Birds of Australia
Wedge Tail Eagle
The wedge-tail eagle is Australia's largest living bird of prey. Its scientific name is Aquila (eagle) audax (bold) and it belongs to the Accipitridae family and the Falconiformes order. The wedge-tail frequents a variety of environments and altitudes from sea level to alpine heights. For preference, it likes open woodland and forest where there is good grassy ground cover.
The wedge-tail eagle is one of Australia's three eagles. The others are the white- breasted sea eagle and the little eagle. The wedge-tail is one of the largest eagles in the world. It is also the most common. Its nearest relatives are the gurney eagle and the golden eagle.Credit: http://commons.wikimedia.org/wiki/File:Wedge_Tailed_Eagle.jpg
The males are smaller than the females and generally weigh about 3.2kg with an odd specimen reaching 4kg. Females weigh between 4.2kg and 5.3kg. The length of the bird can vary from 0.85 to 1.05 metres. The wingspan is huge at 2.5 metres. The birds can soar and glide for up to an hour and a half at a time, reaching heights of over 2000 metres. In the Australian summer, the birds glide on thermals giving an awe-inspiring display of power and majesty.
The bird is named for the wedge shape of the tail. The birds are monogamous and mate for life unless something happens to one of the pair in which case the other will find a new mate. They are territorial and will defend the nest and its immediate vicinity against others. The home range which supplies their food needs is not necessarily defended. Home ranges can overlap. Model planes, hang gliders and helicopters have been attacked by wedge-tails if they perceive themselves as threatened. As they have no real predators, they have little fear of anything and can become roadkill themselves by failing to move out of the way of cars. They normally steer clear of humans.
Location and seasonal conditions play a large part in determining the length of the breeding season. The availability of food also has a large bearing on the breeding cycle. April to September is the usual although sometimes this can extend to December. In drought periods the birds may not breed at all. Nests may be abandoned if the couple are disturbed when about to lay. Pairs bond in various ways. They may position themselves one above the other high in the air. The top one will drop suddenly, slapping the wing of its mate with its own as it drops. Pairs also preen each other. Nest-building, incubation and feeding of the young is undertaken by both birds.
The nest is constructed as a large platform of dead sticks. The fork of a large, high tree is preferred especially if it gives a good view of the surrounds. They will use cliff faces, shrubs or even the ground if no alternatives are available. The nest is built with a shallow indentation and will be repaired and added to each year. After some years, nests can become 1.8m across, 3m deep and weigh 400kg. Dropped sticks will accumulate as litter under nest trees.Credit: http://commons.wikimedia.org/wiki/File:Aquila_audax_-Healesville_Sanctuary,_Victoria,_Australia_-upper_body-8a.jpg
If there is plenty of food in an area, pairs may nest within a kilometre of each other. Usually two eggs are laid at two to four days intervals. The white eggs have purple and brown markings and measure 73 x 59mm. Incubation begins when the first egg is laid and takes 42 to 45 days. The eggs hatch in turn and each chick is progressively smaller. It is common for only the oldest and largest chick to survive. If he runs out of food, he will kill and eat his siblings.
The chicks are covered with white down at first. The first true feathers appear during the second week after hatching. The parents feed the chicks for the first five weeks. They will bring prey to the nest and shred it for the babies. The chicks progress to feeding from the floor of the nest. They will lay flat in the nest if threatened but defend themselves if necessary. Juveniles stay with the parents for around eleven weeks. They then migrate to an area that has food available. Young birds may cover 850km in a 7 to 8 month period while finding and establishing a range for themselves.
The young birds range in colour from sandy to mid-brown. The head and wings are reddish-brown. The birds darken each moult until they reach at least ten years old. Adults are a dark blackish-brown. The female is often slightly lighter. The beak varies from pale pink to cream and the feet are off-white. Plumage continues down the legs to the base of the toes. The iris is dark.
Eagles do most of their hunting around sunrise and sunset. They communicate with whistles and also have a shrill, loud scream. They glide and circle at high altitudes. They have excellent vision and will bear down on their prey in a long, slanting swoop. They either grasp the prey in strong talons and fly off or devour the victim at the site. They hunt singly, in pairs or in groups. Groups will sometimes tackle a full-grown kangaroo. The eagle is very powerful and can lift and fly with 50% of its body weight. They will store uneaten food on branches near the nest.
Their nourishment usually comes from ground-dwelling animals. Rabbits, wallabies, lizards, birds, snakes, possums and feral cats are all prey to the wedge-tail. They perform a useful purpose in cleaning up carrion. Rabbits generally form a large part of the diet of the wedge-tail. Research has shown that, despite the claims of farmers and graziers, lambs make up a very small proportion of their diet. Foxes and crows account for many more lamb deaths than do eagles.
Years ago, eagles were classed as vermin and a bounty was placed on the bird. Thousands were disposed of either by poisoning or shooting. Between 1928 and 1968, over 147,000 wedge-tails were killed in Western Australia and 162,000 in Queensland between 1951 and 1966. Indirect poisoning of the birds occurs through taking dingo baits, pesticides and rabbits infected with the calicivirus.
The wedge-tail is now protected and comes under the jurisdiction of the National Parks and Wildlife Service. It is affected by deliberate persecution and loss of habitat. In more arid areas, nesting resources have been diminished due to extensive clearing.
The wedge-tail eagle is a majestic creature. It is the symbol of the Western Australian 'Eagles' Australian Rules Football Club. Those in Western Australia who follow the team have had the wonderful opportunity to see a wedge-tail flying free over the Subiaco football ground before the start of a game. Although it is supposed to return to its handler before the start of the match, it sometimes goes AWOL for a short time before returning!