Dietary biology of the golden eagle
Owls, which may have dissimilar periods of activity, are not entirely exempt from golden eagle predation either. Several species have been recorded as prey, especially in Northern Europe, Slovakia (where owls make up 2.4% of prey remains) and some parts of the Western United States. Species hunted have ranged in size from tiny Eurasian pygmy owl ("Glaucidium passerinum") and little owl ("Athene noctua") to the formidable, large great horned, Eurasian eagle and great gray owls ("Strix nebulosa"). Barn owls ("Tyto alba") have been predated by golden eagles in both North America and Eurasia. On Santa Rosa and Santa Cruz Islands, barn owls made up 4.5% of the diet, the largest known occurrence of that species in the eagles' diet. However, the great horned owl is the owl species that occurs most regularly in the diet of North American golden eagles. In Europe, golden eagles are relatively minor predators of owls compared to Eurasian eagle-owls and northern goshawks. The primary owls hunted by European eagles are Ural owls ("Strix uralensis") and short-eared owls ("Asio flammeus"). Few eye-witness accounts are known of owl predation, but one violent mid-day confrontation between a golden eagle and an adult great horned owl was witnessed in Jefferson County, Colorado. Although this owl was about a third of the eagle's weight, it was able to land some damaging blows on the larger bird. In the end, despite its wounds, the eagle was victorious over the dead owl. A case of kleptoparasitism by a golden eagle on a great horned owl has been recorded. Although the Eurasian eagle-owl is preyed upon by golden eagles (4 confirmed cases of golden eagles killing eagle-owls in Europe), in one case a golden eagle (age unspecified) was found amongst the prey in an eagle-owl nest.
Eurasian eagle-owl
The male selects breeding sites and advertises their potential to the female by flying to them and kneading out a small depression (if soil is present) and making staccato notes and clucking noises. Several potential sites may be presented, with the female selecting one. Like all owls, Eurasian eagle-owls do not build nests or add material but nest on the surface or material already present. Eurasian eagle-owls normally nest on rocks or boulders, most often utilizing cliff ledges and steep slopes, as well as crevices, gullies, holes or caves. Rocky areas that also prove concealing woodlots as well as, for hunting purposes, that border river valleys and grassy scrubland may be especially attractive. If only low rubble is present, they will nest on the ground between rocks. Often, in more densely forested areas, they’ve been recorded nesting on the ground, often among roots of trees, under large bushes and under fallen tree trunks. Steep slopes with dense vegetation are preferred if nesting on the ground, although some ground nests are surprisingly exposed or in flat spots such as in open spots of the taiga, steppe, ledges of river banks and between wide tree trunks. All Eurasian eagle-owl nests in the largely forested Altai Krai region of Russia were found to be on the ground, usually at the base of pines. This species does not often use other bird’s nests as does the great horned owl, which often prefers nests built by other animals over any other nesting site. The Eurasian eagle-owl has been recorded in singular cases using nests built by common buzzards ("Buteo buteo"), golden eagle, greater spotted ("Clanga clanga") and white-tailed eagles ("Haliaeetus albicilla"), common ravens ("Corvus corax") and black storks ("Ciconia nigra"). Among the eagle-owls of the fairly heavily wooded wildlands of Belarus, they more commonly utilize nests built by other birds than most eagle-owls, i.e. stork or accipitrid nests, but a majority of nests are still located on the ground. This is contrary to the indication that ground nests are selected only if rocky areas or other bird nests are unavailable, as many will utilize ground nests even where large bird nests seem to be accessible. Tree holes being used for nesting sites are even more rarely recorded than nests constructed by other birds. While it may be assumed that the eagle-owl is too large to utilize tree hollows, when other large species like the great grey owl have never been recorded nesting in one, the even more robust Blakiston's fish owl nests exclusively in cavernous hollows. The Eurasian eagle-owl often uses the same nest site year after year.
In Engadin, Switzerland, the male eagle-owl hunts until the young are 4 to 5 weeks old and the female spends all her time brooding at the nest. After this point, the female gradually resumes hunting from both herself and the young and thus provides a greater range of food for the young. While it may seem contrary to the species’ highly territorial nature, there is one verified cases of polygamy in Germany, with a male apparently mating with two females, and cooperative brooding in Spain, with a third adult of undetermined sex helping a breeding pair care for the chicks. The response of Eurasian eagle-owls to humans approaching at the nest is quite variable. The species is often rather less aggressive than some other owls, including related species like the spot-bellied eagle-, great horned and snowy owls, many of the northern "Strix" species and even some rather smaller owl species, which often fearlessly attack any person found to be nearing their nests. Occasionally, if a person climbs to an active nest, the adult female eagle-owl will do a distraction display, in which they feign an injury. This is an uncommon behaviour in most owls and most often associated with small birds trying to falsely drawl the attention of potential predators away from their offspring. More commonly, the female flies off and abandons her nest temporarily, leaving the eggs or small nestlings exposed, when a human approaches it. Occasionally, if cornered both adults and nestlings will do an elaborate threat display, also rare in owls in general, in which the eagle-owls raise their wings into a semi-circle and puff up their feathers, followed by a snapping of their bills. Apparently eagle-owls of uncertain and probably exotic origin in Britain are likely to react aggressively to humans approaching the nest. Also, aggressive encounters involving eagle-owls around their nest, despite being historically rare, apparently have increased in recent decades in Scandinavia. The discrepancy of aggressiveness at the nest between the Eurasian eagle-owl and its Nearctic counterpart may be correlated to variation in the extent of nest predation that the species endured during the evolutionary process.