Wedge-tailed eagle

Name and taxonomy

The English ornithologist John Latham described the species in 1801 as Vultur audax. The Latin word audax means bold, which suited the bird's reputation as a hunter. In fact the eagle is wary of people and often timid near them. It was once placed in a genus of its own, Uroaetus, because of its odd shape.

Two subspecies are recognised, though some researchers doubt the split. The Tasmanian race, named for the naturalist David Fleay in 1954, is larger and deep chocolate brown. Genetic work shows no gene flow between Tasmanian and mainland birds. Aboriginal names for the bird include Waalitj, Wildu, Mibayn and Garriirl.

Evolution and relatives

The wedge-tailed eagle is one of 12 large dark eagles in the genus Aquila. These are the booted eagles, named for the feathers that cover their legs to the toes. Its nearest relative is probably the Gurney's eagle of New Guinea, which looks much like it. Genetic work also puts it close to the Verreaux's eagle of Africa.

Those three sit in a group with the golden eagle, the most widespread eagle of all. The Bonelli's eagle and two African hawk-eagles belong to the same clade. The wedge-tailed eagle faces fewer rivals than any other Aquila, because Australia holds so few large raptors. That freedom probably explains why it uses more habitats than any relative.

Appearance

An adult is dark brown to almost black, with a paler nape and fully feathered legs. The upper mandible is long, and the beak and feet are heavy. Total length runs from 81 to 106 centimetres, and the wings span 182 to 232 cm. A female killed in Tasmania in 1931 spanned 284 centimetres, the record for the species. Claims of much wider birds were checked and rejected as unreliable.

Females weigh 3 to 5.8 kilograms, and the smaller males weigh 2 to 4 kg. One sample from the Nullarbor Plain averaged 3.13 kg for males and 3.8 kg for females. The wings, at more than 65 centimetres, and the tail, at up to 45 cm, are long for the body weight. Nine or ten other eagles outweigh this one, despite its great length and span. Young birds are mid-brown with reddish heads and darken to blackish-brown over about ten years.

Habitat and range

The bird lives across the whole of Australia, from Cape York to the southern capes. It reaches Tasmania, many offshore islands, and the Trans-Fly savanna of southern New Guinea. GBIF holds records from 8 countries, and almost all of them come from Australia. It is scarce only in the driest interior, such as the Lake Eyre Basin.

Few raptors use as many habitats as this one. It lives in open woodland, savanna, heath, grassland, semi-desert, subalpine forest and mountain tops. It prefers rough country with rocky ground, some open ground and stands of eucalypts. Dense forest is avoided, and the bird works the glades and the edges instead. It occurs from sea level up to about 2000 metres with no clear preference for height.

Diet and hunting

This is one of the most powerful bird predators in the world. More than 200 prey species have been recorded for it. Across 21 diet studies, mammals made up 61.3 percent of prey items, birds 21.6 percent and reptiles 13.2 percent. By weight, mammals came to almost 90 percent of everything taken. The bird generally ignores birds and reptiles under 100 grams and mammals under 500 g.

The introduced European rabbit is now the staple over most of the country. Rabbits usually make up 30 to 70 percent of the diet and once reached 92 percent at one site. Native prey means marsupials, above all young, small or sick kangaroos and wallabies. Ravens, Australian magpies, wood ducks and galahs are the birds taken most often. Bearded dragons are the favoured reptile and can dominate the diet in arid country.

The eagle snatches prey from the ground after a glide, a low quartering flight or a slanting stoop. It may spot an animal from more than a kilometre away. Pairs and groups hunt together, and one bird drives prey towards another that waits unseen. One record describes 15 eagles working a mob of kangaroos in relays. A bird can carry prey of at least 5 kilograms.

Carrion matters as much as hunting, and young eagles take most of it. The birds watch ravens gather at a carcass and glide down to take it over. An eagle can swallow 1 to 1.5 kg at one sitting and then go for weeks without another meal. Heavy carcasses are sometimes covered with vegetation and left for later.

Behaviour and social life

Adults perch for much of the day on a tree, a rock or a termite mound with a wide view. Outside the breeding season they spend hours in the air. They soar without a wingbeat, often at 1800 metres and sometimes far higher. Glider pilots have met wedge-tailed eagles above 3000 metres. Adults are sedentary, but a young bird may travel more than 850 kilometres before it settles.

Pairs hold a territory and defend the ground within about 1 kilometre of the nest hardest. Disputes usually stay in the air, with high circling, rolls and talons presented. Young birds are more sociable, and up to 40 have gathered at one carcass. Smaller birds mob the eagle noisily, and magpies are among the boldest. The eagle usually ignores them and only sometimes rolls over to show its feet.

This is the one bird with a name for attacking hang gliders and paragliders. It has torn the fabric of a glider with its talons, though it does not hurt the pilot. Eagles have also destroyed survey drones flown over mine sites. Young birds appear to play, fetching thrown sticks and teasing dogs from the air.

Communication and senses

For a bird of this size the voice is weak, and it is seldom heard. A pair may stay silent for months outside the breeding season. The usual call is a high thin whistle, followed by a short falling note. Yelps, squeals and a rolling series are added near the nest and in display. The female's calls are lower and harsher than the male's.

Sight probably does the work that the voice does not. The eye is about as large as a small human eye and sees roughly three times as sharply. Vision reaches into the ultraviolet, and the bird carries one of the largest pectens of any bird. Flight probably carries the message instead, because a soaring eagle is visible to every neighbour.

Reproduction and life cycle

Pairs stay together for years and probably for life. The breeding season runs from July to December over most of the range. In dry years in Western Australia a pair may skip breeding for up to 4 years. Display is modest for an eagle, with mutual soaring, undulating dives and tandem flights. Mating happens on a bare branch near the nest, not in the air.

The nest is a stick platform in the largest tree of a stand, usually 12 to 30 metres up. It starts at 70 to 100 centimetres across and grows with each season. An old nest can reach 2.5 metres across and nearly 4 m deep, and weigh over 400 kilograms. Green leaves are laid in the cup, as other hawks do. A pair often keeps two or three nests in one territory and uses them in turn.

The clutch is one to three eggs, and about 80 percent of clutches hold two. Each egg weighs about 120 to 150 grams and is blotched purple-brown on white or buff. Incubation lasts 42 to 48 days and falls mostly to the female. Because she starts sitting with the first egg, the first chick hatches ahead of the second. In poor years the older chick kills or starves out the younger one.

Chicks hatch in white down and are nearly feathered by 49 days. They fledge at 75 to 95 days and then depend on the adults for another 3 to 6 months. Goannas take most of the eggs and chicks that are lost, because the parents guard the nest poorly. Birds are mature at about three years but usually do not breed until six. The oldest banded wild bird was about 11 years old, and captive eagles reach 40 years.

Predators and threats

An adult wedge-tailed eagle has no natural predator. Goannas are the main nest robber, and they take eggs and small chicks. White-bellied sea eagles fight wedge-tailed eagles at the edges of their ranges. Those fights can turn into talon grappling and cartwheeling in the air.

The threats that matter now come from people and from land use. Clearing of mature trees removes nest sites, and disturbance makes a pair desert its young. Dingo baits and pesticides poison birds that scavenge. Eagles are also killed by power lines, vehicles, fences and wind turbines. The calicivirus crash in rabbit numbers cut the main food supply in some districts.

Population and conservation

The IUCN Red List rates the species Least Concern. BirdLife International put the total at about 100,000 mature birds in 2009 and called the trend possibly increasing. The range covers more than 10.5 million square kilometres, so the true figure is likely in the hundreds of thousands. Rabbits, thinner forest and roadkill have all helped the species since European settlement.

Tasmania is the exception, and its race is listed as endangered by the state. Numbers there fell from about 140 pairs in the 1980s to 60 or 80 pairs by the mid-1990s. Surveys comparing two periods found a decline of around 28 percent. Tasmanian birds nest in old-growth forest and desert the nest readily when disturbed. Forestry there must now leave a nest reserve of at least 10 hectares and keep out of a 500 metre buffer in the breeding season.

Relationship with humans

Sheep farming turned the eagle into a target from the closing decades of the nineteenth century. In 1903 a single Queensland station said it had poisoned 1060 eagles over eight months. Laws passed between 1909 and 1925 obliged landowners to kill the bird as vermin. Between 1958 and 1967, Queensland and Western Australia paid 120,000 bounties, about 13,000 a year. An estimated 30,000 were killed across Australia in 1969 alone.

The charge was largely false, because the eagle takes lambs that are already dying or sick. Legal protection began in Western Australia in the 1950s and spread later to the other states. Killing still happens, and it accounted for 54 percent of recovered dead eagles in the 1980s. The bird is now protected everywhere in Australia and cannot be hunted or harmed. Falconers appear not to keep it, and only licensed centres hold one.

In culture and myth

For the Kulin nation of central Victoria, the creator Bunjil takes the form of a wedge-tailed eagle. Bunjil pairs with Waang the crow, and the two divide the people into moieties. In one story he lifts his spear and halts a flood, once people promise to keep the law. In another, whirlwinds carry him upward, and he turns into the star Altair. A rock art cave near Stawell, in Gariwerd, carries the name Bunjil's Shelter.

The bird is also an emblem of the Northern Territory and of the New South Wales Police Force. The Australian Defence Force uses it on its flag, and the air force uses it on badges. The Royal Australian Air Force named its Boeing E-7 Wedgetail radar aircraft after the eagle. The 2nd Cavalry Regiment keeps a live eagle called Courage as its mascot. One mascot flew off during training in 1997, was charged with being absent without leave and lost his rank.