and access to the morning sun. Energetics are also a consideration in the siting of nests because adults must carry heavy prey to the nest and active, flapping flight is energetically costly for the eagle. So, for example, although typically nest trees are located about mid-slope, where the slope is particularly high but hunting land is below, the eagles may nest lower on the slope. Another requirement is freedom from disturbance by humans, and eagles tend to nest well away from farm-houses and other human activity. Elevation varies according to landform, and nest height above the ground may be from zero to 50 m or more, usually about three-quarters of the height of the tree and with some canopy above. Provided it is a tall robust tree, with a commanding view, on the lee side of a hill, or riverside, the tree species is unimportant but includes: Leopardwood, Beefwood, White Callitris, River Red Gum, Belah, Mulga and Needlewood on the mainland, and Blue Gum, White Gum, Mountain Ash and stringybark in Tasmania. On rare occasions, a power-pylon is a good tree substitute. In Tasmania, the eagles nest almost exclusively in forest, usually choosing the largest tree on a sheltered slope, as elsewhere; in that State, nests in woodland or isolated forest trees, which are used occasionally, do not stay active for long. Nick Mooney believes that trees with few branches near the ground are preferred. ‘The nest is made of thick sticks piled together in a slovenly fashion, till the entire structure would form a good load for a cart.’ James McDougall in A.J. Campbell (1901).
The nest itself is a bulky structure—often the accumulation of several years of use and refurbishment by the eagles. Old nests may be two metres across and three metres deep, with over 400 kg of wood. Typically nests measure about 1.5 m in diameter and 1 m deep. The structures are probably smaller in more open country than in forest, where it may be less sheltered and there is
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less dead wood. In open, arid country of Western Australia, Ridpath and Brooker measured several: they were 70–90 cm across and 30–80 cm deep. Both members of the pair contribute to the structure but the female may take on the greater burden of the task. They snap off dead branches by landing heavily on them and catching them as they fall, breaking off smaller ones in flight, mostly carrying them in their feet, and arranging them with their beak, tucking the stiff sticks into the growing pile. Occasionally they collect branches from the ground, but take-off is difficult with such awkward cargo. To complete the nest, sprays of eucalypt leaves are brought to line it. The egg cavity is shallow and is deepened by the weight of the bird during incubation eventually measuring about 35 cm across and 10–15 cm deep. Fresh greenery is added throughout incubation and the early nestling period. It has been proposed that this either acts as an insecticide to help keep nest parasites under control, or, because it is obvious from the air, advertises that the nest is occupied. Of course, it could serve both purposes. ‘The Wedge-tailed Eagle has an extraordinary habit of building a nest for some purpose other than breeding.’ Thos. P. Austin in North (1911–1912).
Each year, in about April-May (July-August in Tasmania), a new nest may be built or old one(s) refurbished. The nesting territory may be peppered with nests in various stages of disrepair. A pair has between two and three nests on average, but occasionally up to 10, sometimes several to a tree. In Tasmania, the number of nests per territory ranges from 1–5, and averages 1.4. Of these, in any year between one and three are in good condition. Sometimes the same nest is used repeatedly but, in one study, 48 per cent of nesting attempts were in refurbished nests and only 20 per cent were occupied twice in a 10-year period and then rarely in successive years. In a Tasmanian study, most nests were used repeatedly if undisturbed, even if they failed to produce young; other nests were deserted following failure. Nests can be lost or made ineffective by outside forces such as tree death or falling, the nests themselves can be blown out or become too heavy for the tree, and large branches can fall on the nest. Several of the structures are used as feeding platforms, vantage points or roosts and some are never lined and put to use as a nest. Finches, pardalotes and even possums may nest among the sticks at the base of active nests, perhaps gaining some protection from the presence of the eagles. Other species, such as Black Ducks and Peregrine and Brown Falcons, may also benefit, taking over unused nests for their own breeding purposes.
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An adult Wedge-tailed Eagle.
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A pair of Little Eagles—the only other true, or booted, eagle in Australia—at their nest.
A pair of Wedge-tailed Eagles—the smaller, darker, male at left—with their chick.
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As other eagles arrive at a carcass, a young eagle partially raises its feathers in threat. The buff tips to its feathers are a clue to its age.
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The best view for miles: a downy chick sits safe in its roomy nest in north-west New South Wales.
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A typical clutch of two eggs in a nest lined with eucalypt leaves.
A chick, still bleary-eyed, about four days after hatching.
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A female prepares a small bird to feed her downy nestling.
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Holding the prey with her feet, she tears small slivers of flesh and gently offers them to the nestling, which reaches out to accept.
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A lizard is readied for a 10-day-old nestling’s lunch on a nest littered with the remains of a rabbit from a previous meal.
A downy nestling spreads its wings and rears back as the photographer nears, its crop bulging after a feed. In slightly older nestlings this defensive position frees the feet, with their already powerful talons, to grab at the attacker.
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A female feeds her four-week old nestling, with a crescent of brown feathers breaking through the thick down of its wing and back.
Eagles nest where prey is richest, such as here, along an ephemeral inland river. They will choose the largest and sturdiest tree which, in low woodland, may be only a few metres above the ground.
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A handsome seven-week-old nestling, close to the weight of an adult but still downy on its head, neck, and patchily on its body. The massive nest has been added to for years.
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A near fledgling about 11–12 weeks old energetically tests its wings in preparation for first flight.
A young eagle, its age revealed by the fawn scattered over its upper surface and throat, and barring its feathers, opens the ‘fingers’ of its upturned wings to let air slip though and reduce lift.
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An adult, black with unbarred flight feathers, aligns eyes and feet as it lands on a log, much as it would when about to strike prey.
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When its mighty feet are tucked under its tail, the aerodynamic shape of the eagle’s body becomes obvious. Once airborne its large wing and tail area makes soaring and gliding almost effortless.
Young eagles sometimes gather in large flocks and feed together on large carcasses. Here a very blonde-naped individual shares a kangaroo carcass with another young bird.
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A young female feeds on a feral goat carcass left by shooters.
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An eagle eyes the fox that has joined the queue of eagles attracted by shooters’ spoils.
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Several dead eagles around the carcass of a poisoned sheep, themselves partially eaten by other eagles. Such wanton destruction is illegal.
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The eagles defend the active nest against goannas, striking them repeatedly, and other would-be predators, but are surprisingly timid towards humans. A few will dive from high in the sky, swooshing by at great speed, but have not been known to strike. One view has it that centuries of persecution selected for timid eagles that would flee the gun. There are always exceptions but, in general, the eagles are intolerant of disturbance and will desert the nest if the site is visited when they are preparing the nest or about to lay; as breeding progresses they are less likely to abandon chicks, especially large ones (that have been a large investment). This sensitivity to disturbance is recognised as one of the main threats to the endangered Tasmanian Wedge-tailed Eagle.
When to breed: timing Wedge-tailed Eagles are seasonal breeders; most clutches are laid in winter, July–August (September–October in Tasmania). The season varies with latitude and local conditions and is timed so that the nestling-fledgling periods coincide with expected times of prey abundance. Thus, laying tends to occur earlier in warmer and more northern areas and later in cooler more southern parts. For example, the peak laying month is June in New Guinea and August–September in Tasmania. Presumably daylength plays a part and the shortening days of winter (mainland) or lengthening days of spring (Tasmania) trigger the urge to breed, the timing of which is fine-tuned by food availability and, hence, body condition. Within this three- or four-month window of opportunity to lay, females must attain the necessary level of condition. If they do not put on sufficient weight they will not breed and, if it takes some time to put on the weight, breeding is delayed till later within the window. Thus, depending on seasonal conditions, in any particular area the beginning of the egg-laying season as well at its length may vary: it tends to be earlier and extend longer in years when prey is abundant; similarly individual pairs nesting near prey abundances tend to lay earlier than those in poorer areas, and old experienced birds may lay earlier than novices.
To breed or not to breed: reproductive success Although there is an annual breeding season, not all pairs breed every year. During prolonged drought some pairs may not reproduce for several years. In Tasmania over a three-year period an average of 54 per cent of territories were active, that is, held adults that prepared a nest. Again food is of great influence and in poor areas or seasons relatively few pairs attempt to breed. Pairs catching medium-sized prey that can be carried to the nest are more likely to breed
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and tend to be more successful in raising chicks than those with access only to large prey, very small prey or carrion. Indeed, the eagles tend to avoid feeding carrion to nestlings. Thus, in regions where rainfall is most regular, prey is usually diverse and prey populations relatively stable and eagles can breed regularly. It is also likely that in areas where they can specialise on just a few abundant prey species of medium size the eagles tend to have greater breeding success than where they must draw on a wide range of prey. Because reproductive success is quite variable and can be measured in different ways, it is difficult to generalise. Failure can occur at any stage but mostly either pairs fail to lay or chicks are lost soon after hatching. In studies involving many pairs, of those that laid a clutch between 52 and 90 per cent fledged at least one young. This translated to between 0.2 and 0.5 chick per pair, 0.7 to 1.2 young per clutch, and 1.1 to 1.3 young per brood (0.9 in Tasmania, where the clutch is smaller). At Burrendong Dam, in temperate inland New South Wales, from 1999–2003, 15 pairs produced an average of one chick per territory except in 1998, following a drought, when only 0.4 chick per territory was recorded. At Kinchega National Park, arid New South Wales, in the late 1970s to early 1980s, the number of young per pair averaged 0.99 and varied little over six years, whether conditions were favourable or severely drought affected. This figure could be read to suggest that drought had little effect on eagle breeding. However, the number of pairs ranged from three in the most drought-affected year to 12 in the best year, such that the annual number of fledglings for the population varied four-fold. The three regularly breeding pairs nested near permanent water, where rabbits persisted, thus underlining the importance of a good territory. Adults tend to enjoy greater breeding success than younger birds, at least in part because they can be expected to hold the highest quality territories, as in Imperial Eagles. Like many aspects of eagle life history there is probably a density-dependent relationship between the number of floaters and breeding success of residents, that is, a feedback mechanism regulating the total number of eagles in the population. When the number of floaters is high, the pressure on breeders from intrusions by would-be usurpers is high, which tends to suppress the breeding rate. In the past, one pair is claimed to have raised two broods in a season, but this isn’t possible given that the cycle from laying to fledging takes four months and there is a period of fledgling dependence on top of that. All told, the parents spend 8–10 months, from egg-laying to independence of their
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offspring, which leaves only a couple of months for courtship and nest building. Not surprisingly, some pairs cannot muster the resources to begin again, and take a year off. By contrast, one particularly successful pair under observation bred every year for a decade, fledging 12 young for an average of 1.2 per year.
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9 From egg to adult
‘[The egg] … is clouded with large blotches of pale purple, and small specks and dashes of yellowish umber-brown on a stonecoloured ground, and is three inches in length by two and a half in breadth.’ John Gould (1865).
The eagle lays attractive eggs: buff white, speckled, spotted and blotched with purple-brown or red-brown and lavender. When fresh they are glossy but become more matt with age. Some eggs are very heavily marked and others pure white, but most have scattered markings which sometimes merge into large patches or caps, most often at the more pointed end. The eggs within a clutch do not necessarily match; quite often one is much less heavily marked than the others. The female lays an egg every three days or so until the clutch is complete. Usually, the eggs are rounded oval with one end slightly more pointed than the other, but sometimes almost globular. At 7.3 cm long by 5.8 cm wide, larger in Tasmania and smaller in the northern Australia, they are sizeable. Occasionally there may be a runt among them, as little as half the dimensions of the others, which never hatches. Each egg normally weighs about 120–150 g, equivalent to about three domestic chicken eggs and about 3 per cent of the female eagle’s body weight, 10 per cent for a full clutch of three—a small investment compared with other smaller species. If a clutch is lost or stolen early in incubation, some pairs will replace it, relaying after an interval of about a month.
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One to three eggs make up the clutch, but more than 80 per cent of clutches are of two eggs. Extremely rarely, four eggs are laid. As is often the case with island populations, Tasmania birds lay a smaller clutch than those on the mainland, with a maximum of two eggs. The influence of food abundance, and through it body condition, is illustrated by the finding that clutch size of the Wedge-tailed Eagle decreased after broadscale rabbit control brought about by introduction of the myxoma virus, which had spread throughout the Australian rabbit population by 1954. Clutches of two were more common before control than after. These changes were not due to a shift in laying date (important because clutches laid early in the season tend to be larger than those laid late), suggesting instead an adaptive response to a change in food availability. The incubation period is fairly fixed at 42 to 43 days for each egg. Typically incubation starts after the first egg is laid. Both parents share in this task during the day and the female alone does night duty. They develop a single, highly vascularised brood patch, a strip bare of feathers down the centre of their belly, to allow effective warming of the eggs. Because the surrounding feathers close over it, the brood patch is not evident during other activities. As is typical of birds in general, the eggs lose about 14–15 per cent of their weight throughout incubation. A few days before pipping the chicks start peeping and are encouraged by soft calls from the mother. In these days immediately before hatching the incubating adult quite often stands quickly and look quizzically at the eggs, probably in response to the chicks’ calls or movement. The chicks use their egg tooth to raise a little bump on the surface of the egg, the pip, eventually breaking through and turning to chip in a circle to separate a cap, which they then push open; this is an exhausting process that takes one to two days. Because incubation is not delayed until the clutch is more complete, the second chick hatches about three days later and so on. This creates a family staggered in size. After struggling from the egg, the little eaglets are wet and feeble. Their head is so large and heavy atop a spindly neck and wobbly little body that they hold it up only briefly before it lolls. Their eyes are open but the chicks do not appear to be fully sighted. Only the aquiline beak and relatively large feet give any hint at the chicks’ future strength. Under the mother’s warm breast their sparse down dries quickly. At some nests greenery is brought in immediately and the chicks may be fed within half an hour or so. At other nests they are kept well brooded but may not be fed for the first day or so. ‘
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Infants that commit murder’, A.H. Chisholm (1934) p. 61.
‘The newly-born eaglet was much the smaller of the two, as yet it scarcely had the strength to lift or even move its grotesque, wobbling head on its scrawny neck, and Lanny hoped that, as was often the case, the stronger first-born would not harry and peck the weakling to death.’ Mary Patchett (1960) p. 140.
If the pair has miscalculated and cannot raise as many chicks as they have hatched then siblicide trims the brood to the number that can be fed. This is entirely a matter for the chicks and the parents do not intervene. Inevitably, if a chick has to go, the last hatched and therefore smallest suffers. Within days of hatching it is pecked and kept from food by its sibling until exhausted, and killed outright or forced from the nest. After the first week or so these murderous tendencies subside. In some aquilas this Cain and Abel struggle, named for the biblical brothers, is obligatory and only one chick ever survives. For Wedgetailed Eagles the behaviour is facultative and adjusts the brood to prevailing food resources. Hence, in bumper years, some mainland Wedge-tailed Eagle
around 5 weeks
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around 6 weeks
around 6 weeks
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pairs raise three fat chicks, and two chicks are common, although in most years a single chick is the usual brood size. The situation is similar in Tasmania—only 2 per cent of pairs raise more than one chick—but the maximum brood is two. Pairs living in more equable regions raise more chicks on average, and presumably suffer fewer losses to siblicide than those living in unpredictable climates. A different phenomenon likely explains the small clutch and, hence, smaller average brood size in Tasmania. Island populations that are relatively small and contained tend to have lower adult mortality rates and hence need fewer recruits, so that a small clutch is favoured. This is an ultimate, evolutionary adjustment, rather than a proximate response to local conditions in a particular season. There may well be other parts of Australia where the maximum clutch is also two, for example, northern Australia. ‘When the young are about a fortnight old they are clothed in white down, which is long and hair-like about the head and neck, and there is just an indication of black quills.’ North (1911–1912).
The chicks grow quickly. From about 100 g at hatching their mass increases five-fold to around 500 g in a week, and they are more or less the weight of an adult at fledging. Because the wing grows on almost regardless of body condition, it is useful for roughly estimating age. The formula for nestlings is: age in days = (wing length in cm – 2.37) ÷ 0.6
Measured with the wing folded against the eaglet’s body, from the carpal joint to the tip of the longest flight feather, wing length increases from about 8 cm at ten days of age to 51 cm at 80 days. The first down is white and silky, longish and punk-like on the head, and grows from the same papillae from which the brown juvenile feathers later emerge. A second shorter, slightly greyer, denser down begins to replace it from about 12 days or so and becomes the woolly undercoat for the brown contour feathers. By three weeks of age the chicks are robust and woolly, and have become steady on their feet. They are left for longer periods of the day, brooded only at night and in extreme weather. ‘…the train and primary feathers appeared like the tips of small flat paintbrushes…Each wee brush was tipped by a tiny piece of protective down which gradually peeled off and left the perfect feather showing.’ Mary Patchett (1960) p. 168.
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The pins of the primary (wing) feathers are emerging when the nestlings are about 10 days old and they begin to burst from their quills around 17–20 days. By two weeks of age the black quills of the primaries have begun to sprout, and by 28 days upper wing coverts are appearing. At 35 days dark feathers are appearing on the breast, belly, mantle, back, forehead and earcoverts; most evident as a few dark rufous feathers poking through the head down and a short buff-tipped tail. At seven weeks the nestlings are almost fully feathered but the wing and tail feathers are yet to reach full length; a few wisps of down may remain on their crown or neck. As the nestlings become more active, the female leaves them snoozing flat on the nest or active, for longer periods. She returns with sprays of fresh gumtips, gracefully arching her neck to tuck them into the nest. In rain or during the heat of the day she shields the young with her body, wings extended, until the rain eases or they are in dappled shade again. ‘… then the female would spread her great wings for an umbrella over the young, the black flight feathers clean and shiny and the fawn shawl of upper-wing covert feathers looped across the wing.’ Graham Pizzey (2000).
The male delivers prey several times a day and in years of plenty uneaten carcasses may build up on the nest. He rarely feeds chicks directly but calls as he arrives, prey often dangling from one foot, to pass it to the female at nest, on a nearby perch or in the air. Early in the nestling period she dissects the prey and passes tiny slivers of tender flesh to the youngsters, swallowing the bone and gristle herself. The chicks respond to the sight of her movements and reach up to take the meat she offers. They tire easily and sometimes she must gently coax them to open their beak so that she can place the food in it. Especially when they are tiny, saliva and fluid from the salt gland in her nostril visibly runs over the meat, passing electrolytes, enzymes, helpful bacteria and moisture to the chicks. Within days the chicks are much sturdier and take a more active part in feeding, reaching up eagerly. The eaglets spend much of the day lying prone in the nest between feedings, whether they are brooded or not. As hunger sets in, they awake from their post-feeding torpor and shuffle about cheeping to be fed. In their fourth week, between sleeps, they begin to walk about, and become more animated. They back clumsily but carefully to the nest rim, raise their tail and defecate, with a forceful squirt, over the edge of the nest. This helps to keep the nest clean. When a parent returns the chicks prostrate themselves with head down, tail raised and wings slightly open, in a submissive ritual. If brought a meal,
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after a short silence they begin to cheep, if not, they resume their normal activity. Early in their second month the eaglets are eating pieces of bone, grabbing at food, swallowing whole lizards, and can tear open carcasses themselves even though they seem to prefer the parents to do so. Even when still mainly downy, the gangly youngsters energetically flap their stubby wings and hop about in the nest. Later still, as they grow flight feathers, the by now red-brown eaglets face into the wind and eventually begin to achieve short lift-offs, apparently to their great excitement and surprise.
From eaglet to eagle Sometime around 11 to 12 weeks of age the large eaglets leave the nest, at first returning at night or to be fed. They now have just a few traces of down remaining on the throat, upper breast and vent, but are almost as large as their parents. With their lighter wing loading, males can take to the wing at a slightly younger age than can females. This is because their wings grow at the same rate but the female puts on mass faster, so that as nestlings of the same age they have a similar winglength but the female is markedly heavier. By then it is often November-December (mid- to late January in Tasmania), when many other young birds are also fledging. Ungainly at first, the fledglings gradually learn to fly and hunt and become more independent. At first they are quite vocal, yodelling at returning parents in excitement, ever hopeful of a meal. They perch rather inconspicuously and fly clumsily on longer more flexible wing feathers than their parents, with the secondaries noticeably broader, to give them more lift. As they gain confidence the fledglings perch in the open and begin to soar with their parents. They venture further from the nest, and follow the adults to feeding areas or carrion. The adults continue to catch prey for the fledglings for some months and are often hounded, sometimes quite aggressively, for food. The male has a significant size disadvantage when daughters are intent on monopolising prey he may bring, and both adults tend to deliver the prey and depart quickly when their fledglings are aggressively hungry. Around four to six months after fledging, as pairs start redefining territories for the next breeding season, the juveniles go their own way. If food, particularly carrion, is plentiful, the juveniles they may leave their natal area earlier, just two to three months after fledging. But some appear to stay in the family group for as long as six months, by which time they are nine months old. There is at least one record of a yearling, still being fed by its parents, apparently killing their new nestling. Perhaps because of this danger, it is usual for the parents to eject any remaining previous year’s young well before the parents restart the cycle by laying again.
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Independent young may band together with other youngsters, especially where there is carrion. For the first four to five years they join the so-called floaters, young birds and unattached adults that tend to live where there are few territorial adults. Breeders are recruited from among the floaters, which either fill a vacancy or forcibly evict a territory owner. Typically, vacancies are filled within hours or days, rarely months. New recruits tend to be mature birds (four-plus years) but occasionally younger eagles find an opening, for example, if mortality among breeders has been high. Recruitment is probably density-density dependent, as in the thoroughly studied Imperial Eagle. That is, eagles are recruited into the breeding population at an earlier age when population density is low than when density is high. There have been no studies on the survival of individuals before they are recruited into the breeding population. Possibly about two-thirds of fledglings die before they reach breeding age at three to five years. Almost certainly the eagles are like many raptors, in which males especially tend to return to near their natal area intermittently, hoping for a vacancy in the local breeding population. On average females take up positions among the breeders a good year or two earlier than males. This is probably because the male must supply much of the food for the breeding attempt, so his hunting skills must be more finely tuned by experience than those of the female. Once settled on a territory the chances of survival are much improved. The annual mortality rate may be in the order of 2–3 per cent a year for territorial adults and a lucky few might live for a full half-century. One captive died at a respectable 40 years of age, after a bone embedded in its throat, rather than from old age.
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10 Hunting and prey
‘For the sake of refuse thrown away by the Kangaroo-hunters it will follow them for many miles, and even for days together… I saw no less than thirty or forty assembled together around the carcass of a dead bullock, some gorged to the full perched upon the neighbouring trees, the rest still in enjoyment of the feast.’ John Gould (1865).
It is relatively easy to gather pellets from in and around nests and, in part because of controversy over the eagle’s impact on livestock, it’s breeding diet has been much studied. Researching the diet of eagles that are not breeding is more challenging. Like all raptors the eagle regurgitates a wad of indigestible material: fur, feathers, scales and some bone, much of which can be identified to species of origin. This is what is left after the eagle’s highly acidic stomach (pH about 1.7) has digested the food, and is quite sterile and not unpleasant to sort through to identify its contents. Pellets are ejected about once a day, provided the eagle has fed; that is, about 21–22 hours after a meal. The rest of the waste is excreted, as uric acid and faeces, which are often voided together as white urine with a small amount of darker faecal material. The Wedge-tailed Eagle captures, steals and scrounges a wide range of food, from koalas and kangaroos, to wallabies, foxes, cats, rabbits, goats, lambs, possums, Echidnas, Tasmanian Devils, quolls, reptiles and birds including Galahs and larger parrots, crows, kestrels, magpies, kookaburras and ducks, and even the odd fish. Typically the bird species taken weigh over 100 g and the mammals
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over 500 g, but prey as small as a house mouse or small bat and as large as a Red Kangaroo, Emu or goat have been recorded. Reptiles taken range from large goannas to bearded dragons and shinglebacks, but seldom snakes. In New Guinea, wallabies, bandicoots, Rusa Deer and flying foxes are among its prey. ‘A bat was disturbed from a hollow post during daylight, and it fluttered blindly up and up … toward a soaring Wedge-tail— which languidly thrust out a claw and grasped the tiny wanderer.’ A.H. Chisholm (1934) p. 103.
The eagle’s diet depends on region, season and availability (and age and experience of the eagle). Having such a catholic diet gives the eagle great flexibility and buffers it against shortage by allowing it to switch between prey types. In any particular area the eagles may specialise on one or two prey types, and, where possible, medium-sized mammals are the main component of the diet. Main items in the diet of Wedge-tailed Eagles in different regions, as a percentage of the total number of items found in pellets and prey remains in and around nests. If the weight contributed by the various items is taken into account, birds and reptiles become less important than heavier prey. Not only do different locations and time periods produce different results but different techniques do also. Rabbits
Sheep Lambs
MacroOther pods mammals
Birds
Reptiles No. of food items
Dry west coast WA 1970s
9.6
2.3
9.1
17.7
16.7
23.4
21.1
384
Dry sth. inland WA 1970s
92.2
0.4
0.4
1.6
0.2
3.3
1.7
759 552
Inland SW WA 1970s
33.5
2.2
11.8
2.7
1.3
39.6
8.9
Mildura,Vic 1980s
74.7
1.1
0
0
2.2
22.0
0
91
NE Melbourne, Vic 1980s
48.7
2.3
0
3.5
27.9
17.8
0
250
ACT 1960s
45.1
1.7
6.8
1.9
22.8
18.1
3.5
689
ACT early 2000
25.0
2.9
0
22.1
-
37.5
-
Armidale, NSW 1995
60.0
0
0
16.0
0
16.0
8
Western NSW 1990s
47.8
3.0
0
13.7
63
8+
22.6
2071
Western NSW1 1990s
31.5
24.5
0
9.9
3.7
8.8
21.6
273
Western NSW2 1990s
39.1
5.3
0
11.5
0.8
11.4
31.5
994
NW NSW–SW Qld 1960s
30.6
0.8
6.8
25.5
3.5
15.2
17.6
369
NE NSW 1997
18.3
0
0
0
31.6
26.7
23.3
60
0
0
0
66.7
24.2
0
9.1
33
46.5
0
0
28.9
6.1
7
10.5
114
3
including 4% goat
Idalia Nat. Park, Qld 1990 SW NT 1990s Note: pellets only; 1
2
nest remains only;
25
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On the Nullarbor Plain in the 1970s eagles preferred rabbits even though other prey were available; in the south-west of Western Australia they ate a diverse diet in which birds, kangaroos and sheep were most important; and on the arid west coast, sheep and kangaroos predominated in the diet, but numbers of foxes and cats were also taken. Rabbits are the perfect prey for eagles, and wherever they are abundant they are favoured above other prey. The introduction of the rabbit to Australia may well have allowed an increase in eagle numbers, though native prey in that same size range declined at the same time. In areas where rabbits decreased because of deaths from calicivirus (rabbit haemorrhagic disease), the eagles turned to reptiles, birds, young macropods, and kangaroos and other carcasses left by shooters or killed by vehicles—whatever was available. At Burrendong Dam, a 75 per cent reduction in rabbit numbers had no discernible effect on the reproductive success or numbers of breeding pairs in the local eagle population, even though rabbits had been a staple, and the adults continued to feed rabbits to their chicks even when rabbit numbers were quite low, but themselves ate other prey or carrion. Indeed, in general, live prey is favoured by breeding eagles to feed their chicks, whereas subadults and non-breeders take whatever is easiest, often carrion. ‘I have seen it on more than one occasion rushing through the tall timber, with wings half closed, after large wallabies, but have never witnessed the closing struggles between them; the Aborigines, however, tell me they kill and devour them.’ George Savidge in North (1911–1912).
Eagles typically hunt in savannah and open woodland, or perhaps that is where they are most easily observed. Remarkably adaptable and opportunistic, there are few habitats that they do not use. They hunt possums on the densely forested eastern slopes of the Great Dividing Range, making good use of clearings and road cuttings but also the forest proper, preferring areas with little understorey. They chase currawongs through the canopy of closed forest and pull possums and birds from their nests, but overwhelmingly take prey from the ground. Only rarely do they capture prey on the wing. In the rainforests of north Queensland various species of ringtailed possum and Spotted-tailed Quoll are common prey. Indeed, it has been suggested that, as the only likely major predator of several large tropical parrots such as Palm Cockatoos and Eclectus Parrots, the eagle helps shape the parrots’ life history and behaviour. In Tasmania many of the eagles live almost entirely within forested country.
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‘Although Wedge-tailed Eagles prey to a large extent on rabbits, they do not always confine themselves to small quarry … in Gippsland I have often seen them attack full-grown native bears, and on one occasion a pair attacked a half-grown kangaroo, but I did not see the result as it was in a thickly timbered place.’ K.H. Bennett in North (1911–1912).
Most foraging is by high soaring and prospecting, although the eagle sometimes hunts in a more directed manner, along a transect. It also hunts from a prominent vantage point, attacking in a short, swift glide, often making good use of the element of surprise, sometimes striking from the cover of the blinding sun. It is attracted to fires, catching animals fleeing the flames. Stock, tractors and people can serve the same purpose, to flush prey. The eagle has learnt to trail kangaroo hunters’ vehicles in the hope of discards, and follow moving stock or deliberately rouse it in anticipation of catching any prey it flushes. At carrion, the eagle dominates most other scavengers, except the larger mammalian predators. It sometimes robs other animals, harassing Brown Falcons and other raptors until they drop the prey they are carrying. Foxes with prey are targets for theft and in turn occasionally rob eagles. Goannas are struck repeatedly until they weaken, even as the goanna clings with strong claws to a tree.
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Although the eagle often begins to soar and prospect for a meal around midday, later than most other raptors, it may also hunt in the early morning and late evening, taking nocturnally active animals such as Banded and Rufous Hare-wallabies and Burrowing Bettongs as they emerge for the night. Presumably, it adjusts to the activity of its favoured prey, depending on region, season and conditions, though it sees poorly in twilight. On bright moonlit nights, however, the eagle has in days gone by been seen to remove rabbits from traps and, more recently, to feed on carrion. The eagles are not particularly swift but make up for lack of speed with power, stealth, perseverance and a certain cunning. They use hunting areas in rotation, in response to changes in prey numbers and behaviour. They also use teamwork—a pair of eagles may land near a ewe or sow and attempt to separate its lamb or piglet. Especially where prey is large, such cooperative hunting is common. Of 89 hunts observed in Central Australia, about one-third were cooperative. A pair of eagles, less often a group, cooperates to flush prey into the open. One flies low over spinifex followed by the other a bit behind and above, ready to swoop at unsuspecting animals flushed by the first. When hunting large prey the eagles almost invariably hunt in tandem, taking turns to swoop at the victim until it is exhausted, sometimes bailing it up against a fence. Adult dingoes and kangaroos are overcome in this manner even though they are many times the weight of an eagle (more than ten times). ‘I did not see the commencement of the attack, but when I came across them they had evidently been at the dog for some time, for he was very much exhausted, and was staggering along in an aimless manner. One Eagle was perched on the dog’s neck and flapping its wings, the other perched on his loins; occasionally the latter would turn his head and snap in a feeble manner at the Eagle, who would simply fly up, and the next instant drop on his loins again. This continued for some time, the dingo evidently getting weaker and weaker, until he stumbled, fell and lay perfectly still.’ K.H. Bennett in North (1911–1912).
At carrion, once it is sated the eagle may perch nearby, guarding the carcass. Even innocuous Emus that happen to pass too close get moved along, sometimes losing a tuft of feathers from their rump to the eagle in the process. Kangaroos killed by vehicles are common on outback roads, and many an eagle has been killed in such a situation, caught eith heavy crop bulging, unable to take off quickly or evasively enough to avoid becoming a road casualty themselves.
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‘The average eagle in Scotland is able to fly with a weight of at least nine to eleven pounds. We can do much better than this in Australia.’ A.H. Chisholm (1929), who believed reports of eagles flying off with 40- and 50-pound sheep—a physical impossibility.
The eagle’s crop allows it to gorge at a kill or carrion, packing in as much as one-third of its own bodyweight (1–1.5 kg or more), after which it retires to perch and quietly digest. Capable of killing larger prey, the eagle can carry a maximum of about 5 kg, greater than its own bodyweight. Daily intake in captivity is about 6–10 per cent of body weight, 250–300 g or so. Because it spends much of the day in low-energy pursuits, loafing at a prominent perch or soaring without effort, if necessary, a healthy eagle can live several weeks without feeding, perhaps over a month, especially in warm weather, with little loss of weight. If weight loss does reach a critical level, however, the eagle goes into a state known by falconers as ‘yarak’—when it become extremely aggressive and fearless—with obvious survival value. Also in warm weather the eagle may have a need to drink, scooping water up in its lower bill and tipping its beak up to let the water flow down its throat, but most of its moisture is gained from its prey. ‘Mr W Crisp, of Rock Flat, near Cooma, informs me that one day in November, 1900, he counted forty-two Eagles in one of his paddocks, and many people thought they ought not to be destroyed, for although they did harm in killing lambs, they did more good in killing hares and rabbits.’ A.J. North (1911–1912).
What impact does the eagle have on prey populations? This question is at the heart of accusations of significant damage by eagles and also claims that they control rabbits. Where rabbits are common they are eagles’ most common prey, constituting around 43 per cent to 98 per cent of breeding diet. For a Wedge-tailed Eagle pair, over the ten weeks of the nestling period it has been estimated that about 200 rabbits are brought to the nest. Even so, several studies have concluded that the eagle and other raptors have an insignificant impact on rabbit populations. In subalpine Australian Capital Territory, birds of prey accounted for the deaths of 0.7 per cent of newly emerged rabbit kittens per day, and about 10 per cent of kittens overall, another 20 per cent or so were taken by mammalian predators. Little Eagles nesting around Armidale
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were estimated to take 3–4 per cent of rabbit kittens produced annually. At a Mildura, Victoria, study site, rabbits formed the major prey of eight of the ten raptors and it was estimated that through their combined efforts they removed 7–14 per cent of the year’s production of young rabbits during a four month period. Wedge-tailed Eagles in south-west inland WA selected older rabbits (over nine months) even though juveniles predominated in the rabbit population. At low rabbit densities, predation by the Wedge-tailed Eagle, the only raptor that takes significant numbers of adult rabbits, may have some impact. Most likely, however, the eagles would switch to other more easily caught prey before they had an impact on the few remaining rabbits. ‘… does not justify the continued persecution of a native predator that eats mostly rabbits and has scarcely any adverse impact on the sheep industry.’ Leopold and Wolfe (1970).
Wedge-tailed Eagles are often the secondary cause of lamb deaths, they take lambs left dead, sick or injured, due to natural causes, crow or fox attacks. In the 1960s and 1970s thousands of lambs examined in several studies revealed that, contrary to expectation, a very small percentage were eaten by eagles and an even smaller percentage were healthy when taken. Ian Rowley examined 314 dead lambs at Geary’s Gap, New South Wales, of these 185 had been wounded by predators (fox, crow or eagle). Of the five (1.6 per cent) taken by eagles, two (0.6 per cent) had been healthy when attacked, two were of questionable health and one was starving. About the same time, remarkably similar figures were reported for 981 lambs examined in Queensland. Near Canberra, in a typical 3000 ha range, 4000 lambs were born, of which the eagles removed six or seven. At Carnarvon, Western Australia, lambing was watched for 367 hours from dawn till dusk. Up to 11 eagles were present at a time and one or more was there 64 per cent of observation time, yet only two lambs were killed. How can farmers protect their flocks? Keeping the lambing flock close to the house, in a paddock with shelter trees, helps to minimise eagle predation. Leaving carcasses in a paddock for eagles to feed on, away from the lambing paddock, rather than disposing of them, provides alternative food for any hungry scavengers, including eagles. A few farmers have successfully trained their resident eagles not to bother lambing ewes, by scaring off the eagle with noise and movement each time it approaches the lambing paddock. Certainly, the number of lambs taken recently justifies the removal of eagles.
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‘The Wedge-tailed Eagle (Uroaetus audax) is a scarce bird in the Clarence River district, which is probably due to the kangaroo shooters who poison parts of the carcasses of the kangaroos for the sake of obtaining dingo scalps, and I have seen several of these Eagles dead around them.’ A.J. North (1911–1912).
Natural and unnatural mortality The eagle has few natural predators. Foxes, dingoes, large goannas or belligerent Tasmanian Devils are a match for an eagle and may challenge at a kill, but can themselves become prey. Crows and other aerial predators would kill a young, unguarded eaglet, but this is unlikely unless the parent is kept off the nest by human disturbance or the eaglet falls to the ground. A large goanna could climb to some nests and kill even a nearly grown chick, but would normally be driven off by an adult. After starvation, humans cause most eagle deaths, either directly, through persecution or motor vehicle accidents, or indirectly, through electrocution, secondary poisoning or collisions with high wires and similar obstructions. The eagle is timid and prone to desertion of the nest if disturbed during nest site selection, and from egg-laying up until the nestling is a few weeks old. This wariness may be the result of years of intense persecution, but the fact that there is a general trend for large raptors to abandon more readily than small raptors indicates that it is more likely a trade-off that favours adult
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survival over nestling survival in species that are long-lived. If they abandon one nesting attempt the chance is high that they will have further chances to breed. Wedge-tailed Eagles suspected of lamb depredation are still shot illegally and deliberately poisoned by illicit use of acutely toxic strychnine, lucijet or phosdrin-laced baits, though this in not as prevalent as in the past and is sometimes targeted at corvids, Tasmanian Devils or foxes. Other legally available poisons, such as pindone and alphachloralose for rabbit control are highly toxic to raptors (and other birds) when used improperly or when the carcasses of target animals are not disposed of appropriately. Not least, sub-lethal doses of some of these poisons can predispose eagles to other sources of mortality such collision. The eagle has a relatively high tolerance to 1080 (sodium monofluoroacetate) poison; rarely, it may be the unintentional victim of 1080-laced freshmeat baits layed on the ground (not buried) for foxes, dingoes or pigs. Where 1080 is used routinely to control browsing animals, such as in Tasmanian plantations, eagles continue to live and breed successfully. The insecticide DDT, used in agriculture for 40 years from the late 1940s, caused many raptor species to lay eggs too thin to withstand incubation, but the eagles were unaffected. The eagles’ predominantly mammal diet probably saved them from ingesting the high levels to which bird and fish-eaters were exposed. Lead poisoning has proved to be a problem in some large raptors that feed on carcasses of animals killed using lead shot and left by hunters. However, it has not been identified as a significant problem in Australia. Very occasionally eagles are killed in collisions with aircraft or come to grief attacking helicopters that venture into their territory during the breeding season. Five such incidents with helicopters in Tasmania resulted in five dead eagles but no human fatalities. Electrocution and death or injury from collision with wires, fences and cables claim greater numbers of lives. Large birds like eagles present challenges for powerline companies: eagle collide with cables strung across valleys, and can be electrocuted when they try to perch by certain powerline configurations. Cars claim the lives of many eagles—hundreds if not thousands annually nationwide. The proliferation of wind-turbines will likely cause some eagle losses. In Tasmania, three eagles have been killed in the three years since construction of the first wind farms. Bill Brown has analysed causes of mortality for 84 recorded eagle injuries and deaths in that State, 52 per cent (44 individuals) were attributable to electrocution or collision (including windfarm mortalities), 15.5 per cent to persecution (13 individuals), 11 per cent to natural causes and for 21 per cent the cause was unknown. The risks from many of these threats can be mitigated through careful placement and development of more benign structures, scaring devices,
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installation of safer electricity installation configurations, and burying of cables wherever possible. It should be borne in mind that, like most causes of mortality, even though these unnatural losses are sometimes high, they are not necessarily additive at the population level, that is, they tend to replace other causes of death, such as starvation. A simplified example of this is that if persecution is severe then there is more food for remaining eagles and fewer starve. Particularly if mortality is mainly of non-breeders, as was the case with Wedge-tailed Eagles when they were heavily persecuted (see next section), its impact on a healthy population may be less important than appearances suggest. Even if significant numbers of breeders are removed then the eagles can compensate to some extent by breeding at an earlier age (the average age at maturity of the population would be lowered): that is how the Scottish Golden Eagle population is thought to remain stable in the face of persecution (3–5 per cent annual adult mortality attributed to persecution).
Bad press and persecution ‘Not long ago, there was a very sensational picture in a colonial illustrated paper, representing a poor brush kangaroo attacked by a swarm of eagles; the artist seemed to have drawn as many as his paper would hold, and the account given by the person who saw the incident was very sad and shocking’, Louisa Anne Meredith (1880).
‘It was further recommended that crows and eagle hawks be brought under the head of noxious birds, they being equally destructive [as the dingo], and that bonus be paid for same.’ Among the recommendations for amendments to the Marsupials Destruction Act 1877 by the Marsupial Board, The Settler and South Queensland Pioneer, 24 August 1901.
‘Decided difference of opinion prevails as to whether the Wedgetailed Eagle is a useful or destructive bird. The squatters wage war against it with rifle, trap and poison because they assert that it kills lambs. Others who have studied the bird carefully, and examined their nests containing young ones, claim that it lives almost exclusively on rabbits and hares, where they can be obtained.’ G.A. Keartland in North (1911–1912).
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Humans have always had love-hate relationships with large predators and the eagle is no exception. Reports of eagle predation of livestock in Australia were few until about 1860, after which a labour shortage led to a switch from shepherding to open range grazing. About the same time, the first rabbits were released and within twenty years had colonised most of south-east Australia. Presumably the rabbits allowed an increase in eagle numbers and, by 1880, exaggerated reports of lamb killing were rife and the eagle had become one of the pastoralists’ major enemies. As early as 1892 the Western Australian government offered a reward of two shillings a head and in 1919 the eagles were declared vermin under the Vermin Act. They were trapped, poisoned and shot without restraint. ‘… we used seven [lambing] pens with two white fowls and fourteen rabbit traps set around each…we accounted for 411 eagles in seven weeks’ Manager of a sheep station near Bourke, NSW, 1920s, quoted in Fleay-Thomson (2002) p. 55.
‘for several years now I have been studying the wedge-tailed eagles which are so numerous in the sheep country, and I have come to the conclusion that they are more sinned against than sinning.’ G.H. Victoria, 1939, quoted in Fleay-Thomson 1960 p. 56.
The irrational hatred of eagles by some farmers was unbounded. In a letter to the The Australasian of 22 April 1939, a Mrs Philip Russell, of Carngham, Victoria, wrote of ‘very handsome but extremely cruel’ eagles tearing the throat from lambs and preferring the warm blood of ‘the largest and strongest’. She enclosed three photographs of an eagle caught in the act of killing ‘a healthy young lamb’. In a reply published in April, well-known naturalist-author David Fleay questioned the ‘badly tattered and frayed’ plumage, odd juxtaposition of predator and prey, and closeness of the photographer to the subject. Unrepentant, Mrs Russell replied on 6 May, that the photos were indeed faked—an eagle was shot and posed over a [dying] lamb—but claimed that her actions were warranted as ‘propaganda’ against the eagle, because the images were visually educational! Fleay’s reasoned defence of the eagle continued for many years. In 1955, an article appeared in the 14 July Courier Mail, with the news that the Commonwealth External Affairs Minister, Mr Casey, had gone hunting eagles from a light plane. It was accompanied by a photograph of the plane ‘skim-
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ming along at an altitude of 50 ft’ with passenger side door removed, and Casey taking aim. A second image shows the injured eagle with Casey in the background about to take its photograph. Fleay called the claimed slaughter of lambs ‘an excuse for a thrilling sport’. Casey responded quickly enough to indicate his embarrassment, but was unapologetic for his callousness. Beginning in the closing decades of the nineteenth century, with the establishment of broadscale sheep farming, the eagle quickly became the most relentlessly persecuted of raptors worldwide. Steel-jawed rabbit traps were set around carcasses and funnel traps could trap several eagles at once. It is claimed that 1060 eagles were poisoned on a single Queensland station over an eight-month period in 1903. Between 1958 and 1967 bounties were paid on 120 000 eagles in just two states, Queensland and Western Australia, averaging more than 13 000 a year. In New South Wales the eagles were ‘Noxious Animals’ and the Pastures Protection Board paid nearly 8000 bonuses in 1899. Bills, claws and even eggs were proof of destruction. By 1907 the take was down to 835. As recently as 1967–1976 in arid Western Australia, killing by humans accounted for 54 per cent of banded eagles that were recovered dead, mostly juveniles. Thirty thousand was the estimated toll Australia-wide in 1969. Tasmania too joined the slaughter, with a private organization ‘The Spring Bay Eagle and Tiger Extermination Society’ which claimed the eagle was imported into the State. The Society helped drive the lamentable tiger to extinction; fortunately the eagle proved more resilient. ‘Like a dog with a bad name, the Wedge-tailed Eagle apparently is saddled permanently with the reputation of a lamb-killer. The food of this formidable bird is mostly rabbits, carrion, lizards and, as with the Tawny Eagle of India, prey bullied from hawks and hunting animals. Some State Governments continue to offer blood-money and many landholders still think no method is too barbarous to exterminate their country’s most majestic bird. Despite exact evidence that the eagle does more good than harm, eager exaggerations of damage, and unmerciful slaughter, continue.’ Alan Bell (1956).
‘Wedge-tailed Eagles, with wingspans up to 9’ are slaughtering thousands of new-born lambs … It might be hard to believe but they dive bombed in formation of 20–25 striking hard with their talons…’ Sydney Morning Herald 6 April 1967.
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Although the eagle is no longer perceived to be a pariah, eagle-nonsense raised its head in another form at the height of the spread of the escaped calicivirus that promised to rid the country of rabbits. Concern for the welfare of the eagle in the wake of decimation of its main prey were real, but reports of excessive numbers of starving eagles lining outback roads in the hope of finding roadkills and crashing through windscreens and striking at the vehicles’ occupants were at best apocryphal. In 1975, the Northern Territory fittingly proclaimed the eagle as its emblem. After a century of determined persecution, in 1979 the eagle, along with most native animals, was given full protection by the Commonwealth government. The States and Territories also brought in full protection, but some issue a small number of permits to kill eagles causing problems. Eagle carcasses are no longer seen strung grotesquely along fences and the shift in perception has been strong. Eagles are more likely to be viewed with tolerance if not interest or even pride, a legacy of the conservation movement and ever more intimate wildlife documentaries. Of course, not all landowners are as accepting. In the late 1980s, it was estimated that 30 per cent of immature Wedge-tailed Eagles and 5–10 per cent of adults were shot or poisoned annually in Tasmania; recent figures suggest an improvement and most human-related eagle mortalities are now accidental rather than deliberate. There will also be individuals that hold unreasonable prejudices. A recent case of cruelty in New South Wales involved the trapping of at least 14 eagles, lured to a carcass by four eagles, pinioned and chained to logs, and surrounded by 48 steel-jawed rabbit traps.
Conservation With persecution no longer the problem it has been almost since European settlement, habitat destruction is now by far the major threat to the eagle population at large. Intensive agriculture and extensive urbanisation are incompatible with eagles. In temperate areas, the eagle apparently has difficulty nesting when hillsides have been cleared of potential nest trees. Inland, they have less need of large trees in elevated positions because they are more often assisted by thermals. Where habitat clearance and degradation is severe, prey may be insufficient to support eagles. Clearing or logging of trees is especially critical in Tasmania, where the eagle is by and large a forest-dependent breeder. There the eagle mostly nests in emergent trees in old-growth native forest exposed to early morning sun and sheltered from prevailing strong winds and cold spring winds. With the island’s population numbering in the hundreds and perhaps declining, as evidenced by slow replacement of lost pair members, the subspecies is listed as
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endangered. The subspecies requires forest areas greater than 10 ha in which to breed and is very prone to desert its nest when disturbed. In State forest, protocols are in place to detect eagle nests and protect them by creating an obligatory nest reserve of at least 10 ha and restricting forestry operation during the breeding season to outside a buffer zone of 500 m, extending to 1 km if the proposed work is in line-of-sight. In the past, surveys with fixed wing aircraft have been used to check the activity of a few less accessible sites (active nests are lined with green leaves, which are readily observed from the air, or contain eggs or chicks) and cover large areas efficiently. Now, however, searches for nests, often by helicopter, are routinely conducted in the nonbreeding season, targeting not only coupes proposed for logging but also access roads and other infrastructure. Computer habitat modelling techniques using Geographical Information Systems (GIS) are being used to successfully predict the locations of new sites, allowing searchers to focus on high priority habitat to detect nests early in the planning process. The protection is proving effective, provided that reserves are of adequate size and design. As with many conservation issues, a few dedicated individuals have done much to promote awareness and tolerance among the Tasmanian public, formulate habitat management plans, and gain co-operation and engender a sense of ownership among forest managers and landholders. Thankfully, the Australian eagle population has shown a remarkable resilience, enabling it to survive the swings in human perception that take it from pest to icon and back, the good times and bad, as eagles everywhere throughout history.
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List of scientific names The aquilas Steppe Eagle Aquila nipalensis Tawny Eagle Aquila rapax Golden Eagle Aquila chrysaetos Gurney’s Eagle Aquila gurneyi Imperial Eagle Aquila heliaca Indian Spotted Eagle Aquila hastata Lesser Spotted Eagle Aquila pomarina Spanish Eagle Aquila adalberti Spotted Eagle Aquila clanga Verreaux’s Eagle Aquila verreauxii Wahlberg’s Eagle Aquila wahlbergi Other species Bearded Dragon Amphibolurus barbatus Goannas Varanus spp. Shingleback or Stumpy-tailed Lizard Tiliqua rugosa Cunningham’s Skink Ergernia cunninghami Australian Kestrel Falco cenchroides Bearded Vulture (Lamergeier) Gypaetus barbatus Black Kite Milvus migrans Black-breasted Buzzard Hamirostra melanosternon Brown Falcon Falco berigora Corvids Corvus spp. Eclectus Parrot Eclectus roratus Egyptian Vulture Neophron percnopterus Emu Dromaius novaehollandiae Galah Cacatua roseicapilla Harpy Eagle Harpia harpyja Laughing Kookaburra Dacelo noveaguineae Little Eagle Hieraaetus morphnoides Australian Magpie Gymnorhina tibicen Magpie-lark Grallina cyanoleuca Martial Eagle Polemaetus bellicosus Palm Cockatoo Probosciger aterrimus Peregrine Falcon Falco peregrinus Rufous Fantail Rhipidura rufifrons
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Steller’s Sea-Eagle Haliaeetus pelagicus Whistling Kite Haliastur sphenurus White-bellied Sea-Eagle Haliaetus leucogaster Willy-Wagtail Rhipidura leucophrys Banded Hare-wallabies Lagostrophus fasciatus Burrowing Bettongs Bettongia lesueur Cat Felis catus Common Ringtailed Possum Pseudocheirus peregrinus Crescent Nailtail Wallaby Onychogalea lunata Daintree River Ringtail Pseudocheirus cinereus Dingo Canis lupus European Rabbit Oryctolagus cunniculus Flying-foxes Pteropus spp. Goat Capra hircus Green Ringtail Pseudocheirops archeri Hare-wallaby Lagorchestes spp. Herbert River Ringtail Pseudocheirus herbertensis House Mouse Mus domesticus Koala Phascolarctos cinereus Pig Sus scrofa Red Kangaroo Macropus rufus Rufous Hare-wallabies Lagorchestes hirsutus Rusa Deer Cervus rusa Sheep Ovis aries Spotted-tailed Quoll Dasyurus maculatus Tasmanian Devil Sarcophilus harrisii Beefwood Grevillea striata Belah Casuarina cristata Blue Gum Eucalyptus globulus Leopardwood Flindersia maculosa Mountain Ash Eucalyptus regnans White Gum Eucalyptus viminalis Mulga Acacia aneura Needlewood Hakea leucoptera River Red Gum camaldulensis Messmate Stringybark Eucalyptus obliqua White Callitris Callitris glaucophylla White Gum Eucalyptus viminalis
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Baker-Gabb, D.J. (1989). BOP Watch update no. 6. RAOU Newsletter 81: 11. Baker-Gabb, D.J. and Fitzherbert, K. (1989). An overview of raptor movements and wintering places in Australia and New Zealand. In: B.-U. Meyburg and R.D. Chancellor (eds) Raptors in the Modern World. World Working Group on Birds of Prey, London. pp. 159–166. Baldwin, M. (1977). Mating of Wedge-tailed Eagles. NSW Field Ornithologists’ Club Newsletter 22: 3. Barrett, C.L. (1945). Australian Bird Life. Oxford University Press, Melbourne. Batey, I. (1907). Wedge-tailed eagle and lambs. Emu 7: 43–45. Beehler, B.M., Pratt, T.K. and Zimmerman, D.A. (1986). Birds of New Guinea. Princeton University Press, Princeton. Bell, A. (1956). Some Common Australian Birds. Oxford University Press, Melbourne. Bell, P.J. and Mooney, N.J. (1999). The Wedge-tailed Eagle Recovery Plan 1998–2003. Tasmanian Parks and Wildlife Service, Hobart. Berney, F.L. (1903). The Wedge-tailed Eagle in North Queensland. Emu 3: 123–124. Berney, F.L. (1906). Eagles and rats. Emu 6: 13. Beruldsen, G. (1956). Hawks of Yorke Peninsula. South Australian Ornithologist 22: 23–25. Beruldsen, G. (1980). A Field Guide to Nests and Eggs of Australian Birds. Rigby, Adelaide. Birks, N. (1998). Wild and Free: Australia’s Natural World through the Lens of Nicholas Birks. Reed New Holland, Sydney. Black, J. (1922). Eagle and cockatoo. Emu 21: 197. Blakers, M., Davies, S.J.J.F. and Reilly, P.N. (1984). The Atlas of Australian Birds. RAOU and Melbourne University Press, Melbourne. Blows, J.M. (1995). Eagle and Crow: An Explanation of an Australian Myth. Garland, New York. Bochenski, Z.M., Huhtala, K., Sulkava, S. and Tornberg, R. (1999). Fragmentation and preservation of bird bones in food remains of the Golden Eagle Aquila chrysaetos. Archaeofauna. 8: 31–39. Bonnin, M. (1978). A day at Rawlinna Station, South Australia. Australian Bird Watcher 7: 152–154. Boyle, B. (1993). Wedge-tailed Eagle attacks raven. Boobook 14: 30. Brandon, T. (1938). Nesting of the Wedge-tailed Eagle (Uroaetus audax) in 1937, etc. South Australian Ornithologist 14: 117–121. Brandon, T. (1948). Bird notes from Wilmington. South Australian Ornithologist 19: 3–5. Breckwoldt, R. (1983). Wildlife in the Home Paddock: Nature Conservation for Australian Farmers. Australian Natural Science Library. Angus and Roberston, North Ryde. Brickhill, J. (1993). Abundance of raptors in the Riverina, 1978–1987. In: ‘Australian Raptor Studies’. (Ed. P. Olsen). Proceedings 10th Anniversary Conference Australasian Raptor Association, Canberra, September 21–22 1989. Australasian Raptor Association, Royal Australasian Ornithologists Union, Moonee Ponds, Victoria, Australia. pp. 262–272. Brodkorb, P. (1955). Number of feathers and weights of various systems in a Bald Eagle. Wilson Bulletin 67: 142.
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Index Page references in bold refer to photos; page references in italics refer to illustrations Aborigines 7–9, 9, 81 allopreening 44 asynchronous hatching 70 beak 30, 32 breeding 41–77 age of 77, 89 courtship 41, 43, 44, 45 courtship feeding 44 copulation 44, 46 success 65–67 timing, season 65 castings see pellets clutch see eggs conservation 2, 17, 92–93 crop 84 crows 8, 25, 79, 85, 87–89 description, first 12 diet 79–80 and age 80 carrion 9, 16, 21, 25, 27, 29, 66, 75, 76, 77, 81, 83, 84, 91 see also prey dimorphism 29, 40 dispersal of juveniles 21 distribution 19–20 drinking 84 Eagle Golden 13, 15, 16, 27, 30, 36, 89 Gurney’s 16, 21, 36 Little 15, 16, 50, 84 Martial 30 Steller’s Sea-Eagle 30 Verreaux’s 16, 27, 30, 36 White-bellied Sea-Eagle 15, 16, 24, 29, 42, 47 Eagle-Crow legend 8
eggs 53, 69–70 clutch size 44, 69–70, 73 hatching success 66 incubation 69–70 laying 69–70 size 17, 69 eyes colour 24, structure/function 37–39 Falcon Brown 25, 48, 82 Peregrine 24, 25, 40, 42, 48 feeding 32, 61, 62, 78 feet 30, 32 fidelity 46 fledgling 59, 75–76 flight 23, 28, 34, 35–37, 59, 61 foraging 82–83 fox 25, 63, 79, 81, 82, 85 gliders 36 goanna 80, 82, 87 growth see nestling habitat 19, 20, 42, 43, 81 loss 42, 92, 93 hearing 39 home range see territory hunting see also predation 18, 79–83, 82 cooperative 83 methods 81–83 identification 23–24 incubation see eggs interactions, aggressive interspecific 6, 24, 25, 24–26 intraspecific 41–42 kangaroos 12, 80–83, 82 Kite, Whistling 15, 25, 36, 47
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Index
longevity
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mate choice 46 mate replacement 77 measurements 29–32 mortality 73, 77, 87–89 movements 17, 21 names 11–13 nest 46–49, 52, 57 density 42–43 desertion 13, 48, 65, 87, 93 location 42, number 48 size 47–48 spacing 42–43 nestling 70–75, 51–57, 58 ageing 73 growth 68, 71, 72, 46, 68–75 measurements 73–74 nomenclature see names pair bond 44, 46 parrots 81 persecution 20, 65, 89–92 pellets 79 pesticides 88 plumage 23, 26–28 age changes 23, 26–27, 30 care 27–28 nestling 26, 73–74 poisoning 8, 64, 87–88, 90–92 population density 20, 43 numbers 20, 43 Port Jackson Painter 12, 12
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predation impact 84–85 predators 87 preening see plumage, care prey types 79–83 weight 16, 83, 84 radio-tagging 43 rabbits 20, 37, 67, 70, 80–81, 83–85, 90, 92 recruitment 73, 77 relatives see taxonomy reproduction see breeding sexual dimorphism see dimorphism sheep 1, 80–81, 85, 90–91 siblicide 71–72 sight see vision size 29–30, see also measurements starvation 87, 89, 92 survival see mortality taste 39 taxonomy 15–17 territory size 42–43 defence 41–42 threats 87–92 vision 1, 16, 37–39 voice 32–33 vulture 11–13 weight adult 15, 29 nestling 73
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