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.
Eurasian eagle-owl
The first egg hatches after 31 to 36 days of incubation. The eggs hatch successively; although the average interval between egg-laying is three days, the young tend to hatch no more than a day or two apart. Like all owls that nest in the open, the downy young are often a mottled grey with some white and buff, which provides camouflage. They open their eyes at 4 days of age. The chicks grow rapidly, being able to consume small prey whole after roughly three weeks. In Andalusia, it was found that the most noticeable development of the young before they leave the nest was the increase of body size, which was the highest growth rate of any studied owl and faster than either snowy or great horned owls. Body mass increased fourteen times over from 5 days old to 60 days old in this study. The male continues to bring prey, leaving in on or around the nest, and the female feeds the nestlings, tearing up the food into suitably-sized pieces. The female resumes hunting after about three weeks which increases the food supply to the chicks. Siblicide has been recorded widely in Eurasian eagle-owls and, according to some authorities, is almost a rule in the species. Many nesting attempts produce two fledglings, indicating that siblicide is not as common as in other birds of prey, especially some eagles. It has been theorized in Spain that males are likely to be the first egg laid to reduce the likelihood of sibling aggression due to the size difference, thus the younger female hatchling is less likely to be killed since it is similar in size to its older sibling.