Australasian Eagles and Eagle-like Birds

Stephen Debus

• Publisher: CSIRO PUBLISHING
• Year: 2017
• ISBN:
  9781486306947
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

Eagles are awe-inspiring birds that have influenced much human endeavour. Australia is home to three eagle species, and in Melanesia there are four additional endemic species. A further three large Australian hawks are eagle-like. Eagles, being at the top of the food chain, are sensitive ecological barometers of human impact on the Earth's ecosystem services, and all of the six Australian species covered in this book are threatened in at least some states (one also nationally). Three of the four Melanesian tropical forest endemics are threatened or near-threatened.

In Australasian Eagles and Eagle-like Birds, Dr Stephen Debus provides a 25-year update of knowledge on these 10 species as a supplement to the Handbook of Australian, New Zealand and Antarctic Birds (HANZAB) and recent global treatises, based partly on his own field studies. Included are the first nest or prey records for some Melanesian species. This book places the Australasian species in their regional and global context, reviews their population status and threats, provides new information on their ecology, and suggests what needs to be done in order to ensure the future of these magnificent birds.

Australasian Eagles and Eagle-like Birds is an invaluable resource for raptor biologists, birdwatchers, wildlife rescuers and carers, raptor rehabilitators and zookeepers.

PART I

Sea-eagles

The White-bellied Sea-Eagle Haliaeetus leucogaster was described by John Latham in 1781 from a specimen collected in Java, but formally named by Johann Gmelin in 1788. Illustrated by the Port Jackson Painter, this eagle must have been encountered by the First Fleet in Sydney waters in 1788. As related by Graham Pizzey (1985), Matthew Flinders had a close encounter with an inquisitive pair, which had perhaps never seen humans, or at least Europeans, on an island in Spencer Gulf (South Australia) in 1802.

The White-bellied Sea-Eagle is arguably one of the most attractively plumaged of the world’s sea-eagle species, and is certainly a conspicuous and imposing sight around all Australian coastlines. It is surprising, therefore, that most Australians (other than birdwatchers) seem to have little concept of what it looks like, or how it differs from the Osprey Pandion haliaetus. On the other hand, I would guess that most Americans know what a Bald Eagle looks like, and many Australians seem to as well, via American television. Some apparently even think we have Bald Eagles in Australia. For instance, the logo of the Manly-Warringah rugby team (Sydney, NSW) most resembles a caricature of the Bald Eagle, apparently with a bit of Brahminy Kite Haliastur indus thrown in (mainly the colour). All this despite the fact that Brahminy Kites do not occur in Manly-Warringah, whereas White-bellied Sea-Eagles do, and surely provide a more appropriate model for a ‘sea-eagles’ logo. Some Australian business or other logos also feature some sort of stylised Bald Eagle, as if we have no idea what our own eagles look like, or which continent the Bald Eagle inhabits. The classic case of such ignorance and confusion must be a certain ‘Golden Eagle Fishing Club’ in New South Wales (NSW), whose logo is a stylised Bald Eagle!

Despair at the level of public consciousness of Australian eagles prompted a letter to the Sydney Morning Herald in late 2003, titled ‘The Optus eagle’:

About the new highway billboards announcing better mobile [phone] coverage on the NSW coast, under the banner of a soaring American Bald Eagle. Are the ad people and company publicists just plain ignorant of Australia’s own iconic eagle – the Wedge-tailed Eagle – and the natural distribution of the Bald Eagle? Or are we to read something into the symbol of Uncle Sam spreading itself over more of our country?

Whether or not the letter had any influence, if indeed it was ever published, the billboard pictures of the Bald Eagle soon disappeared. (And lest the name ‘Bald Eagle’ be thought utterly inappropriate for an eagle that is not bald, or even remotely looks so, ‘bald’ in that sense is an old word meaning white [-headed], as in piebald, etc.)

Early studies on the White-bellied Sea-Eagle were pioneered by David Fleay and Norman Favaloro. The first serious field studies were conducted in Victoria by Bill Emison and Roger Bilney in the early 1980s, but research really only took off since 1990 (once it was realised that the Sea-Eagle was threatened in Tasmania and South Australia), and positively bloomed since 2000. Credit must go to Bill Emison (an expat American with the Victorian wildlife service) for kick-starting the process. Nick Mooney had also been collecting data in Tasmania, summarised in HANZAB and some published in the popular (rather than scientific) literature. Credit must go to another expat American, Jerry Olsen, for taking a suitably sceptical view of the Australian raptor folklore and busting a few myths, including on the Sea-Eagle’s supposed dietary link with the introduced European Rabbit Orcytolagus cuniculus.

Sanford’s Sea-Eagle is included here, because of the debate about its taxonomic status, and to highlight some recent observations that shed new light on its ecology. Ernst Mayr was the first to describe and name it as a distinct species, having noted museum specimens that were mistaken for juvenile White-bellied Sea-Eagles, as well as having observed and collected Sanford’s Sea-Eagles in the field himself around 1930. Until 1950, his field information was all that was known about it, and there is still much to learn. The DNA evidence for lumping it as a subspecies of the White-bellied Sea-Eagle is limited, and in any case Sanford’s Sea-Eagle is an isolated, morphologically distinct entity worthy of study and conservation in its own right.

White-bellied Sea-Eagle

Haliaeetus leucogaster

This species, in the strict sense, is regarded as monotypic (no subspecies), and the genetic evidence supports this treatment on the basis of minimal population structuring across its Australasian range (Shephard et al. 2005b). ‘Demotion’ of Sanford’s Sea-Eagle to a subspecies of the White-bellied, on minimal genetic evidence, has not found favour with field observers (Pikacha et al. 2012).

Field identification

Despite the exhaustive treatment in HANZAB and its derivative field guide (Debus 2012), as well as current bird guides, it is apparent that juveniles of this species are still confused with the juvenile and immature (‘brown’) stages of the Wedge-tailed Eagle. This confusion clouds issues such as trying to elucidate the breeding status of the White-bellied Sea-Eagle in the Australian Capital Territory (Debus 2005). The salient identification points (versus the Wedge-tailed Eagle) are the Sea-Eagle’s bare tarsi, short pale tail and broader, more rounded wings.

Similarly, it is apparent that immature White-bellied Sea-Eagles in their ‘osprey’ plumage stage, with a breast-band, are still often confused with the Osprey, and that, conversely, Ospreys are called (or assumed to be) ‘sea-eagles’ by laypeople unfamiliar with the differences. The key points are plumage characters (dark eye-stripe, barred flight and tail feathers in the Osprey), tail shape (square-tipped in Osprey, wedge-shaped in Sea-Eagle), and wing carriage in soaring and gliding flight (bowed in Osprey, strongly upswept in Sea-Eagle). Ospreys also dive almost vertically into water, submerging, which Sea-Eagles seldom do (instead, they typically snatch prey from the surface in a low pass, wetting only their feet).

Adult White-bellied Sea-Eagle (left) and its dependent juvenile; the juvenile has a full crop from a recent feed. Photo: Chris Field.

Habitat

The Sea-Eagle’s breeding habitat is mostly sea cliffs in treeless parts of coastal South Australia (Dennis et al. 2011a,b, 2012), offshore islands in Western Australia (Johnstone and Storr 1998), and forest or woodland in treed areas of Tasmania and various other parts of Australia (Debus 2008; Thurstans 2009a,b; Bluff and Bedford 2011; Corbett and Hertog 2011, 2012; Hodge and Hodge 2011; O’Donnell and Debus 2012; Debus et al. 2014; O’Brien and Lacey 2016). Tree nests are located within 1 km of a waterbody (Corbett and Hertog 2011, 2012; O’Donnell and Debus 2012). Breeding habitat is increasingly subject to human disturbance in the form of recreation, and clearing for urbanisation (Thurstans 2009a,b; Dennis et al. 2011b; O’Donnell and Debus 2012; Debus et al. 2014). Sea-Eagles avoid perching in urbanised areas, even where there are suitable tall trees or structures, instead preferring to perch in undeveloped areas of coastline (Spencer and Lynch 2005), although some pairs have recently taken to nesting within view of urban areas (Debus et al. 2014).

Population

The Australian population of this species has been crudely and conservatively estimated as 6000 pairs, based on published estimates of 800 pairs in NSW, 100 (or perhaps 200) pairs in Victoria, 200 pairs in Tasmania, 80 pairs in South Australia, and extrapolated estimates of 1000+ pairs in Western Australia, 1000+ pairs in the Northern Territory, and 2500+ pairs in Queensland (Debus et al. 2014). Similarly, the global population has been estimated as in the low tens of thousands of individuals (Ferguson-Lees and Christie 2001), or perhaps therefore between 15 000 and 20 000 pairs, of which the Australian population occupies about one-third of the Sea-Eagle’s global geographical range. Thus, both estimates agree rather well.

The Sea-Eagle population, or its breeding productivity, is declining in urbanising and other human-affected coastal areas of southern Australia, with the relentless and increasing human pressure on its breeding habitat and nest sites (Thurstans 2009a,b; Dennis et al. 2011a,b; O’Donnell and Debus 2012; Cooper et al. 2014). Declines have been measured as a 22% loss of occupied breeding territories in mainland South Australia in recent decades (Dennis et al. 2011a), and a 25% decline in sighting frequency (atlas reporting rate) over 20 years on the NSW north coast (Cooper et al. 2014).

The Sea-Eagle is state-listed as Endangered in South Australia, and as Vulnerable in Tasmania, Victoria and NSW. It is also listed as protected on the ‘Marine’ schedule of the federal Environment Protection and Biodiversity Conservation Act 1999. However, it has been delisted from the ‘Migratory’ schedule following its removal from the China–Australia Migratory Birds Agreement, and its disqualification by the Bonn Convention criteria for true (international, biannual return) migration.

Threats include disturbance to or logging/clearing of breeding habitat and nest sites, fire (including prescribed fire), chemical pollution, oiling (from oil spills and commercial fish offal dumps), entanglement in fishing gear and fish-farm netting, illegal persecution (shooting, poisoning, vandalism of nest trees), collisions (e.g. with windfarm turbines, vehicles, powerlines), degradation of wetlands by feral ungulates, and competition with the Wedge-tailed Eagle for remaining breeding space and nest sites (Mooney and Wiersma 1995; Debus 2005; Spencer 2005; Manning et al. 2008; Bluff and Bedford 2011; Hodge and Hodge 2011; Dennis et al. 2011a,b; Corbett and Hertog 2012; Debus et al. 2014; Debus 2015; O’Brien and Lacey 2016). The Sea-Eagle’s eggshell thickness deceased by an average of 6% (locally a maximum of 25% in areas of heavy pesticide use) in the DDT era, with several clutches so thin-shelled that eggs were likely to break during incubation and so cause reproductive failure, although unlikely to cause widespread population declines (Olsen et al. 1993). An emerging issue in urban areas is disease, including possibly frounce (trichomoniasis) and viral beak and feather disease (Anon. 2016): the former possibly related to predation on diseased or avicide-affected feral pigeons; the latter possibly related to human feeding of over-abundant parrots.

Juvenile White-bellied Sea-Eagle soaring. Photo: Ákos Lumnitzer.

Movements

Adult, breeding White-bellied Sea-Eagles are sedentary in large, permanent home ranges (Wiersma and Richardson 2009; Mooney in Debus 2015). Juveniles are dispersive, being capable of traversing the continent (1800 km linear displacement, or 3000 km via the coastline), though usually travelling lesser distances (average 300 km, mostly less than 140 km from the natal nest or territory, in apparently random directions: Debus 2015).

Banding. Of 196 birds banded in the Australian Bird and Bat Banding Scheme, 11 (6%) were recovered (Debus 2015). Of 60 Sea-Eagles banded, and in some cases wing-tagged, in a study in the Northern Territory, four (7%) were recovered or resighted (Corbett and Hertog in Debus 2015). Birds banded/tagged as juveniles or immatures were recovered or resighted up to 3.5–4 years later. One banded female lived for 14 years as a resident adult in her territory, and therefore at least 19 years in total (Debus 2015).

Food

The Sea-Eagle’s foraging and prey have been studied recently on inland waters by Olsen et al. (2006a,b, 2013), Debus (2008) and Corbett and Hertog (2011, 2012), and on coastal waters by Wiersma and Richardson (2009), Hodge and Hodge (2011), O’Donnell and Debus (2012) and Harrington (2013). The Sea-Eagle feeds mainly on fish, waterbirds and aquatic reptiles, supplemented with mammals and carrion. In the Bismarck Archipelago, in the absence of large forest raptors, this species has undergone ecological release, where its prey includes arboreal forest mammals reportedly taken from tree branches (Heinsohn 2000). Nevertheless, in the Bismarcks it rarely hunts over inland forest, although it regularly soars there (G. Dutson). There is no evidence that the introduced European Rabbit is an important food source or has influenced the Sea-Eagle’s inland distribution or abundance in Australia (contra Shephard et al. 2005a), and rabbit remains in Sea-Eagle nests may be largely attributable to prior occupation by Wedge-tailed Eagles (Debus 2005).

Foraging behaviour. White-bellied Sea-Eagles use a variety of searching methods: long-stay perch-hunting, short-stay perch-hunting (with flights between successive perches), high quartering, and soaring and prospecting (Debus 2008; Wiersma and Richardson 2009). For breeding males in Tasmania, primary hunting areas were within 4 km of the nest, and males transported prey ~2 km to nests; short-stay perching forays averaged 1.2 km and soaring foraging flights 16 km in natural (non-aquaculture) habitats, whereas in areas with marine fish f...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Foreword
• Contents
• Preface
• Photographer’s acknowledgements
• Introduction
• Part I: Sea-eagles
• Part II: Harpy eagles
• Part III: Booted eagles
• Part IV: Australian eagle-like hawks
• Epilogue
• Bibliography
• Other sources of information
• Index