Dietary biology of the Eurasian eagle-owl
In the Iberian peninsula, the Eurasian eagle-owl is part of a complex guild of predators that have evolved to survive mostly on European rabbits. Among this guild, golden eagles ("Aquila chrysaetos") were recorded per one study to rely on rabbits for 40% of the diet, the eagle-owl for 49% of the diet, the Spanish imperial eagle ("Aquila adalberti") for 50% of the diet, the Bonelli's eagle ("Aquila fasciata") for 61% of the diet and the Iberian lynx ("Lynx pardinus") for 79% of the diet. Elsewhere, the Spanish imperial eagle and eagle-owl are considered to outflank the golden and even the Bonelli's eagle as the most specialized avian predator of rabbits in the Iberian region. Other predators, such as common buzzard ("Buteo buteo"), northern goshawk ("Accipiter gentilis") black kite ("Milvus migrans") Iberian wildcat ("Felis silvestris tartessia"), red fox, stone marten ("Martes foina") and introduced Egyptian mongoose also prey heavily on rabbits in Spain, but are more generalized and less reliant than the above predators. All these powerful predators do not generally compete directly for food but conflicts may ensue over the rights to territories and nesting sites amongst the birds. Being nocturnal in activity, however, the Eurasian eagle-owl does not tend to compete as directly as do the three eagle species and even these are discreetly segregated by habitat preferences, the goldens preferring open, steep cliffs, the Bonelli's favoring densely vegetated areas mixed with rocky spots and the imperial favoring relatively flat open woodlands. Among these eagles, the golden eagle tends to have the most similar habitat preferences to the Eurasian eagle-owl and across a broad band of overlapping distribution, the two are considered nearly ecological equivalents by day and night. However, good-sized female golden eagles are up to twice as heavy as an average eagle-owl, so seem to have a considerable advantage in size and power. Conversely, to support their large size and large territory needs, the golden eagle cannot survive on small and diverse prey nearly as successfully as the eagle-owl. Among the Iberian peninsula rabbit-eating guild, the species with wider ranges, the eagle-owl and the golden and Bonelli's eagles, have had some degree of success living off of alternate prey following the devastation of the rabbit population due to rabbit haemorrhagic disease. The two rabbit predators endemic to Iberia, the Iberian lynx and Spanish imperial eagle, have been devastated by the reduction and other causes and both are threatened species today.
Dietary biology of the golden eagle
Other raptors, large and small, are often outright dominated by golden eagles. In Eurasia, the relative paucity of mid-sized "Buteo" species is thought to be due to the radiation of large "Aquila" eagles that already occupied the large raptor niche and outcompeted them. In North America, where there are no "Aquila" eagles other than the golden eagle, "Buteo" species are more diverse with at least 3 large hawks (the red-tailed, Swainson's ("Buteo swainsoni") and ferruginous hawks ("Buteo regalis")) that occur in similar habitats and sometimes eat similar prey as both each other and golden eagles. On occasion, golden eagles will kleptoparasitize or prey upon these three "Buteo"s. Beyond the aforementioned North American "Buteo", other small to medium-sized diurnal raptors are known to occasionally be hunted by golden eagles, including ospreys ("Pandion haliaetus"), black kite ("Milvus migrans"), hen harriers ("Circus cyaneus"), Eurasian sparrowhawks ("Accipiter nisus"), Cooper's hawks ("Accipter cooperii"), northern goshawks ("Accipiter gentilis"), red-shouldered hawks ("Buteo lineatus"), common buzzards ("Buteo buteo"), long-legged buzzards ("Buteo rufinus"), upland buzzards ("Buteo hemilasius"), rough-legged buzzards, Eurasian kestrels ("Falco tinnunculus"), American kestrels ("Falco sparverius"), merlins ("Falco columbarius"), prairie falcons and gyrfalcons ("Falco rusticolus"). Occasionally, golden eagles may even boldly predate these smaller species during migration in mid-flight, as was recorded at Hawk Mountain Sanctuary with a red-shouldered hawk. It is thought that on rare occasions, more nimble, smaller raptors like rough-legged buzzards and harriers can kleptoparatisize golden eagles. Northern goshawks in Scandinavia rely on grouse to about the same extent as golden eagles there and even hunt in similar forest edge habitats there. However, the goshawks primarily hunt neonatal and fledgling grouse (30% and 53% of their take of grouse) rather than adult grouse (17% of their prey) in late summer when grouse dominate other birds in their diet, whereas the eagles seem to mainly adult grouse and hunt them aseasonally. Similarly, hen harriers may become somewhat specialized at taking young red grouse, whereas the eagles are more likely to take adults.