Bonelli's eagle

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Bonelli's eagle
An adult Bonelli's eagle perched in Spain

Least Concern  (IUCN 3.1)[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Accipitriformes
Family: Accipitridae
Genus: Aquila
Species:
A. fasciata
Binomial name
Aquila fasciata
Vieillot
, 1822
Subspecies[2]
  • A. f. fasciata Vieillot, 1822
  • A. f. renschi (Stresemann, 1932)
Distribution of A. fasciata
Synonyms
  • Hieraaetus fasciatus
  • Aquila fasciatus

Bonelli's eagle (Aquila fasciata) is a large

electricity pylons as well as persistent persecution.[9][10]

Taxonomy

Bonelli's eagle was described in 1822 by French ornithologist

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.[11][18][21][22] 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.[6][23]

Description

Size and form

Perched on a tree near a wetland in Biligiriranga Hills

Bonelli's eagle is a fairly large bird of prey and a medium-sized eagle. When still classified as a member of the genus

Aquila it is amongst the smallest-bodied species.[6][24][13] 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.[6][25] 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.[6] 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).[26][27][28] 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.[27][29] 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).[26] 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 6.1 in) in wingspan.[26] 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.[24][30]

Bonelli's eagles are medium-sized eagles and have distinctly elongated legs and massive feet and talons.

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.[6][31] 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.[6][24] 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).[6][26][24][32] 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).[6] 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.[24][33][34] 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.[13][35] 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).[26]

Colouring and identification

Juvenile Bonelli's eagle in flight, in rural Sangli district.

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.[6][27][36] 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.[13][26] 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.[6][27][36] 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.[6][27][37][38] 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.[6][27]

In flight, Bonelli's eagle is a largish raptor with a well projecting head and broad, long and somewhat 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 long and broad but if pinched in can appear surprisingly narrow.[6][24][37] 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.[39] At nearly all times of the year, Bonelli's eagles quite often flies in pairs.[24] 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.[6][27][37][39]

An adult Bonelli's eagle in flight shows its ample white mantle.

Bonelli's eagles are generally unmistakable when shape, flight and plumage are considered in combination.

short-toed eagle (Circaetus gallicus) which roughly matches the size of Bonelli's eagles but the short-toed has larger and differently rather wedge-shaped wings with a much less dark overall pattern as well as a shorter tail, a rounder head on a shorter neck and usually a dark rather than light throat.[37][39] Also, goshawks, short-toed eagles and European honey buzzards usually frequent different habitats, more often living in more wooded and lowland habitats.[6] 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 moreover 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.[24][39] 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 somewhat smaller and comparatively short winged and longer tailed (the total length is similar between the species due to the African's longer tail but Bonelli's can be visibly larger bodied and rather heavier). In the African species, the adult plumage is a more contrasting, with dark slate grey upperparts, purer white underparts with dark streaking. In African hawk-eagles, juveniles compared to Bonelli's are darker above with pale wing-windows.[6][41]

Vocalizations

Call of Bonelli's eagle.

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.[6][42][43] The main call of 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-out 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).[6][13][38] 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.[6][34]

Distribution and habitat

Typically rugged environment with mixed vegetation that Bonelli's eagle is often at home in. Here in Andalusia's Sierra de las Nieves.

Bonelli's eagle have a spotty and sparse worldwide distribution currently. The species is distributed in

IUCN and others have mapped out a semi-regular wintering range for Bonelli's eagles, in coastal west Africa from southern Morocco down through Western Sahara, Mauritania and northwestern Senegal (rarely also east to Mali), although little more is reportedly known about this population and its origins and altogether the species is considered largely non-migratory.[1][47][48][49] Additionally, the species has been recorded as a vagrant in east Africa in Somalia as well.[50]

In southern Europe, they range patchily through different parts of

southeastern Europe, an isolated population possibly persists in Croatia as well as in northern and southern Macedonia (with the further possibility of spilling over into Kosovo) and spottily through different areas of Greece (possibly spilling over the borders in the west in Albania and in the east in Bulgaria), as well as Crete.[1][54][55][56][57]

Out of Europe, they may be found in western and southern

extirpated), the isle of Cyprus, Lebanon, Israel, western Jordan, northeastern Egypt (rarely in northern half of Sinai Peninsula), possibly but not certainly in spots in the west and south of Saudi Arabia, and elsewhere in the Arabian Peninsula to Yemen, Oman, and the United Arab Emirates.[1][6][58][59][60][61][62][63][64][65] Elsewhere in the Middle East, their range includes eastern Iraq and west, south, and northeastern Iran, extending somewhat into Turkmenistan in the Kopet Dag range.[1][6][66][67]

Further east into Asia, their distribution includes eastern

Yangtze River, as well as rarely into Hong Kong.[1][6][68][69][70][71] Their isolated Indonesian population range is in the Lesser Sunda Islands, including at least Sumbawa, Timor, Wetar, Luang, and Flores, however records show they've turned up on as many as 20 islands in the Lesser Sundas.[6][23][72]

Historically speaking, research published in

Mediterranean Basin and that they spread there with the help of humans around 50,000 years ago.[73][74] Genetic analyses indicate that the Mediterranean Bonelli's eagle population likely comprised a few individuals around the last glacial maximum, which later thrived as the temperature in the Mediterranean Basin rose, and the human population grew and became sedentary.[73]

Mechanistically, the three bilateral interactions among Bonelli's eagles,

golden eagles, and the human population are jointly what allowed Bonelli's eagles to move from the Middle East into the Mediterranean Basin at this time – and also what prevented them from moving there sooner.[73][74] Golden eagles are less tolerant of humans than Bonelli's eagles are.[73] Additionally, between the two eagle species, the golden eagle is the dominant species, while Bonelli's eagle is the subordinate one.[73] With these interactions in mind, the authors of the 2024 People and Nature study hypothesize and test with data-driven mathematical models that with the arrival of the first anatomically modern humans in Europe, some of the golden eagle territories closest to human settlements were abandoned, and these 'vacant' territories began to be occupied by Bonelli's eagles from the Middle East.[73][74] That is, Bonelli's eagles could not have established themselves in the Mediterranean before the arrival of the first Homo sapiens because the competitive pressure exerted by golden eagles and other species would have been too overwhelming, but the arrival of humans displaced the golden eagles and provided an empty niche in the localities' ecosystems that Bonelli's eagles were then able to neatly fill.[73][74]

Bonelli's eagles are mostly residential throughout their range but juvenile can disperse up to over several hundred kilometres. Sometimes, they are recorded at migration sites and at spots where not known to breed in winter.

Korean Peninsula, Malaysia and Cochinchina in Vietnam, as well as a record in winter 1996 on the isle of Yamdena, the latter presumably from the Lesser Sunda population.[6][23][81][82][83][84]

Habitat

The Bonelli's eagle tends to dwell in similar habitat types across its range and lives in lands hugging large bodies of water, largely the

semi-desert, especially in areas such as Israel and India where moister valleys intersect with deserts.[6][24][87][90] Juveniles may take up temporary residence over dry cultivation, small wetland areas, coastlines or surprisingly deep woodlands.[6][24][86][87] In winter, these eagles may occur at times at lower elevation levels and more open habitats in semi-deserts and plains, where they can appear surprisingly at home, but often prefer wetter habitats such as large river mouths, marshlands and lakes, especially where these fall in existing home range, as prey is more likely to be concentrated in such areas.[6][24][86] In some areas such as southeastern Asia, some of the Indian subcontinent and in the Lesser Sundas, Bonelli's eagles may reside around tropical rainforest that is much wetter and more humid than their typical habitats, and in such areas are attracted to more sparse and rocky areas such as slopes and cliffs as well as alternately open mosaics and glades.[6][24] Usually Bonelli's eagles live at an elevation of 1,500 m (4,900 ft) or lower in Europe, to 2,000 m (6,600 ft) in their African Atlas mountain homes and to as high an elevation 3,000 m (9,800 ft) in Asia and even 3,750 m (12,300 ft) in residence in Bhutan. The main elevation where the species resides in the Himalayas falls between 1,200 and 2,000 m (3,900 and 6,600 ft).[6][91]

Behaviour and dietary ecology

Pair of Bonelli's eagle at Raigad

This species is very aerial, often given to soaring and circling over its home range. Like most raptorial birds, it mainly lives solitarily or in a breeding pair.

Aquila eagles in order to capture water birds incidentally flushed during their flybys.[24][95]

A Bonelli's eagle that has caught a rock dove, one of the favoured prey species.

Overall, Bonelli's eagles take a fairly wide variety of prey. Across its wide range, their prey spectrum has been known to include perhaps up to nearly 200 prey species. Dietary studies have primarily been conducted in

pigeons and reduced numbers of rabbits.[94] Thus, on average, prey sizes average about 20–45% of Bonelli's eagles own weight.[94][98][99][100] Furthermore, the latter Spanish study found hunting success of Bonelli's eagles to average around 28.5%, a slightly higher hunting success rate than golden eagles (20%) or lesser spotted eagles (Clanga pomarina) (24%) but slightly lower than greater spotted eagles (Clanga clanga) (34%).[94]

Rabbits and other lagomorphs

European rabbits are strongly favoured prey for Bonelli's eagles in western Europe.

More than any other, the

lagomorphs may be taken, extending to occasional Granada hares (Lepus granatensis) as well as accounts of Bonelli's eagles hunting European hares (Lepus europaeus) in the Greek isles and Indian hares (Lepus nigricollis) in the lower Himalayas.[24][108][109]

Gamebirds and pigeons

Bonelli's eagle with a freshly caught grey junglefowl. Gamebirds such as junglefowl are favoured in the diet whenever available.

The main secondary wild prey species associated with Bonelli's eagles is the

pigeon fanciers frequently persecute this eagle due its allegedly heavily predation of domestic birds). Of the 1497 prey items overall, feral pigeons were found to comprise 30.1% of the food by number and 26% of the biomass while the domestic types made up only 9.7% of the diet by number and 7.2% of the biomass.[103] In Catalonia, Spain, unidentified pigeons made up 17.8% of the foods and 17.4% of the biomass while identified common wood pigeons made up a further 6.24% of the number and 6.54% of the biomass, while a smaller study from the same area boosted wood pigeons to make up 11.3% of 524 prey items.[101][111] In Cyprus, rock and common wood pigeons collectively made up 27.7% of the diet.[96]

Other birds

A Bonelli's eagle being mobbed by house crows in Pakistan, as the eagle is a common predator of corvids.

Other medium-sized birds are taken widely by Bonelli's eagles. A surprisingly popular dietary choice in

south of France, 67.7% in Georgia and 62.6% in Catalonia, Spain.[8][101][102][111][112][116]

Other assorted prey

Beyond the high significance of rabbits (and sometimes other lagomorphs), other mammals are rarely as important or diverse in the diet of Bonelli's eagles as birds are. A couple of rodents can be locally significant secondary prey, however. The

starred agamas (Laudakia stellio) comprised 5.9% of the food, unidentified Lacerta lizards 10.76% of diet in Georgia (and reptiles altogether adding up to 16.9% of the food by number). Relatively large adult specimens of ocellated lizard (Timon lepidus), at 228 g (8.0 oz) in mean body mass, made up 3.97% of the biomass and 7.05% by number in Catalonia, Spain.[101][96][116] Desert monitor (Varanus griseus) and probably assorted other monitor lizards were reportedly amongst the leading prey for Bonelli's eagles in several parts of India.[24][110][120] Minor prey includes toads and possibly a few other types of amphibian. Potentially insects and/or other invertebrates may be taken but these may incidentally consumed (i.e. undigested food from the stomachs of prey).[102][123]

Interspecies predatory relationships

While other birds of prey present some competition for Bonelli's eagles, intraspecific conflicts are the most significant. Here an adult escorts out a wandering juvenile from its range.

Bonelli's eagles frequently occur in range with multiple other eagles and competing predators. Almost certainly the most direct competitor from their European range to the

griffon vulture (Gyps fulvus) was apparently a routine territory and nest usurper of other birds of prey, displacing golden eagles, bearded vultures (Gypaetus barbatus) and Egyptian vultures (Neophron percnopterus) from their nests as well as 9 out of 23 eyries built by Bonelli's eagles in the study area.[136] Despite their prior claimed "dominance" over the swift falcons, at least three cases have been observed of peregrine falcons usurping Bonelli's eagle (presumably through routine harassment and dive-bombing) nests in Spain.[137] Beyond golden eagles, peregrines and griffon vultures, tawny owls (Strix aluco) have been known to take over old Bonelli's eagle nests.[138]

European rabbits have a huge range of predators in the

Iberian peninsula, with at least 30 different species known to hunt the once densely populated lagomorph.[8][106][105] Besides the overlapping ranges of Bonelli's and golden eagles, most other birds of prey that hunt rabbits extensively are partitioned from the potential depletive effect of competition by differences in habitat preferences, hunting techniques and temporal activity. Beyond the specialized mammalian predator, the Iberian lynx (Lynx pardinus), some of the other most specialized predators of wild rabbits are Bonelli's eagles, golden eagles, Spanish imperial eagles (Aquila adalberti) and Eurasian eagle-owls.[8][106][105][139][140][141] A comparative study indicated that the golden eagle diet was comprised 40% by rabbits, while they made up 49% for eagle-owls, 50% for Spanish imperial eagles and 61% for Bonelli's eagle.[142] Elsewhere, higher import has been applied for rabbits in the local diet of golden eagles as well as for Spanish imperial eagles.[98][143] The mean size of rabbits taken increases more or less with the size of the avian predator: 662 g (1.459 lb) for Eurasian goshawks, 857 g (1.889 lb) for Bonelli's eagles, 1,000 g (2.2 lb) for Eurasian eagle-owls and 1,360 g (3.00 lb) for golden eagles.[21][98][144][145]

Along with northern goshawks, golden eagles and Eurasian eagle-owls, Bonelli's eagle is considered a "super predator" in the European region due to its habit of hunting other predators. In contrast to the other birds of prey, they are somewhat less commonly at high predator status compared to goshawks (most common predator of other diurnal raptors in studies), golden eagles (most common predator of

Stone martens are also counted amongst the predators of nests (exclusively as egg thieves) in Spain.[154]

Breeding

mating on the Southern bank of the Chambal River, Uttar Pradesh, India.

Pair formation and nest distribution

Bonelli's eagles, like most but not all raptorial birds, generally lives solitarily or in pairs. They usually mate for life.[155] Territories are maintained 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 original altitude and diving again. The sky-dance sequence may be repeated up to 5–10 times. Occasionally but usually infrequently, territorial exclusions escalate into talon grappling between a territorial bird and an intruder.[6][156] Aerial display extend with diminished frequency into the incubation and early nestling periods.[24] In Spain, the average estimated size of a pairs home range was a very large 44.2 km2 (17.1 sq mi), though only 27.3% of their home ranges on average were used in all seasons.[157][158] Home ranges in Portugal were estimated to average up to 130 km2 (50 sq mi).[159] On Cyprus, the mean nearest neighbor distance was 7.4 km (4.6 mi) with 0.52–0.65 pairs per 100 km2 (39 sq mi).[59] Contrary to many other raptor species, it was found that were no significant relationship between the density of their main prey species and the distance of the neighbouring pairs.[160] A dead or missing mate may be quickly replaced and mature adults have additionally been seen breeding with subadults.[24]

Paired Bonelli's eagles

Nests

A Bonelli's eagle's egg, from the collection Museum Wiesbaden
MHNT

The breeding season of Bonelli's eagles is from late January/February to July in the western part of the range and November–August/September (peaking December–May) in the

chir pine (Pinus roxburghii) near villages. In Pakistan, the species has been known to use seaside cliffs. Also, in the Indian desert zone, Bonelli's eagles are known to use relatively low rocky hills with shallow gradients, making the nest ledges relatively easy to access.[24][166] On Cyprus, 70% of nests were in Turkish pines (Pinus brutia) at a mean elevation of 625 m (2,051 ft).[59] Often this eagle uses the same nest in successive years but also alternate nest may be used. Often somewhere between 1 and 5 nests may be built by the species on their home range. Like other birds of prey, the presence of alternate nest may be a strategy to cope with ectoparasitic infestations within the nest.[13][137] The construction of a new nest takes roughly a month's time.[13]

Development of young

The clutch size is usually 2, though one egg is not infrequently laid. This species seldom lays three eggs, though there are now a few records of this and even three large eaglets recorded in a nest.

Caches of food are often stored early on but are quickly depleted and rarely still present latter in the nesting period when the eaglets growth accelerates.[24][170] The female lingers near nest even after brooding stage. However, the female also tends to take part in prey capture relatively early in fledging period compared to many other eagles.[13] In the latter third of the post-fledging period, the parents are rarely in attendance except when bringing food. The dependence of the young eagles may extend for about 8 to 11 weeks, but has individually varied from 50 to over 120 days. Research on conditions and habitat composition has indicated variation in the post-fledgling dependence stage is derived from habitat quality or from parental skill. However, fledgling body condition seemed to play no major role in this stage. On evidence, the young eagles drift from their parents care independently.[6][13][171][172][173][169]

Fully fledged juvenile eagles lingering in the vicinity of their nest.

Post-breeding

One of the most significant portions of Bonelli's eagles lifecycle is the dispersal stage. The dispersal and post-dispersal stage has been studied at length in western Europe, with a surprising amount of individual variation being found. Here, dispersal occurred at an average age of 142 days (occasionally up to 163 days old) with a varying distance of dispersal from nest to settlement area were from 50 to 536 km (31 to 333 mi). The average distance of dispersal in France was 158 km (98 mi). Anywhere from 58% of 47 to 87% of 7 juveniles survived per these radio-tagged studies.[174][175][176] The high distance dispersal of the juvenile Bonelli's eagles may potentially benefit gene flow.[177][178][179] At least 20 communal roosts for post-dispersal juvenile Bonelli's eagles were found in Spain. Each were found to house between 2 and 11 eagles of the species, with mean of 5.1. It was also found the juveniles were usually sharing many of the roosts with Spanish imperial eagle juveniles as well (in 91.4% of roost) though each species clustered separately in different parts of the trees or bushes. More infrequently, assorted other species of raptor would join the juvenile eagle roosts at dusk.[180]

Breeding success and causes of failures

Bonelli's eagle feeding its eaglet with a partridge.

The breeding success of Bonelli's eagles may vary considerably. Mean fledgling success on

Kumaun division, they often produce just one. Fledgling number here was thought to be driven primarily by prey carrying capacity of a given area.[24] When an almost fledged young was stolen by village children in India, 15 hours later, researchers introduced another which was accepted by parents. In a similar case, another Indian pair rejected its own nearly fledged eaglet after it had been stolen, however with repeated attempts was accepted and successfully fledged.[189] When poachers stole some eagles in Spain, a couple of pairs were found to successfully lay replacement clutches (each with the typical 2 eggs) some 25–30 days later.[190][191] A western European review of 1052 breeding attempts indicated a negative correlation with colder temperatures and heavier rains during nesting. Therefore, in more temperate areas such as northern Spain, the average breeding success was lower and the young often dispersed southwards due to the cooler climate.[192][193]

Conservation and rehabilitation

Artwork from a 19th-century German Natural History book

Bonelli's eagles have sharply declined, at least locally inexorably and drastically, in much of their range. In the 1990s, it was estimated that the entire west

IUCN from uplisting Bonelli's eagle to a more severe status is due to lack of extensive research on their population in the Asian range. Strong declines in Asia may be occurring as well.[1][6] A bird survey of a large area of Uttarakhand, India where the species was historically present failed to find any signs of remaining Bonelli's eagles.[201] In Gujarat, India, an analysis from the 1990s determined that the species was increasingly scarce due to human disturbance and logging.[202]

In multiple parts of the range, certainly in

northeastern Africa has indicated low genetic diversity in these populations, which cause concerns of a population bottleneck for the species in these former strongholds.[215]

Conservation efforts

Research has indicated that the most significant predicted cause to a strong recovery for Bonelli's eagles in Europe would be conservation of appropriate habitats, followed by higher survival rates for territorial and non-territorial eagles.[216][217] It was suggested in 2008 that reducing risk of electric powerline collisions and reducing persecution are the most immediate and significant measures that should be taken to retain Bonelli's eagles in Spain.[176][218][219] Research indicated that 99% of avian mortality would be reduced by modifying only 27% of the pylons in areas inhabited by the eagles.[210] As reported by 2015, biologists in coordination with local authorities started to properly insulate dangerous powerlines in green areas in order to help converse this and other threatened birds. It was shown that the local population growth rates increased quickly as a result (from 0.82 to 0.98). However, this study showed an apparent increase of anthropogenic mortality from other causes, such as car collisions, in sync with reduced electrocution.[220] It was estimated that for stage of 2008–2014, 0.28 and 0.64 of mortality was still due to electrocution for territorial and non-territorial eagles.[221] In further efforts to converse the species locally, Spanish researchers have provided supplemental feedings to these eagles, which may improve their odds of successfully producing young.[222]

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