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.

Some antiquated texts also refer to it as the crestless hawk-eagle.

Taxonomy

Bonelli's eagle was described in 1822 by Louis Pierre Vieillot. 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. 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

thumb|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 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) (albeit with rather shorter wings than the tawny), slightly larger than the African hawk-eagle and notably larger than the Cassin's hawk-eagle. 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. 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 .

thumb|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. 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. Two males from the A. f. renschi race measured in wing chord length and a single female measured . 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 . 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. 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. At nearly all times of the year, Bonelli's eagles quite often flies in pairs.

Another unlikely confusion species is the 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.

Vocalizations

thumb|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. 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). Beyond its African breeding range, the IUCN Red List 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. Additionally, it has been recorded as a vagrant in east Africa in Somalia.

In southern Europe, they range patchily through different parts of Portugal and Spain into southern France as far north as the department of Drôme. Discontinuously, they are now seemingly solely left as breeding bird in Italy on the islands of Sardinia and Sicily. They at least were known to live in Aspromonte National Park in Calabria, near the far southwestern tip of the Italian Peninsula, directly across the narrow Strait of Messina from Sicily. In 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.

Out of Europe, they may be found in western and southern Turkey, Syria (possibly but most likely 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. 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.

Further east into Asia, their distribution includes eastern Afghanistan and Pakistan through most of the Indian subcontinent, where generally it is uncommon but more locally common near Nepal. On the other hand, they are absent in eastern India and only occur as a vagrant to Sri Lanka and Bangladesh. 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.

Historically speaking, research published in People and Nature in 2024 by scientists from the University of Granada and Miguel Hernández University of Elche (UMH) indicates that Bonelli's eagle is a relative newcomer to Europe and the Mediterranean Basin and that they spread there with the help of humans around 50,000 years ago. 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. Wanderings include around north of their regular range in France near the coast of English Channel, far from their normal haunts in Regensburg, Germany and, probably both from the Italian island populations, to northwestern Italy and Slovenia. From their Iberian Peninsula range presumably, vagrants have been reported in the Canary Islands. Beyond Sri Lanka, other areas the species has been known to vagrate (or perhaps rarely winter) in Asia have included Kazakhstan, the 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.

Habitat

Bonelli's eagle usually lives at an elevation of or lower in Europe, in the Himalayas at and up to in Bhutan, at in the Atlas Mountains.

It tends to dwell in similar habitat types across its range and lives in lands hugging large bodies of water, largely the Mediterranean Sea and northern Indian Ocean. To a lesser extent, it lives near the coast of the Atlantic and the Pacific Oceans and near the Caspian Sea. Despite often being near seas and oceans it mostly occurs in fairly arid habitats and in quite sunny areas. It prefers rocky areas including lower mountains, foothills with plentiful cliffs and steep sided canyons; it is very skilled at hunting in craggy, irregular rocky terrain. Usually, extensive garrigue-type habitat such as low bushes or more substantial vegetation such as scattered trees are a common feature of residential ranges but also at times even denser woodlands. Such scrubby areas are key since they hold prey concentration in Mediterranean habitats. However, excessive ground cover may limit hunting success, so it avoids dense scrub. In the Mediterranean region, it visits pine or sclerophyll forests, but avoids deep forests. In Spain and Sicily, it shows a preference for agricultural arable land and other human-modified habitats, probably because it preys on feral pigeons. Bonelli's eagles also hunt in a quartering flying style relatively close to the ground (in a fashion reminiscent of a harrier) or patrols hillsides for prey activity. Compared to most other booted eagles, Bonelli's eagle takes a great majority of its prey alive and seldom comes to carrion or pirates foods from other raptors.

thumb|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 western Europe, though some study has gone into their food habits elsewhere (being well known in Cyprus and, less so, India). Brown & Amadon (1986) considered Bonelli's eagles prey size range as nearly as extensive as the most massive booted eagles, such as the golden eagle and the martial eagle (Polemaetus bellicosus) (but mainly may have been describing the African hawk-eagle that was lumped at the time). A subsequent study in Spain, however, posited the mean prey size as lower than in the past, stating that prey taken by males averaged an estimated and by females at , probably due to increased importance of pigeons and reduced numbers of rabbits. 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%). In the third largest western European study, rabbits were secondary in number to pigeons (at 18.4% of 1641 prey items) but were still the largest contributors of biomass, at 33.2%. Even where non-native, such as the Aegean islands of Greece, the European rabbit dominated the foods of this eagle, comprising 40.8% by number and 46.6% by biomass of the foods.

Research determines that Bonelli's eagle are often attracted to scrub areas during hunting forays to catch sight of rabbits foraying out of vegetative cover. Since young juvenile and yearling rabbits are forced out to more open feeding spots by dominant adult rabbits, they are disproportionately often selected by Bonelli's eagles and other avian predators. Rabbits become more commonly caught during the summer when the young rabbits tend to disperse. On the contrary, 86.2% of the rabbits taken in southwest Portugal were reportedly adults. In additional, significant numbers of other 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.

Gamebirds and pigeons

thumb|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 red-legged partridge (Alectoris rufa). Although at times capable of evading the attentions of eagles, this partridge occurs in conveys in the same mixed scrub that hold rabbits and is taken whenever the eagles are lucky enough to have the element of surprise. About 383 red-legged partridges were estimated to be hunted annually in one study area of southwestern Spain. At times, even adult Indian peafowl (Pavo cristatus), potentially weighing up to , have been dispatched by this species. In Cyprus, rock and common wood pigeons collectively made up 27.7% of the diet. Water birds taken Bonelli's eagles may vary in size from wading birds as small as common sandpiper (Acitis hypoleucos) and diving birds as small as little grebes (Tachybaptus ruficollis) to those as large as adults of painted storks (Ciconia leucocephala), greylag goose (Anser anser) (though reportedly taken while injured by buckshot in India), and common crane (Grus grus). Corvids, of a dozen or more species and up to the size of the common raven (Corvus corax), are taken in considerable numbers in differing parts of the range.

In Provence, Eurasian magpie (Pica pica) and western jackdaw (Corvus monedula) made up 10.17% and 9.95% of the diet respectively. In the Aegean islands, carrion crows comprised 14.1% of the prey by number and 8.8% of the biomass, while south of Turkey in Cyprus, western jackdaw comprised 7.6% of the foods. and various larks, swallows, Iberian grey shrike, accentors, at least 10 Old World flycatcher species, thrushes, pipits, starlings, buntings, finches and house sparrows. In total, about 130 bird species may be taken and birds as a whole almost always form the most ample part of the diet compared to other classes: 69.5% and 80.97% of the biomass in the south of France, 67.7% in Georgia and 62.6% in Catalonia. Other rodent species known in the diet of Bonelli's eagles have included other squirrels, gundis, assorted mice, voles, dormice and blind mole rats. Beyond a few species of hedgehogs, additional mammalian prey for this species, although seldom taken, can be relatively large. They have been known to attack the young of various ungulates include blackbuck (Antilope cervicapra), chinkara (Gazella bennettii), domestic goats (Capra aegagrus hircus) and domestic sheep (Ovis aries). In the Aegean islands, live-caught but often young and small goat kids comprised 8.5% of the foods and 24.3% of the biomass at nests. In France and Spain, mammals overall comprised 34.8% and 26.1% of the diet, respectively, whereas in Georgia they made up 15.4% of the diet.

Cases of golden eagles taking over prior Bonelli's eagles territories have been reported but usually golden eagles only takes up the prior Bonelli's territory when the latter vanishes due to unrelated (often anthropogenic) causes not direct competition or usurpation. A minor negative effect has been probably correlated with golden eagles not infrequently attacking and displacing juvenile and subadult Bonelli's eagles and can tend to be behaviourally dominant in keeping with its larger size. This in turn presumably hampers the ability of Bonelli's to expand their range after declines and stabilize their population. Further east, in Israel, Bonelli's and golden eagles are competitors as well. In the dry, barren Negev desert, golden eagles nests were found apart and Bonelli's were scarce. In the Judean desert, which has more annual rainfall and more available prey, the distance between golden eagle nests averaged and Bonelli's eagle easily outnumbered them. Apparently, Bonelli's eagle exceptionally outcompeted its larger cousin here due to a subtle topographic variation in the habitat. In Spain, Bonelli's eagles share cliff habitats beyond golden eagles also with peregrine falcons (Falco peregrinus), common ravens, Eurasian eagle-owls (Bubo bubo) and three species of vulture. The eagles tend to dominate the smaller carnivorous birds in most circumstances, even the swifter peregrine. However, the still larger 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. 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. Beyond golden eagles, peregrines and griffon vultures, tawny owls (Strix aluco) have been known to take over old Bonelli's eagle nests.

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. 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. Elsewhere, higher import has been applied for rabbits in the local diet of golden eagles as well as for Spanish imperial eagles. The mean size of rabbits taken increases more or less with the size of the avian predator: for Eurasian goshawks, for Bonelli's eagles, for Eurasian eagle-owls and for golden eagles.

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 mesopredator mammals), and eagle-owl (most common predator of other owls). However, they are relatively common predators of other diurnal birds of prey, per overall analysis they took such prey somewhat more regularly than did golden eagles in Europe. Among the other accipitrids that Bonelli's eagle have been known to hunt include the Indian spotted eagle (Clanga hastata), European honey buzzard (Pernis apivorus), red kite (Milvus milvus), black kite (Milvus migrans), western marsh harrier (Circus aeruginosus), Montagu's harrier (Circus pygargus), hen harrier (Circus cyaenus), Eurasian sparrowhawk (Accipiter nisus), shikra (Accipiter badius), Eurasian goshawk, long-legged buzzard and common buzzard (Buteo buteo). Among falcons, they have been known to prey upon common kestrel (Falco tinnunculus), lesser kestrel (Falco naumanni) and peregrine falcon and as for owls, tawny owl (Strix alucco), little owl (Athene noctua), long-eared owl (Asio otus), short-eared owl (Asio flammeus) and most impressively of all, in at least one instance, an adult Eurasian eagle-owl. Although usually classed as an apex predator, as in most cases of apex predators in competitive environments, Bonelli's eagles sometimes infrequently fall victim to interspecific killings and predation as well. Eurasian eagle-owls have been known to prey on Bonelli's nestlings a few times and possibly also an adult at least once. In one case, a subadult male golden eagle preyed upon an adult male Bonelli's eagle. Stone martens are also counted amongst the predators of nests taking eggs in southeastern Spain.

Breeding

thumb|left|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. 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. Aerial display extend with diminished frequency into the incubation and early nestling periods. Home ranges in Portugal were estimated to average up to . On Cyprus, the mean nearest neighbor distance was with 0.52–0.65 pairs per . A dead or missing mate may be quickly replaced and mature adults have additionally been seen breeding with subadults. The trees selected are often the tallest and/or most densely foliaged in a given stand. However, up to 52 tree nests for the species have now been recorded in southwest Portugal. Often the Portuguese nesting eagles used invasive Tasmanian blue gum (Eucalyptus globulus) (44.2% of the time) while a further 21.2% were on cork oak (Quercus suber) not to mention some that were placed on large shrubs, i.e. strawberry trees (Arbutus unedo). The mean height of Portuguese tree nest was . The Portuguese study further found 67.3% of the tree nests to be on hill slopes and average height for lowest branch, both presumably as anti-predator measure. In the 1990s, the first ever tree nest was found in the relative Bonelli's eagle stronghold of Catalonia, Spain, while another singular tree nest was also found in the south of France.

In India, Bonelli's eagles seem to readily switch between tree and cliff nests. The eagles of areas such as Maharashtra and the Western Ghats are usually partial to nesting in trees while in the Deccan Peninsula, Indo-Gangetic Plain and Himalayan foothills, the eagles alternated between nesting on cliffs and lofty trees including red silk cotton (Bombax ceiba), sacred fig (Ficus religiosa), Javan plum (Syzygium cumini) or Dalbergia ssp. Bonelli's eagles in India may also nest close to human habitations if disturbance is low, such as in Saurashtra and in Himalayan foothills, in the latter often in large 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. On Cyprus, 70% of nests were in Turkish pines (Pinus brutia) at a mean elevation of . Their eggs are largely white but are often sparingly spotted and streaked with brown. In a sample of 120, egg height was measured as , with an average of by in diameter, with an average of .

Female broods about 90% of the time for first two weeks after first hatching but this decreases to 50% by the end of those weeks. The female attacks potential predators that come near the nest including other raptors. In the Indian subcontinent, they have been seen to escort oriental honey buzzards, crested serpent eagles (Spilornis cheela), bearded vultures, as well as Gyps vultures, conspecifics and corvids away from the eyrie vicinity while the presence of northern plains grey langurs (Semnopithecus entellus) was observed to provoke a fierce defensive attack. However, unlike African hawk-eagles, Bonelli's eagles rarely attack humans at the nest. 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. The high distance dispersal of the juvenile Bonelli's eagles may potentially benefit gene flow. 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.

Breeding success and causes of failures

thumb|left|Bonelli's eagle feeding its eaglet with a partridge.

The breeding success of Bonelli's eagles may vary considerably. Mean fledgling success on Cyprus was found to be 1.44 per pair. Of 1506 breeding attempts in western Europe, 65.7% were successful. Of these successful ones, 39.8% produced one fledgling, 59.7% produced two and only 0.5% produced three fledglings. Like many birds of prey, siblicide or cainism has occurred, wherein the eldest nestling repeatedly attacks, often killing and occasionally eating their younger siblings. In about 20% of nest, the second chick survives, therefore this species is classed as a facultative cainist rather than an obligate one. On evidence, egg laying and hatching may grow more asynchronous when frequently interrelated outside stressors such as food supply, habitat disturbance and poor weather are applied, all of which may increase the likelihood of cainism. Whether the young have died by siblicide or via other means, Bonelli's eagles have been known to consume their own dead nestlings on a couple of occasions. On evidence, the younger eaglets of Bonelli's eagles and other species in areas where threatened may too survive by human intervention, wherein they remove the chicks and either raise them in semi-captivity or introduce them to a new set of parents.

In India, habitat and the resulting prey composition were found to be the most significant drivers of breeding success. In protected areas such as Ranthambore National Park, nest often produce two fledglings, while in degraded areas such as the Kumaun division, they often produce just one. Fledgling number here was thought to be driven primarily by prey carrying capacity of a given area. 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. 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.

Conservation and rehabilitation

thumb|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 Palearctic held about 2000–3000 pairs with the Iberian peninsula (750–845 pairs) and northwestern Africa (1000 or so) being the core areas. In the mid-1990s, it was indicated that there were 938–1039 pairs in all of Europe, about 75–80% of which in Spain with an estimated 75–90 in Portugal, 35–45 in Greece, 29 in France, 15–20 in Italy, and a handful each in Croatia and Albania. Although listed today on the IUCN Red List as breeding species there, Bonelli's eagle may be extinct as a nesting species in Bosnia and Herzegovina.

As of 2010, 20 to 22 breeding territories have been found in Sicily and it was thought that this population holds about 95% of the remaining Italian population. Sicilian eagles per study were shown to have high adult mortality (10.2%) and at least 17 pairs in 2010 failed to breed altogether. In the Region of Murcia, Spain, Bonelli's eagle was considered the second most threatened raptor species, behind only the lesser kestrel. In the Province of Burgos in northern Spain the number of pairs reduced from 25–27 to 10 between 1980 and 1996. Of 100 breeding attempts from 1988 to 1996, only 0.3 were successful and average success rate was only 0.35, despite surplus feeding beginning after 1992. From 200 or more pairs in Greece in the early 1980s, the population has fallen to less than 50. It was estimated that the maximum number in Asia is likely around 35,000 pairs but it could be well less than half of that. Perhaps the only factor preventing authorities such as the IUCN Red List 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. In Gujarat, India, an analysis from the 1990s determined that the species was increasingly scarce due to human disturbance and logging.

In multiple parts of the range, certainly in western Europe as well as Cyprus, Bonelli's eagles face a high degree of 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 this eagle as they may alter their range to avoid such activity. From 1990 to 1996, 424 dead Bonelli's eagles in Spain were recorded, 55% died due to electrocution and 26% due to poisoning and shooting. Adults were mainly killed via persecution whereas most juveniles died by electrocution. In Catalonia and central Spain, 50% and 86% due to electrocution whereas persecution was more major in Levante and Green Spain (accounting for 52% and 43% of deaths). Abandonment of territories could not be correlated to interspecific competition but was linked to human influence and persecution.

In Sicily, the main threats are thought to be habitat fragmentation and intensifying agriculture. Previously egg-collectors were semi-regularly exacerbating the reduction of the species on Sicily, but this behaviour has seemingly declined mercifully in recent years. Given its relative scarcity in Crete, only a small number of Bonelli's eagles were recovered dead from persecution when compared to other raptors. However, death through shooting and poisoning is surely not sustainable given the low population there. Increasing overhead power line collisions resulting in electrocution from highly dangerous pylons are a major cause of mortality, resulting in unsustainably high population turnover. In one Spanish study area, 56% of juveniles and 13% of adults were killed by electrocution. In France, 44% of radio-tagged post-dispersal juveniles were killed by electrocution. Wind farms in Spain are a potential growing source of changed territories and deaths for Bonelli's eagles but they are likely to be less effected locally than golden 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. Research from western Europe and northeastern Africa has indicated low genetic diversity in these populations, which cause concerns of a population bottleneck for the species in these former strongholds.

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. 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. Research indicated that 99% of avian mortality would be reduced by modifying only 27% of the pylons in areas inhabited by the eagles. 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. 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.

References

  • Bonelli's eagle in Spain
  • Bonelli's eagle Conservation Biology Team of the University of Barcelona
  • Ageing and sexing (PDF; 5.4 MB) by Javier Blasco-Zumeta & Gerd-Michael Heinze
  • Life Bonelli