The Bonelli's eagle (Aquila fasciata) is a large bird of prey. Its common name commemorates the Italian ornithologist and collector Franco Andrea Bonelli. This species has declined precipitously in various parts of its range, including almost all of its European distribution, and may face potential local extinction.
Adult Bonelli's eagles are dark brown above, from a cold hue similar to dark chocolate to an amber hue, 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 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 color 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. The streaking on this eagle is normally strongest on the breast and upper flanks while the lower belly and crissum are typically either plain or only faintly marked. Juveniles are a lighter medium brown above with variable paler edges, sometimes with a creamy patch on the back (not the mantle as in the adults) and uppertail coverts. Generally, juveniles have a rusty-brown head with a darker brown around and behind their eyes. The juvenile eagle's crown is either darkly streaked or, occasionally, plain greyish. The tail of young birds is more clearly barred than the adults while the subterminal band is only negligibly thicker than the other bars. Like adults, the juvenile Bonelli's eagle's tail has a thin white tip. The juvenile is light rufous to warm buff below with minimal black streaks, which are normally confined to chest-sides. By their 2nd summer, the young eagles are still largely the same in coloring but tend to become more patchy below with increased heavy streaking. Among the bare parts, the adult's eyes are yellow to yellow-orange while those of the juvenile are hazel-brown. Adult plumage is obtained between the 4th and 5th years. At all ages, the cere and feet are both pale yellow.
Bonelli's eagles breed from Southern Europe, Africa on the montane perimeter of the Sahara Desert and across the Indian Subcontinent to Indonesia. In Eurasia, they may be found as far west as Portugal and as far east as southeastern China and Thailand. Bonelli's eagles are mostly resident throughout their range but juveniles can disperse up to over several hundred kilometers. Sometimes, they are recorded at migration sites and at spots where not known to breed in winter. These birds prefer rocky areas including lower mountains and foothills with plentiful cliffs, as well as steep-sided river valleys and gorges. They are very skilled at hunting in craggy, irregular rocky terrain. Bonelli's eagles also occur in garrigue-type habitats such as low bushes or scattered trees, and even denser woodlands. In the Mediterranean region, they like either pine forests or sclerophyll forests. Bonelli's eagles may also range into timbered plains, barren slopes, semi-desert, dry cultivation, small wetland areas, coastlines, or surprisingly deep woodlands.
Bonelli's eagles usually live singly or in breeding pairs. They are very aerial, often seen soaring and circling over their home range. When gliding, they do so often on flat wings with well-spread feathers and the carpals pressed slightly forward but more so when entering a fast glide. At nearly all times of the year, Bonelli's eagles quite often fly in pairs. They often perch 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. Bonelli's eagles are powerful predators, and have been described as rather “bold and rapacious”. They hunt during the daylight hours waiting patiently from a concealed tree perch or a lofty spot in irregular rocky terrain. Upon spotting its quarry, the eagle often dashes out rapidly to take birds as they take off or a mammal as it runs for cover, at times making lengthy tail-chase that may continue between trees or into tree stands or bushes. The eagle may occasionally walk on the ground to obtain the prey. Bonelli's eagles also hunt in a quartering flying style relatively close to the ground or patrol hillsides for prey activity. They will also occasionally stoop from a soaring height onto prey. Bonelli's eagles are typically silent outside of breeding season. Their main call is done during the courtship display and, sometimes, also at the nest. It consists of a loud, shrill, somewhat far-carrying scream, 'yuiii-yuiii-gii-gii' or a 'drawn-oout heeeeii-heeeeii' with slight regional or even individual variations. Other recorded vocalizations have included a fluted, low-pitched 'klu-klu-klu' as well as a repeated 'ki ki ki' in alarm. Also, other barking, gurgling, and grunting sounds have been heard at or near the nests.
Bonelli's eagles are carnivores and take perhaps up to nearly 200 prey species. They mainly hunt birds, mammals, and less often lizards and snakes. In Western Europe, Bonelli's eagles are considered specialist predators of rabbits and partridges, as well as pigeons, gulls, and corvids. They also take various water birds, including gulls, rails, stone curlews, lapwings, sandpipers, tubenoses, cormorants, and herons. Among passerines, they hunt various larks, shrikes, swallows, accentors, flycatchers, thrushes, pipits, starlings, buntings, finches, and sparrows. Mammal prey includes squirrels, gundis, assorted mice, voles, dormice, and Blind mole rats. Bonelli's eagles also attack the young of blackbuck, chinkara, domestic goats and domestic sheep, Red foxes, wildcats, Stone martens, and assorted weasels.
Bonelli's eagles are monogamous and usually mate for life. They breed from late January/February to July in the western part of the range and in November-August/September in the Indian subcontinent and Myanmar. During this period pairs maintain territories through aerial displays which often involve calling, single or mutual high circling and, most frequently, sky-dancing in the area of eyrie. During this species’ sky-dances, one or other of the eagle pair plunges headlong from a great height, with its wings almost closed, before checking and rising again on stiff wings, circling to regain its original altitude, and diving again. The sky-dance sequence may be repeated up to 5-10 times. Sometimes, territorial exclusions escalate into talon grappling between a territorial bird and an intruder. The nest is a huge structure of branches and sticks often located high on a cliff ledge or at 5 to 40 m (16 to 131 ft) above the ground in large trees. The female typically lays 2 eggs and incubates them for 37 to 45 days while the male mainly captures food. The eaglets hatch altricial. The first feathers start appearing through the white down at 25-35 days and practically cover the body by 45 days. The young fledge between 56 and 65 days of age and become independent when they are about 8-11 weeks old.
Bonelli's eagles have sharply declined in much of their range. In multiple parts of their range, certainly in western Europe as well as Cyprus, Bonelli's eagles suffer from persecution by hunters, gamekeepers, and pigeon-fanciers. Shooting and poisoning of this species persist extensively into the 21st century. Habitat alteration and destruction (e.g. development of roads, intensified agriculture, irrigation of dry fields) in addition to reduced prey numbers and human disturbance in the nesting area are ongoing and increasing threats everywhere for this eagle. Even human activity such as large quantities of people on holiday has been shown to have a negative effect on Bonelli's eagles as they may alter their range to avoid such activity. Increasing overhead power line collisions resulting in electrocution from highly dangerous pylons are a major cause of mortality. Wind farms in Spain are a potential growing source of changed territories and deaths for Bonelli's eagles. Lead poisoning from bullets in injured small game, which have been associated with high lead levels in eagle feathers in several parts of their range.
According to the IUCN Red List, the total population size of the Bonelli's eagle is 21,000-24,000 mature individuals. The European population consists of 1,100-1,200 pairs, which equates to 2,100-2,400 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.
