Bonelli's eagle (Aquila fasciata) is a large bird of prey. The common name of the bird commemorates the Italian ornithologist and collector Franco Andrea Bonelli. Bonelli is credited with gathering the type specimen, most likely from an exploration of Sardinia. Like all eagles, Bonelli's eagle belongs to the family Accipitridae. Its feathered legs marked it as member of the Aquilinae or booted eagle subfamily. This species breeds from southern Europe, Africa on the montane perimeter of the Sahara Desert, and across the Indian subcontinent to Indonesia. In Eurasia, this species may be found as far west as Portugal and as far east as southeastern China and Thailand. It is usually a resident breeder.
Bonelli's eagle often occurs in hilly or mountainous habitats, with rocky walls or crags, from sea level to . Habitats are often open to wooded land and can occur in arid to semi-moist climate. It can be considered partially opportunistic, but is a specialist predator of the European rabbit, galliforms and pigeons. On evidence, when staple prey populations decline or are locally scarce, Bonelli's eagle switches to being an opportunistic predator of a wide variety of birds. Despite its persistence over a large range and its continued classification as a least concern species by the IUCN Red List, Bonelli's eagle has declined precipitously in various parts of its range, including almost all of its European distribution, and may face potential local extinction. Its decline is due to widespread habitat destruction, electrocution from electricity pylons as well as persistent persecution.
Etymology The generic name Aquila is Latin for "eagle", possibly derived from aquilus, "dark in colour", while the specific name fasciata comes from the Late Latin word "fascia", meaning "band" or "stripe". The common name Bonelli's eagle is for the collector of the type specimen, Franco Andrea Bonelli.Aimassi, G. (2015). The original description of Bonelli's Eagle Aquila fasciata Vieillot (Aves: Accipitridae). Zoological Bibliography 4(1): 1-15. Some antiquated texts also refer to it as the crestless hawk-eagle.Jerdon, T. C. (1862). The birds of India (Vol. 1).
Taxonomy Bonelli's eagle was described in 1822 by Louis Pierre Vieillot. It is a member of the Aquilinae or booted eagles, a monophyletic subfamily of the accipitrid family. At least 38 species are currently part of this subfamily, all with signature well-feathered tarsi. The African hawk-eagle (Aquila spilogaster) was once lumped with Bonelli's eagle, with most accounts until about the 1990s listing the species as monotypical. However, several morphological differences between the two species, life history discrepancies and their considerably allopatric distribution lead them to being considered separate species. Despite the differences between Bonelli's eagle and the African hawk-eagle, the two species are visibly similar and are still considered to form a species complex.Simmons, R.E. (2005). African Hawk-Eagle Aquila spilogaster. Pp. 533-534 in P.A.R. Hockey, W.R.J. Dean, and P.G. Ryan (eds.), Roberts Birds of Southern Africa. 7th ed. Trustees of the John Voelcker Bird Book Fund, Cape Town, South Africa. However, genetic studies have indicated that they are not closely related relative to other species pairs of the booted eagle subfamily.
DNA research resulted in the two species being moved from the genus Hieraaetus to Aquila, along with another dissimilar species, the Cassin's hawk-eagle (Aquila africana). Bonelli's, African hawk- and Cassin's hawk-eagles were found to be genetically closely related to the golden eagle (Aquila chrysaetos) species complex, which also includes Verreaux's eagle (Aquila verreauxii), Gurney's eagle (Aquila gurneyi) and wedge-tailed eagle (Aquila audax). These species are all conspicuously larger than Bonelli's and African hawk-eagles with differing proportions to their wings, tail and legs (in adaptation to their open country habits) and much darker coloured plumages. Furthermore, the four other traditional members of the genus Aquila have been revealed to be a separate species complex despite showing superficial similarity to the golden eagle group, i.e. being relatively large and long winged with usually dark colouring.
Beyond the nominate subspecies of Bonelli's eagle, which is found throughout its range in Eurasia, a second subspecies dwells in the Lesser Sunda Islands, A. f. renschi. The latter race is linearly smaller, and compared to other Bonelli's eagles tends to have more strikingly barred remiges and tail, the belly, thighs and crissum more boldly marked. At one time, its restricted and very isolated range have caused authors to suggest A. f. renschi may be a full species but recent studies have indicated that it is not genetically distinct enough to be considered a separate species. Furthermore, the most recent analysis couldn't rule out early introductions (possibly by ancient falconers) at least playing a part in the species presence in the Lesser Sundas, as some other established wild birds on those islands are certain to have reached there by early human introductions.
Description Size and form
Bonelli's eagle is a fairly large bird of prey and a medium-sized eagle. When still classified as a member of the genus Hieraaetus, it was considered the largest extant species therein, however, as a member of Aquila it is amongst the smallest-bodied species.Naoroji, R., & Schmitt, N. J. (2007). Birds of prey of the Indian subcontinent. Om Books International.Brown, Leslie and Amadon, Dean (1986) Eagles, Hawks and Falcons of the World. The Wellfleet Press. . Amongst the currently accepted species of Aquila eagles, it is of similar size to the tawny eagle (Aquila rapax) (albeit with rather shorter wings than the tawny), slightly larger than the African hawk-eagle and notably larger than the Cassin's hawk-eagle.CRC Handbook of Avian Body Masses, 2nd Edition by John B. Dunning Jr. (Editor). CRC Press (2008), . Like most birds of prey, Bonelli's eagle displays reverse sexual dimorphism as the female is larger than the male to the contrary of most other kinds of birds, in this case she may average about 10% larger overall.
Total length in fully-grown eagles of the species can vary from . Wingspan in males can vary from while that of the female may vary from . Prior claims put the weight of this species as , however this probably slightly underrepresents both their size and the sexual dimorphism of this eagle. A large sample of full grown males from western Europe were found to average , with a range of (sample size of 91), while 87 females were found to average , with a range of . Mature males from western Europe were found to have averaged in total length and in wingspan while mature females averaged in total length and in wingspan. Although the linear measurements reportedly increase slightly in average size in the eastern Asian part of the range, body weight was similar or slightly lower at a mean of and for males and females from the Indian subcontinent, respectively, though the sample size is unknown in this case.Dementiev, G. P., Gladkov, N. A., Ptushenko, E. S., Spangenberg, E. P., & Sudilovskaya, A. M. (1966). Birds of the Soviet Union, vol. 1. Israel Program for Scientific Translations, Jerusalem.
Bonelli's eagles in general form appear to have a medium-sized head on a fairly long neck, a strong bill, a mid-to-longish tail and exceptionally long and well feathered legs. The combination of its well-proportioned, stout body and elongated legs may lend to descriptions of the species as "athletic" in appearance.Rasmussen, P. C., & Anderton, J. C. (2005). Birds of south Asia: the Ripley guide. Washington, DC. This eagle often perches with a very upright carriage, at times openly on a rock, a crag, tree branches or some form of post but also in the foliage of tree cover, especially when actively hunting. When perched, the wing tips tend to fall a bit short of the tail tip. Among standard linear measurements, the wing chord of males varies from , with an average in western Europe of , in tail length from , with an average of , in tarsus length from , with an average of and in total bill length from , with an average of . Meanwhile, females vary in wing chord from , in tail length from , with an average of , in tarsus length from , with an average of , and in total bill length from , with an average of .Ali, S., & Ripley, S. D. (1983). Handbook of the Birds of India and Pakistan: Together with Those of Bangladesh, Nepal, Bhutan and Sri Lanka. Oxford University Press. Two males from the A. f. renschi race measured in wing chord length and a single female measured .
Bonelli's eagle is intermediate in its wing lengths and tail length proportionately between the shorter-tailed and longer-winged eagles of open country and longer-tailed and shorter-winged forest eagles, which allows to vary its hunting between short-burst, agile surprise attacks in trees and ample ground-covering pursuits in the open.Whistler, H. (1940). How do large raptorial birds hunt their prey? Ibis, 4: 732–735.Brown, L. (1977). Eagles of the World. Universe Books. Its talons and feet are proportionately very large and presumably rather powerful for the eagle's size. In particular the elongated talon on its rear toe (used as a killing apparatus by almost all accipitrids), or hallux claw, is longer than that of the much larger eastern imperial eagle (Aquila heliaca) and proportionately slightly larger even than its bigger sympatric competitor, the golden eagle. Hallux claw lengths in Bonelli's eagles from western Europe averaged in males and in females averaged , and could farther range up to .
Colouring and identification
Adult are dark brown above, from a cold hue similar to dark chocolate to an umber hue depending on their state of molt as well as to some extent individual and regional variances, with pale margins to most feathers. These pale margins are especially broad on the median wing coverts (which thus appear lighter brown overall). Adults also have a variably sized, irregular white patch on the mantle that can vary from nearly absent (though almost never completely so) to being quite large and extending to the upper back. The adult's tail is grey with obscure darker brown thin bars, with a broad blackish subterminal band and creamy white tip. The adult Bonelli's head is dark brown with a paler, often streaky neck and a white throat. The underside has a cream base colour with variable amounts of sparse blackish-brown streaks or drop shaped markings. The adult female averages darker and more heavily patterned than the adult male, particular on the underside, a case of colour sexual dimorphism otherwise seemingly rare in booted eagles.