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.
Verreaux's eagle-owl
So far as is known, Verreaux's eagle-owl is the only living owl who predates multiple species of primate, although isolated incidents of predation (normally on young primates) has been reported in two to three other large, tropical owls. Multiple cases of predation against galagos have been reported, unsurprisingly as they represent all nocturnal primates in Africa, although they are seldom identified to species. Known galago prey species have ranged from the Thomas's bushbaby ("Galagoides thomasi") to the brown greater galago ("Otolemur crassicaudatus"). Monkeys are also predated opportunistically. Particularly often reported in foods of the Verreaux's eagle-owl as primates go is the vervet monkey ("Chlorocebus pygerythrus"). Incidents of successful predation have included vervets that were half-grown, which the eagle-owl was able to fly off with (despite being about as heavy as the eagle-owl itself), and an adult vervet of an estimated weight of , which an eagle-owl took on the ground and subsequently dismembered. However, considering the formidable gauntlet of predators that vervet monkeys face, the Verreaux's eagle-owl is one of its more minor predators and attacks on them may be considered incidental, due in part to the monkey's primarily diurnal activities. Other monkey species believed to be occasionally vulnerable to attacks include the blue monkey ("Cercopithecus mitis"), which is similar in size to the vervet, patas monkeys ("Erythrocebus patas") and the young of the chacma baboon ("Papio ursinus"). Adult patas monkeys, averaging some , can be even larger than vervet monkeys but whether they take prime adults of the species is questionable. There are a few verified cases of Verreaux's eagle-owls feeding on ungulates, however some authors such as Avery, et al. (1985) feel that these generally represent cases of scavenging on carrion. The remains of an adult grysbok ("Raphicerus melanotis"), weighing an estimated , was opined with certainty to have been taken as carrion per this study. However, Steyn (1982) accepted that this species could take live prey weighing up to on rare occasions, however he stated in a case of an adult common duiker ("Sylvicapra grimmia") being fed on by an eagle-owl that the duiker was likely roadkill. Scavenging on carrion is generally a rare behaviour in owls and has been reported in only a few cases where large owls are exceptionally hungry. Live ungulates verified to have been hunted have included piglets of common warthogs ("Phacochoerus africanus"), which have an average birth weight of only but grow to over in just a couple weeks. Adult Kirk's dik-diks ("Madoqua kirkii"), one of the smallest antelope species at an average of have also been hunted by Verreaux's eagle-owl.