The wedge-tailed eagle (Aquila audax) is the largest bird of prey in the continent of Australia. It is also found in southern New Guinea to the north and is distributed as far south as the state of Tasmania. Adults of the species have long, broad wings, fully feathered legs, an unmistakable, wedge-shaped tail, an elongated upper mandible, a strong beak, and powerful feet. The wedge-tailed eagle is one of 12 species of large, predominantly dark-coloured booted eagles in the genus Aquila found worldwide. Genetic research clearly has indicated that the wedge-tailed eagle is fairly closely related to other, generally large members of the Aquila genus. A large, brown-to-black bird of prey, it has a maximum reported wingspan of and a length up to .
Wedge-tailed eagles are some of their native continent's most generalised birds of prey.Debus, S. (2017). Australasian Eagles and Eagle-like Birds. CSIRO Publishing. They reside in most habitats present in Australia, ranging from desert and semidesert to plains to mountainous areas to forests, even sometimes tropical rainforests. Their preferred habitats, however, tend towards those that have a fairly varied topography, including rocky areas, some open terrain, and native woodlots such as Eucalyptus stands.
The wedge-tailed eagle is one of the world's most powerful avian predators. Although a true generalist, which hunts a wide range of prey, including birds, reptiles, and rarely other taxa, the species is, by and large, a mammal predator. The introduction of the European rabbit (Oryctolagus cuniculus) has been a boon to the wedge-tailed eagle and they hunt these and other invasive species in large volume, although the wedge-tailed eagle otherwise generally lives on marsupials, including many surprisingly large macropods. Additionally, the wedge-tailed eagle often eats carrion, especially while young. The species tends to pair for several years, possibly mating for life.
Wedge-tailed eagles usually construct a large stick nest in an ample tree, normally the largest in a stand, and lay one to four eggs, though typically only two. Usually, breeding efforts manage to produce one or two fledglings, which after a few months more, tend to disperse widely. Nesting failures are usually attributable to human interference, such as logging activity and other alterations, which both degrade habitats and cause disturbances. The species is known to be highly sensitive to human disturbance at the nest, which may lead to abandonment of the young.
Although historically heavily persecuted by humans through poisoning and shooting, mostly for alleged predation on sheep, wedge-tailed eagles have proved to be exceptionally resilient, and their numbers have quickly rebounded to being similar or even higher numbers than before European colonisation, due in part to humans inadvertently providing several food sources, such as rabbits and a large volume of roadkill.
Taxonomy
The species was first described in 1801 by English ornithologist John Latham under the binomial name Vultur audax. At one time, the wedge-tailed eagle was classified in its own monotypical genus Uroaetus, perhaps due to its unique form.
Today, the genus Vultur is used only for a completely unrelated bird of the New World vulture family, the Andean condor (Vultur gryphus). The specific scientific name for the species, audax, is derived from the Latin , meaning "bold", indicative of their perceived disposition, perhaps when hunting, although the species is, in general, highly wary and even timid around humans.
The species is quite similar in many aspects of its morphology, appearance, behaviour, and life history to other species in the Aquila genus. The eagles of the Aquila genus are part of the subfamily Aquilinae, within the larger Accipitridae family. The subfamily is commonly referred to as booted eagles or sometimes as true eagles. Those species may be distinguished from most other accipitrids by the feathering covering their legs, regardless of distribution. With some 39 or so species, the Aquilinae are present on every continent except Antarctica.
By a variety of phylogenetic testing, largely via mitochondrial DNA and nuclear DNA, the wedge-tailed eagle clusters genetically with certain other Aquila eagles. The species found to share the most genetic similarities is Verreaux's eagle (A. verreauxii) of Africa. However, Gurney's eagle (A. gurneyi), a mostly allopatric but outwardly fairly similar eagle, is clearly a very close relation of the wedge-tailed eagle, and the two are likely sister species, most probably originating from the same radiation across the Indo-Pacific region.
The wedge-tailed, Gurney's, and Verreaux's eagles form a clade or a species complex with the well-known golden eagle (A. chrysaetos), the most widely distributed species in the entire accipitrid family, as well as outwardly dissimilar (smaller and paler-bellied yet also powerful) eagles like Bonelli's eagle (A. fasciata), the African hawk-eagle (A. spilogaster), and Cassin's hawk-eagle (A. africanus), the latter three having once been considered members of a different genus.
Beyond the aforementioned species, based on genetic testing, the four other Aquila species, although outwardly similar to golden and wedge-tailed eagles, being large, dark, and brownish, with long wings, are thought to form a separate clade, and are paraphyletic from the members of what can be called the golden eagle clade. Other related outliers from outside the Aquila genus are the small to mid-sized Clanga or spotted eagle species, and the widely found and quite small Hieraeetus eagles. One member of the latter genus contains the only other widely found Aquilinae eagle in Australia, the little eagle (Hieraaetus morphnoides).
Subspecies
Two subspecies of wedge-tailed eagle are recognised. However, the separation of the two subspecies has been called into question, largely because the reported differences in both size and coloration can be attributed to clinal variation, and some of the insular populations may still be at an intermediate stage of subspecific formation. A. a. audax (Latham, 1801) – This subspecies resides in the entire continent of Australia, as well as in southern New Guinea. It is the typical wedge-tailed eagle as subsequently described. A. a. fleayi (Condon & Amadon, 1954) – This race is endemic to Tasmania. The subspecies is named in honour of David Fleay, an Australian naturalist who was the first to propose the difference of the insular race. A. a. fleayi differs from mainland wedge-tailed eagles mainly via size and colouring. It is larger than the mainland eagle and is said to have particularly outsized talon dimensions compared to mainland eagles. Furthermore, it has a deep chocolate brown overall colour rather than blackish, with a whitish-buff colouring to the nape rather than tawny-rufous feathers there. The juvenile is altogether paler and sandier than an equivalent-aged wedge-tailed eagle on mainland Australia. Although the validity of the subspecies has been questioned, genetic studies have determined that no gene flow or introgression occurs between Tasmanian and other wedge-tailed eagles. Furthermore, the insular race was likely formed by marine dispersals, a process in which wedge-tailed eagles may continue to engage despite usually avoiding large bodies of water, albeit usually in narrower straits.
Description
Wedge-tailed eagles are large birds with a characteristically black appearance, although they can appear tar to charcoal brown, depending on lighting and individual variation. They have large bills, but possess relatively smaller and flatter heads, with a long, almost vulturine necks. Furthermore, they are distinctive for their prominent carpals and baggy-feathered "trousers". The species tends to perch conspicuously on dead trees, telegraph poles, rocks, or at times, the open ground. Between the bill size, elongated shape, and prominent shoulders, the species is highly distinctive. While perched, their wings extend down to a long and markedly wedge-tipped tail.
They have a large proportion of bare facial skin, which is thought to be an adaptation to the warm climate of their habitats rather than for eating carrion. Against the blackish plumage, the tawny-rufous hackles on the neck, forming a lanceolate shape, as well as the pale brown to rufous crissum, and narrow, mottled grey-brown band across the greater wing coverts, all stand out well. The sexes are indistinguishable by plumage.
The juvenile is mainly darkish brown, with extensive rufous feather edging, and a paler, fairly streaky head. Furthermore, the juvenile has a lighter-brown crissum, and a light reddish-brown to golden nape, with similar colouring extending sometimes to the back and wing band. The wing band is considerably more prominent than those of adults, extending to the median and sometimes the lesser coverts. Rarely, a juvenile may be all dull black, lacking rufous edges or a wing band. Young eagles are much the same by the second through to the fourth years, though they may be almost invariably visibly in moult and with a narrowing wing band. They become darker around the fifth year, with a red-brown nape and a still-narrowing wing band. Full mature plumage is not attained until the seventh or eighth year, although sexual maturity can be considered as early as five.
Adults have dark brown eyes, while juveniles usually have similar but slightly darker eyes. Wedge-tailed eagles are typically creamy white on the cere and feet, although those can be dull yellow, more so in juveniles than adults. The wedge-tailed eagle has a unique moult process in that they moult almost continuously and very slowly, and three or more years may be needed for an eagle of the species to complete a moult. Moults are arrested only at times of famine, and happen gradually, so that they do not impede the bird's flight or hunting capabilities.
In flight, wedge-tailed eagles appear as a very large, dark raptor, with a protruding head, long and relatively narrow-looking wings, more-or-less parallel edged when soaring, and most distinctly, a long, diamond-shaped tail. The shape is dissimilar to any other raptor in the world. Juveniles tend to be broader-winged by comparison. The wingspan is around 2.2 times greater than the bird's total length. They tend to fly with rather loose but deep and powerful beats. Wedge-tailed eagles spend much time sailing along, looking quite stable and controlled even in strong winds. The species glides and soars on upswept wings with long, splayed primaries. The ample tail may be upcurved, or "dished", at the edges. The eagle often spreads its deep wing emarginations to reduce drag in high winds. Contrary to their superlative and controlled appearance once on the wing, flight for wedge-tailed eagles can be a struggle even in normal circumstances, unless it is from a pinnacle or winds are somewhat high, and within the forest, they may clamber about, with a "lack of grace", to reach the canopy. Gorged birds on the ground can be vulnerable, being practically grounded, which was an advantage historically to Aboriginal hunters. Human gliders have encountered wedge-tailed eagles at more than .