Bonelli's Eagle

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. The Bonelli's eagle is a powerful predator, and has been described as rather “bold and rapacious”. Its primary hunting methods recall those of a powerful Accipiter such as a goshawk. Most commonly, this eagle still-hunts, often either utilizing a concealed tree perch or a lofty spot in irregular rocky terrain to watch for prey activity. Upon spotting its quarry, it 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. Not infrequently as a latter part of a tail chase, these eagles (again reminiscent of a goshawk) will occasionally walk on the ground to obtain their prey. 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. Bonelli's eagles will also occasionally stoop from a soaring height onto prey. Mostly, this predator takes birds from on or near the ground but some snatched them from bushes and, seldomly, water. It has been known to have sufficient agility to catch some birds from active flight. In one case, a Bonelli's eagle was observed to fly below a jackdaw and swoop upwards to grab it from below. Tandem hunting by a lifelong pair is quite common, almost exceptionally so in this species. One eagle tends to fly directly above the other, with several cases of one eagle scattering a bird flock for the other eagle to quickly single out, in a similar style to tandem-hunting laggar falcons (Falco jugger). However, per Spanish studies, apparently tandem hunting neither improved hunting success nor were the eagles able to capture larger prey (in fact the estimated prey size by pairs was slightly lower than that taken by each mate hunting by itself) while hunting in tandem. It was hypothesized that tandem hunting is more important to the socio-sexual relations of the pair rather than capture of a significant amount of prey. 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. However, it will readily come to previously injured prey, especially water birds shot by duck hunters, and will readily take young animals across most prey classes. Also, in Keoladeo National Park, India, Bonelli's eagles were observed to habitually follow harriers, spotted eagles and other Aquila eagles in order to capture water birds incidentally flushed during their flybys. 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 fairly well known in Cyprus and, less so, India). Brown & Amadon (1986) considered the 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). Bonelli's eagles mainly hunts birds and mammals, taking reptiles and other prey types on a more local and sporadic basis. In western Europe, it is considered something of a specialist predator on rabbits and partridges, though other birds such as pigeons, gulls and corvids sometimes are taken as much or more so depending on local prey population trends. Pellet analysis is considered the most reliable way to get a complete picture of Bonelli's eagle dietary habits. Despite its predaceous power, typically the average size of prey taken are within average range for a raptorial bird and it may take smaller prey on average than its mildly smaller cousin, the African hawk-eagle. In Sierra Morena, Spain, the mean size of prey taken was estimated at 630 g (1.39 lb), while in Greece the mean prey size was estimated at 877 g (1.933 lb). 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 416 g (14.7 oz) and by females at 459 g (1.012 lb), probably due to increased importance of pigeons and reduced numbers of rabbits. Thus, on average, prey sizes average about 20-45% of the Bonelli's eagles own weight. 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%).

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. In Spain, the average estimated size of a pairs home range was a very large 44.2 km (17.1 sq mi), though only 27.3% of their home ranges on average were used in all seasons. Home ranges in Portugal were estimated to average up to 130 km (50 sq mi). On Cyprus, the mean nearest neighbor distance was 7.4 km (4.6 mi) with 0.52-0.65 pairs per 100 km (39 sq mi). 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. A dead or missing mate may be quickly replaced and mature adults have additionally been seen breeding with subadults.

Bonelli's eagles have sharply decline, 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. By the 2000s, with some continued declines and minor local recoveries (as well as more comprehensive surveying) resulted in an estimate of 1500 pairs in Europe, still far less than historic numbers (at least a 30% reduction since the 1950s) and qualifying the species for local critically endangered status. Local extinction is probable in many if not all populations yet there is no overarching action plan for conserving the species. On evidence, populations in core protected areas have increased but fringe areas, important especially to vagrant juveniles, continue to show strong declines and high mortality rates. Although listed today by the IUCN as breeding species there, the 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 their Spanish stronghold, the species has declined or disappeared in 27 out of 40 provinces since 1980, with over 20% reduction in north and central parts. The coastal sierras of east and south Iberia hold highest European densities at 1 pair per 100–200 km (39–77 sq mi), but once formerly it held a pair per 60 km (23 sq mi) in the 1970s. In the Region of Murcia, Spain, the 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. What was roughly estimated to be about 50 pairs (estimated earlier at up to 100), in Turkey in the late 1980s to the 1990s, has recently been revised based on research to only 20-35 pairs in isolated small pockets. In Israel, 28 pairs of Bonelli's eagle were known to be present in 1989 but little information has been obtained from the rest of Middle East and from Asia. Israeli populations have been roughly halved in size. By 2001, only 15 pairs were known to breed in Israel. Besides the four species that have become locally extinct in Israel, it is likely that the Bonelli's eagle is the most endangered Israeli raptor. 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 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. 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. In Gujarat, India, an analysis from the 1990s determined that the species was increasingly scarce due to human disturbance and logging. Very little information is known about the current status of Bonelli's eagles in northern Africa (with the last survey known of the species in Morocco dating to the mid-1980s), although the trends seen in western Europe may be roughly reflected there as well. 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 northern 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 powerline 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 genetic bottleneck for the species in these former strongholds. Research has indicated that the most significant predicted cause to a strong recovery for Bonellis 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. 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. 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.