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 Eurasian eagle-owl
Most dietary studies place the average weight of Eurasian eagle-owl prey in the zone of , thus the species fits well within the rule of thumb that most prey weigh no more than 20% of the raptorial bird's own weight. It should be noted that the average prey weight in comparison to the eagle-owl's body mass is reportedly average and unremarkable for both owls and raptors in general, the average prey per one study was 7.2% of the eagle-owls’ body mass. The European raptorial birds most likely to attack prey of a relatively large size compared to themselves were the black kite ("Milvus migrans"), whose average prey was 35.7% of its own weight, and the Eurasian pygmy owl ("Glaucidium passerinum"), whose average prey was 31.7% of its own weight. On the other hand, when rabbit numbers were at their peak, the average prey mass in Spain may range up to at least 55% of the eagle-owls’ own body mass in that population. A raptorial prey comparison estimated that the mean prey weight for eagle-owls in Europe was as a mean among 5 main regions, the highest of any owl but similar or slightly lighter than the European average for northern goshawks ("Accipiter gentilis") and considerably lower only than two widespread European eagles. Per that study the average prey weight of the great horned owl in 4 North American regions was much lower (averaging only ). In Mediterranean scrub, the average mammalian prey of the Eurasian eagle-owl was nearly five times heavier than those of great horned owls from similar habitats in Chile, California and Colorado, where in combination the horned owl's mammalian food averaged . However, a comparison between eagle- and horned owls based upon six comparable biomes in the Americas and Eurasia found the great horned owl's prey averaged , improbably slightly higher than the eagle-owl at . If known Asian studies are mixed with European ones, the average prey weight for the Eurasian eagle-owl overall may also roughly be . Among sympatric owls in Europe, the mean prey weight of long-eared owls ("Asio otus") was found to be , the tawny owl ("Strix aluco") took prey weighing a mean of (though locally can be as high as ) and short-eared owls ("Asio flammeus") mean prey weigh was . A study in Finland seeking to distinguish the differences in prey taken by females and smaller male eagle-owls found that the two sexes prey averaged and , respectively. The species taken as food by the two sexes there were largely the same but the female simply often caught the larger prey individuals.