The Bonelli’s eagle (Aquila fasciata) was officially named in 1822 by the French ornithologist Louis Jean Pierre Vieillot (1748-1831) in honour of Franco Andrea Bonelli (1784-1830), a northern Italian ornithologist and entomologist who first documented the species in 1819. Although the name Bonelli’s eagle remains the same, the scientific taxonomy of Bonelli’s eagle was revised a few years ago from Hieraaetus fasciatus to Aquila fasciata as a result of DNA research that shifted the species’ genus from Hieraaetus to Aquila. The eagle’s current scientific name is derived from the Latin words aquila, meaning eagle, and fascia meaning a band.
One unique feature of the remarkable history of the Bonelli’s eagle is that, although not a falcon, the eagle’s magnificent appearance and characteristic behaviour at one time led to its use in falconry. More recently, the Bonelli’s eagle has been regarded as an emblematic raptor of the Mediterranean region despite its vast global distribution.
In 2010, however, this eagle was described as one of the rarest birds of prey in Europe where it has suffered a significant population decline in recent decades, especially between 1970-1990. The global population is more stable than the population in Europe where the species has disappeared from some of its former geographical areas such as the entire country of Croatia and northern parts of Greece where the eagle was once quite common. In France, it was reported in 2009 that there were only 29 pairs remaining in the country thereby rendering this eagle as one of France’s most threatened species. As a result, the European Commission financially supports many projects that, in whole or in part, are scientifically aimed at rehabilitating the European Union population of the resplendent Bonelli’s eagle, such as the 2011-2015 EU LIFE+ Nature project on Andros Island, Greece.
While observing Bonelli’s eagles during their foraging behaviour, their strength, graceful flight and unique feather patterns become immediately noticeable. Although deemed by many to be a small to medium-sized raptor 65-72 centimetres (cm) in length, their wingspan of approximately 150-160 cm for males and 165-180 cm for females is truly impressive. Their weight ranges from 1.6 to 2.5 kilograms. This eagle has a relatively long life span of up to 30 years in its natural environment and one eagle in France has been recorded with an age of at least 32 years according to the Languedoc-Roussillon Conservatory of Natural Areas (France).
A Bonelli’s eagle can be recognised by its dark brown plumage on its upper parts and its white underside that features elongated, vertical dark streaks in a noticeable pattern that gives this eagle such a distinctive and beautiful appearance. The eagle has a long tail that is brown on top and white below with a single broad, black terminal band. Its feet and eyes are distinctly yellow, and a light yellow colour also appears faintly around its beak. Juvenile Bonelli’s eagles can be distinguished from the adults by their less striking plumage, beige underbelly and the absence of the black band on their tail.
According to the International Union for Conservation of Nature (IUCN), the global distribution of Bonelli’s eagles is very extensive, ranging through southern Europe (including Spain, Portugal, France, Italy, Serbia, Croatia, Montenegro, Albania, Bulgaria, Greece, Cyprus), Turkey, northern Africa (including Morocco, Algeria, Tunisia, Libya, Egypt), the Middle East (including Israel, Palestine, Lebanon, Syria, Jordan, Yemen) and many parts of Asia (including China, Honk Kong, Myanmar, Thailand, Vietnam, Indonesia). The largest European populations of Bonelli’s eagles are found in the Iberian Peninsula.
Bonelli’s eagles are distributed along habitats that are neither completely open nor densely forested. Instead, the eagles prefer wooded, often hilly country with open areas and rocky terrain. Where available, their territory includes steep cliffs, mountains, gorges and even savannah consisting of open grasslands provided that there are some large trees for many of the Bonelli’s eagle African populations.
According to BirdLife International, the global population of Bonelli’s eagles is estimated to be 10,000 mature individuals; however, the species’ European population was recently estimated to be only 1,000-1,200 breeding pairs with the Iberian Peninsula hosting at least 75% of the breeding pairs and Greece hosting 100-140 pairs, or roughly 10% of the European population.
In Greece where the Bonelli’s eagle was once quite common, the species today has a southern distribution nesting on many Aegean islands and, to a much lesser extent, in parts of the Peloponnese where it is localised and relatively scarce.
While Bonelli’s eagles generally live in rocky and hilly or mountainous habitats with wooded areas, their hunting grounds also include relatively open areas enabling these agile hunters to emerge from cover in pursuit of their prey which can be seized from the ground or in the air.
Bonelli’s eagle (Aquila fasciata). Constantine Alexander All Rights Reserved
The Bonelli’s eagle diet will vary somewhat depending on their habitat and prey species availability; however, the eagles generally forage on small to medium sized mammals and birds from a wide range of species. Rabbits, partridges (such as Red-legged and Chukar partridges) and pigeons are among their favourite prey, although Bonelli’s eagles also prey on hares, squirrels, rodents, ducks, crows, gulls and lizards. In Greece, for example, Bonelli’s eagles principally prey on Chukar partridges (Alectoris chukar) and rabbits (Oryctolagus cuniculus). During the non-breeding season when the eagles extend their foraging range beyond the territory in which they forage during breeding season, Bonelli’s eagles can be found in lowlands and wetlands where they hunt waterfowl if such habitat is available in the overall region which they inhabit.
Breeding and Habitat:
Bonelli’s eagles establish their domain in a very expansive spatial territory that can cover 50 square kilometres during the breeding season, although there may be small overlaps with neighbouring Bonelli’s eagles in the event of a concentrated or dense local breeding population relative to the size of the overall available territory such as within an island.
During the non-breeding season, the eagles often expand their home ranges as they venture farther from their nests for foraging and roosting; however, the eagles are, nevertheless, highly dependent on their breeding area throughout the year, especially as nesting sites retain their importance for shelter and rest.
Globally, Bonelli’s eagles live in a variety of rugged habitats that generally include wooded and often hilly or mountainous country with rocky terrain and open areas. While the eagles customarily live at low to medium altitudes averaging 660 metres but infrequently exceeding 1,500 metres above sea level, they have been known to inhabit higher altitude areas reaching as high as 2,000 metres in Africa. The eagles’ territory is often characterised by short or sparse vegetation, including dry grassland, bushes and scrub; rocky terrain; and open areas that may adjoin or include non-intensive crops, vineyards, olive groves, non-irrigated orchards, small woodlands and pasture.
The eagles usually build their nests in remote cliffs, ledges, crags and gorges. Some are constructed in large trees, as, for example, in Portugal where 64% of the population nest in trees such as Cork Oaks, pines and large Eucalyptus, while in other places, such as in France, some Bonelli’s eagles also nest in caves. Nests have even been built on power poles such as in Castilla la Mancha (Spain). In Greece, the eagles favour nesting in steep cliffs, ledges and rock crevices sometimes located just over the sea as well as remote gorges away from human settlements.
For breeding, Bonelli’s eagles build as many as 2 to 5 or more alternative nests. These nests are sturdily built with plant material that varies from region to region. In Spain, for example, which hosts a majority of the Bonelli’s eagle European population, University of Granada research revealed that nests associated with favourable breeding success rates were often lined with remnants of Maritime Pine (Pinus pinaster), which contains a chemical that serves as an insect repellant. This nest construction component is significant as it relates to parasitic infestation, to which birds’ nests are exposed. The breeding pairs labour at nest construction over a period of years since many are used alternately in successive years or intermittent years as Bonelli’s eagles pairs do not necessarily breed every year. Over time, the nests can become increasingly visible as they have been known to reach 1.8 metres in height and 2 metres in diameter. These architectural efforts are worthwhile as the nests also retain their value as an available place of shelter and rest during the non-breeding season.
Globally, the breeding season extends from late January or early February to mid-March depending on the geographical location. In Greece, the eagles lay their eggs in early February although on the Aegean island of Crete egg-laying has been recorded as early as January 25th. Clutch sizes range from 1-2 eggs although 3 egg clutches are not uncommon. Following an approximate 40 day incubation period, the eggs hatch and the chicks are reared for 2-2.5 months after which they fledge in late May or early June. For a minimum of 3-4 weeks following fledging, the juveniles remain close to their natal territory as they continue to depend on their parents for food. During June and July, juveniles can often be observed in flight along with their parents in the vicinity of the nesting area. The juveniles continue to develop their independent foraging skills until September/October at which time the juveniles usually disperse to new areas that are not territorially claimed by breeding adults where they will spend their first winter. The breeding pairs remain together even outside of the breeding season.
Globally, Bonelli’s eagles are subject to a growing number of threats to their conservation status, the most serious of which are:
- direct persecution;
- habitat degradation, fragmentation and destruction;
- shortage of prey;
- electrocution from power lines; and
- human disturbances.
Bonelli’s eagles have experienced significant mortality as a result of direct persecution from shooting, trapping and poisoning the birds, especially by hunters, game wardens and shepherds who perceive the eagles as a threat to livestock and game animals, such as rabbits and partridges.
The eagles’ breeding habitats are particularly vulnerable to habit degradation, fragmentation and destruction as well as disturbances from human activities that are conducted close to the nesting sites, such as land clearance; logging (such as in Portugal where the eagles nest in trees); the construction of buildings, roads and utilitiy lines; forestry activities; quarries; mining; and recreational activities, including rock climbing, hiking, mountain biking, hunting and camping. The increasing development and operation of wind farms are also contributing to the eagles’ territorial disturbances as well as the premature mortality of the individuals which suffer collisions with the wind turbines.
In many areas, Bonelli’s eagle populations are also threatened from a shortage of prey, especially from habitat changes related to rural abandonment when natural woodlands take over and through the intensification of agriculture which corresponds to increasing pesticide use. Prey shortages also result from overexploitation of game (e.g. partridges, rabbits, pigeons) from hunting, which reduces the prey availability for the eagles. Diseases such as myxomatosis and haemorrhagic pneumonia that afflict the rabbit species Oryctolagus cuniculus, which is found in southern Europe and northwest Africa, have also contributed to food shortages for Bonelli’s eagles as well as for Spanish Imperial eagles and the Iberian lynx.
One of the most persistent causes of premature mortality in certain parts of Europe such as Spain is electrocution from power lines which frequently results from juvenile eagles endeavouring to use the lines as a perching site during their hunt for prey; however, mortality also results from the eagles’ collisions with power and telephone cables. Reports of Bonelli’s eagle deaths from electrocution in power lines from Spain in 2007 and France in 2009 documenting alarming 50% and 47% mortality rates, respectively, have raised awareness about the need to ensure the safety of power lines in the known eagle territories.
A European Union Species Action Plan for Bonelli’s Eagle adopted in 1999 by the European Commission Ornis Committee revealed extremely high Bonelli’s eagle juvenile mortality rates - 77% during the first 15 months of life or 90% from fledging to recruitment. These mortality rates underscore the importance of addressing the threats to the species which affect their conservation status by adversely impacting the eagles’ breeding and foraging behaviour as well as their sense of security within their territory that can lead to territorial abandonment. These threats can also lead to premature mortality, while collectively they inhibit the rehabilitation of the Bonelli’s eagle population, especially in areas of Europe that have suffered from Bonelli’s eagle population declines in recent decades.
The EU Andros LIFE+ Nature project is designed in part to mitigate the current threats to the breeding and non-breeding eagles in the Andros Island SPA by improving the eagles’ foraging, breeding and roosting opportunities as well as developing and launching the implementation of a SPA Management Plan in conjunction with stakeholder consultation in order to balance and satisfy the long term beneficial needs of the island’s society and threatened wildlife.
Based on the relative stability of the global population, the Bonelli’s eagle is listed as a species of Least Concern in the IUCN Red List of Threatened Species; however, due to the species’ recent severe population decline in Europe that first prompted serious concern during the last quarter of the 20th century, the Bonelli’s eagle has received legal protection under certain laws, treaties and conventions, including those set forth below.
- European Union Bird’s Directive (Council Directive 2009/147/EC), Annex I;
- Bern Convention on the Conservation of European Wildlife and Natural Habitats, Appendix III;
- Bonn Convention on Migratory Species, Appendix II;
- Convention on International Trade in Endangered Species (CITES), Appendix II;
- Endangered (SPEC 3) listing under Criteria C1 in the European IUCN Red List;
- African Convention on the Conservation of Nature and Natural Resources (June 1969) – Class B listing “all birds of prey and all owls not listed in Class A” - for total protection except with authorisation by competent authority; and
- Hellenic Joint Ministerial Decision 414985/85 and classification as Vulnerable in the Greek Red Data Book.
Contact: Constantine Alexander
copyright © 2012 Constantine Alexander