Bonelli's eagle

The 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, who is credited with gathering the type specimen, mostly likely from an exploration of Sardinia. Some antiquated texts also refer to this species as the crestless hawk-eagle. 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 Middle East and South Asia to Indonesia. On the great Eurasian continent, this may be found a nesting species as far west as Portugal and as far east as southeastern China and Thailand. It is usually a resident breeder. The Bonelli's eagle is often found in hilly or mountainous habitats, with rocky walls or crags and open to wooded land, in arid to semi-moist climate, from sea level to 1,500 m (4,900 ft). This eagle, though it can be considered partially opportunistic, is something of a special predator of certain birds and mammals, especially rabbits, galliforms and pigeons. On evidence, when staple prey populations decline or are locally scarce, Bonelli's eagle switch 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, the 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 due to widespread habitat destruction, electrocution from electricity pylons and persistent persecution. Bonelli's eagle is a member of the Aquilinae or booted eagles, a rather monophyletic subfamily of the accipitrid family. At least 38 species are currently housed in the subfamily, all with signature well-feathered tarsi. The African hawk-eagle (Aquila spilogaster) was once lumped with the Bonelli's eagle but 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 the Bonelli's eagle and the African hawk-eagle the two species are visibly similar and are still considered sister species. Recent DNA research resulted in the two species being moved, in 2014, to the genus Aquila from Hieraaetus, along with another dissimilar species, the Cassin's hawk-eagle (Aquila africana). More specifically and surprisingly, 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 rather larger than the 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 rather dark colours. 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. The 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. Amongst the currently accepted species of Aquila eagles, it is of similar size to the tawny eagle (Aquila rapax), slightly larger than the African hawk-eagle and notably larger than the Cassin's hawk-eagle. Like most birds of prey, the 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 55 to 74 cm (22 to 29 in). Wingspan in males can vary from 143 to 163 cm (4 ft 8 in to 5 ft 4 in) while that of the female may vary from 156 to 180 cm (5 ft 1 in to 5 ft 11 in). Prior claims put the weight of this species as 1.4 to 2.4 kg (3.1 to 5.3 lb), 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 1.94 kg (4.3 lb), with a range of 1.4 to 2.24 kg (3.1 to 4.9 lb) (sample size of 91), while 87 females were found to average 2.62 kg (5.8 lb), with a range of 2.1 to 3.03 kg (4.6 to 6.7 lb). Mature males from western Europe were found to have averaged 65 cm (26 in) in total length and 155 cm (5 ft 1 in) in wingspan while mature females averaged 70.7 cm (27.8 in) in total length and 167.8 cm (5 ft 6 in) 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 1.5 kg (3.3 lb) and 2.5 kg (5.5 lb) for males and females from the Indian subcontinent, respectively, though the sample size is unknown in this case. Bonelli's eagles in general form appear to have a medium-sized head on a fairly long neck, a strong bill, a fairly long tail and exceptionally long and well feathered legs. The combination of its well-proportioned, fairly stout body and elongated legs may lend to descriptions of the species as “athletic” in appearance. 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 458 to 542 mm (18.0 to 21.3 in), with an average in western Europe of 480.4 mm (18.91 in), in tail length from 237 to 287 mm (9.3 to 11.3 in), with an average of 268.1 mm (10.56 in), in tarsus length from 93 to 120 mm (3.7 to 4.7 in), with an average of 99.5 mm (3.92 in) and in total bill length from 40.4 to 45.3 mm (1.59 to 1.78 in), with an average of 43.3 mm (1.70 in). Meanwhile, females vary in wing chord from 478 to 560 mm (18.8 to 22.0 in), in tail length from 246 to 319 mm (9.7 to 12.6 in), with an average of 288.5 mm (11.36 in), in tarsus length from 93 to 127 mm (3.7 to 5.0 in), with an average of 119.1 mm (4.69 in), and in total bill length from 41.3 to 51.8 mm (1.63 to 2.04 in), with an average of 46.6 mm (1.83 in). Two males from the A. f. renschi race measured 444 and 452 mm (17.5 and 17.8 in) in wing chord length and a single female measured 493 mm (19.4 in). The Bonelli's eagle is roughly 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. 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 powerful sympatric competitor, the twice as massive golden eagle. Hallux claw lengths in Bonelli's eagles from western Europe averaged 37.21 mm (1.465 in) in males and in females averaged 43.1 mm (1.70 in), and could farther range up to 47 mm (1.9 in). 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. 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 colouring but tend to become more patchy below with increased heavy streaking. During the gradual further development through subsequent molts, the immature eagles develop a thicker subterminal band and a paler ground colour below. Among the bare parts, 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. In flight, the Bonelli's eagle is a largish raptor with a well projecting head and fairly broad, long and rather square ended wings which are slightly pinched in at body with a little tapering at tips. Feather molts can make the wings look quite variable in shape with some individuals appearing rather longer and narrower winged than others. In flight, the tail appears fairly long and broad but if pinched in can appear surprisingly narrow. This species tends to fly with powerful but loose shallow beats. 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. This species soars infrequently on flat or slightly raised wings. At nearly all times of the year, Bonelli's eagles quite often flies in pairs. In colouring, the flying adult is dark above with a variable amount of the white marking on the mantle. The tail has faded barring (rarely perceptible) on grey with a big blotchy subterminal band and a white tip above. The markings on the tail look more or less the same when seen both from below and above. Adult Bonelli's eagles have white lesser coverts which along with the greyish tail stand out in contrast against blackish central wing band over the greater and median coverts. Also the flight feathers are faintly and thinly barred light grey-brown with paler bases, which often become paler (to a whitish hue) on the primaries inside blackish tips and leading wing coverts. In flight, juveniles are brown above with slightly darker wing ends and tips to greater coverts and greyer primary windows. Occasionally, juveniles manifest a creamy patch on back and obscure narrow U above barred tail, which even if present are only sometimes visible. Below the juvenile’s wing linings are light rufous to warm buff like the body colour. Usually juveniles appear with darker tips to greater coverts forming wing-diagonals (sometimes lacking or confined to carpal area) and a small but distinct area of white on primaries against the blackish tips. Until the 3rd year, the young eagles appear more like a 1st year juvenile than an adult, but begin developing more streaks below and darker greater underwing coverts. By the 4th year, the subadult Bonelli's are increasingly similar to the adult, with an increasing subterminal band, a whiter underbody and fairly prominent underwing-diagonals. However, subadults are often still appear with a mix of paler barred juvenile type feathers and plainer darker adult feathers on the flight feathers. Bonelli's eagles are generally unmistakable when shape, flight and plumage are considered in combination. In poor light, it is possible to mistake one with a honey buzzard, one mainly in Europe and another mainly in Asia, as these raptors are extremely polymorphic and can come surprisingly close to approximating the plumage of various more powerful raptors. The wing shape in Bonelli's eagles can at times appear similar to that of honey buzzard but the latter raptor type are usually distinctly slimmer and slighter bodied with a much smaller, slimmer head. In flight, honey buzzards often have notched rather than square ended tails, less emarginated primaries and typically they fly with their wings held more at an angle. The sympatric species of honey buzzard tend to have bolder barring on the tail and underwings, broader dark trailing wing edges and all have no pale mantle patch or darker underwing-diagonals. An unlikely source of confusion is the northern goshawk (Accipiter gentilis), which is usually visibly smaller with much shorter wings, a slightly longer tail, different level flight style and many distinctive plumage characteristics. Distant juvenile Bonelli's could conceivably be mistaken for the long-legged buzzard (Buteo rufinus), but the buzzard is also smaller and is shorter tailed with prominent dark carpal patches and dark trailing wing edges. Furthermore, the buzzard usually holds its wings in a noticeable dihedral when flying in a soar. Adult Bonelli's eagles may be mistaken for pale morph adult booted eagle (Hieraeetus pennatus) by inexperienced observers but, beyond being a third larger and more than twice as heavy, Bonelli's eagles are rather distinct in plumage. Overall Bonelli's are much darker and streaker especially compared to the clearly paler flight feathers of booted eagles. Booted eagles are both whiter and more contrasting on the upperwing coverts, have whitish uppertail-coverts and have white on the wing’s patagium. More similar in plumage is the juvenile Bonelli's eagle are the rare rufous morph of the booted eagle but the latter can still be told by the booted species’ narrower wings and smaller size. In southern part of the Red Sea, vagrants (largely juveniles) may possibly come into range of the closely related and more similar African hawk-eagle, but the latter is comparatively short winged and longer tailed, with the adult plumage being more contrasting and juveniles being darker above with pale wing-windows. Bonelli's eagle is largely silent outside of breeding season and is a somewhat infrequent vocalizer even in the context of breeding. Its calls are less well studied than those of the African hawk-eagle, which although a tropical species, also generally ceases to call outside of the breeding season. The main call of the Bonelli's eagle is done during the courtship display and, sometimes, also at the nest. Its main call 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. Its call is farther carrying than the “puppy-like” one of the golden eagle and is reminiscent in pitch of that of the red-tailed hawk (Buteo jamaicensis). The call may be given by both sexes. However, the female Bonelli's eagle calls most intensely when the male is delivering prey unlike the preference for vocalizing in aerial display as the male usually does. 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 reported by researchers at or near the nests.