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.
White-tailed eagle
Mammals are usually a quite secondary component of the diet. Although usually better represented than other non-fish and bird prey, their regional importance is variable. In known dietary studies, the mammalian contribution can vary from 0.49% to 41% of prey by number. When mammals are most significant to the diet, it usually due to local white-tailed eagles preying on rabbits and hares with some regularity. In some studies from Scotland, the European rabbit ("Oryctolagus cuniculus") and mountain hare ("Lepus timidus") were found to contribute as much as nearly 25% of the diet. A similar contribution of European hares ("Lepus europaeus") was found in the diet from Austria, where hares were the leading prey species making up 24.35% of 349 prey items (with mammals making up 34.67% of the overall diet). In studies from Germany, European hares were taken fairly often but were numerically secondary prey. Given that these rabbits and hares have average mature weights from in rabbits to in European and Arctic hares ("Lepus arcticus"), they can make a hearty contribution to the prey biomass when available (although certainly juveniles are attacked as much if not more so than adult rabbits and hares). Other large mammalian prey can include foxes, with Arctic foxes ("Vulpes lagopus") being the 6th most regular prey species in Greenland and about a dozen red foxes ("Vulpes vulpes") recorded as taken or scavenged in Norway (predaceous attacks on adult red foxes have been reportedly witnessed). In both the offshore islands of Norway, where Arctic hare can be important prey, and Greenland, where the Arctic fox are prey, large numbers of both are taken when their pelage remains white but snow melts prematurely. A white-tailed eagle was once observed to kill and feed on a live adult golden jackal ("Canis auereus") that was trapped in a furbearer's trap. White-tailed eagles are known to prey on seal pups but most are likely sickly and perhaps both adult and pup seals are most likely to be eaten as carrion. Four Baikal seal ("Pusa sibirica") pups were taken in Lake Baikal. They are considered a predator even for live grey seal ("Halichoerus grypus") pups that weigh at birth. In one instance, a white-tailed eagle reportedly tried to attack an adult harbor seal ("Phoca vitulina") but was immediately dragged into the water and soon surfaced dying with a broken wing. Similarly, in one anecdote, an eagle drowned while apparently attacking an adult harbor porpoise ("Phocoena phocoena"). At least 7 species of mustelid have been recorded in the white-tailed eagle's diet, from the smallest weasel to, rarely, even adult European otters ("Lutra lutra"). Another mustelid, the American mink ("Neovison vison") was introduced as a furbearer to Finland but then became an invasive pest, as a fast-breeding killer that threatens many native species. In turn, Finnish white-tailed eagles have become the main natural control and may inhibit the mink from breeding via heavy predation. Conversely, the white-tailed eagle have not been known to prey on the critically endangered, native European mink ("Mustela lutreola") (perhaps due to its shier habits) which the American mink are known to have been outcompeting in some areas (but the European has mainly declined due to massive overhunting by humans as a furbearer). Kits of larger mammals such as Eurasian ("Castor fiber") and North American beavers ("Castor canadensis") (introduced) and of European badger ("Meles meles") have been found as prey.