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.
Dietary biology of the Eurasian eagle-owl
The Eurasian eagle-owl may hunt at least 300 species of bird. In Europe, the most common avian prey species are the normally feral or domestic rock pigeons ("Columba livia"), although the common wood pigeon ("Columba palumbus") can be locally even more significant. In the Netherlands, the common wood pigeon is the most important prey species, making up 37.3% by number and 38.3% by biomass. Yet another, smaller study of the Netherlands found the rock pigeon to be the primary prey species. In a dietary study from Luxembourg, the common wood pigeon was the most frequent prey species, making 19% of the remains by number. A study from Romania listed the rock pigeon was the second most frequently recorded prey behind only the Romanian hamster and the largest contributed of biomass. Unusually, in the Netherlands, birds were the primary food in both number (77.6%) and biomass (72%). Also, in Luxembourg, eagle-owls hunted birds in general were more regularly than mammals, as nearly 70% of prey remains there were avian. At least six other species of pigeon and dove have also been recorded as prey. However, perhaps the most significant contributor of prey at the family level is the corvids and all available species have been targeted. The carrion crow ("Corvus corone") is the second most widely recorded avian prey species in Europe and, locally, good numbers are also taken of hooded crow ("Corvus cornix"), western jackdaws ("Corvus monedula"), Eurasian jays ("Garrulus glandarius"), common ravens ("Corvus corax"), rooks ("Corvus corone"), two species each of magpie and chough and the spotted nutcracker ("Nucifraga caryocatactes"), in rough descending order of reportage in the eagle-owl's diet. In Baden-Württemberg, Germany, the carrion crow was the most frequently recorded bird among an exceptionally diverse recorded prey base including than 65 bird species there (although birds only comprised 27% of the prey numbers overall). Unlike smaller passerines, corvids roost in numbers in relatively open spots, which make them fairly vulnerable to a predator as stealthy and powerful as the eagle-owl. More than 17 species of gamebirds have been recorded in the Eurasian eagle-owl's diet but they are normally taken in small numbers. There are, however, exceptions. In one study conducted in lower Austria, the most frequently identified prey species was the grey partridge ("Perdix perdix"), which is the most widely reported gamebird prey for eagle-owls in Europe. One study of a “primeval” forested region of Finland found various grouse species to be the most significant contributor of biomass, namely in descending order the western capercaillie ("Tetrao urogallus"), the black grouse ("Tetrao tetrix") and the hazel grouse ("Bonasa bonasia"). In inland areas of Norway, the large forest grouse were also significant biomass contributors. The same forest grouse species were also found to be significant to diet along the Pechora River in Russia. Other gamebirds can be important secondary prey species in different parts of Europe: the red-legged partridge ("Alectoris rufa") in western Europe, the rock partridge ("Alectoris graeca") and the rock ptarmigan ("Lagopus mutus") in alpine highlands, the ring-necked pheasant ("Phasianus colchicus") in eastern Europe and the willow ptarmigan ("Lagopus lagopus") in the sub-Arctic zone. In Turkey, the most significant prey by number was the chukar ("Alectoris chukar"), comprising 20.5% of remains. A Korean study in two habitat types (forest and field) found that ring-necked pheasants were the main prey species, making up an average of 19.33% by number and 34.81% by biomass. Other birds of prey are perhaps second only to pigeons and corvids amongst avian contributors to the diet (their ecological relationships with eagle-owls are explored in more detail below).
Among coastal and some wetland areas, various water birds can come to contribute a large portion of both prey numbers and prey biomass. This may include more than 50 species of shorebird (from the one of the smallest sandpipers to the largest species of gull), more than 30 species of waterfowl, more than 10 species of herons, more than 8 species of rails and any grebes, cormorants or other water birds that are available. The most regularly reported water bird prey in Europe were, roughly in this order, the common moorhens ("Gallinula chloropus"), the Eurasian coot ("Fulica atra"), mallards ("Anas platyrhynchos"), black-headed gull ("Chroicocephalus ridibundus"), and the Eurasian teal ("Anas crecca"). The diet of a handful of eagle-owl pairs in the Riau Islands Province of France were found to be dominated by water birds, especially the yellow-legged gull ("Larus michahellis"), the colonial abundance of which allowed the eagle-owls to atypically occupy these small islands. The diet of eagle-owls in Norway was dominated in coastal areas by water birds, overall for the nation 53% of the food was made of birds, the species most commonly identified as caught being the mew gull ("Larus canus"), common puffin ("Fratercula arctica") and common eider ("Somateria mollissima"). Despite access to large seabird breeding colonies, almost all large bird species hunted in Norway, including large forest grouse in more inland areas, were apparently fully-grown adults and most water birds were caught while resting on open coastal waters. Eagle-owls in northwestern Poland, an area heavily dotted with lakes, relied on birds for about 64% of the diet, more than half of which were water birds. The main prey species there was the Eurasian coot, at nearly 15% of the prey numbers. In Primorsky Krai in Russia, 53.2% of the food for the eagle-owls were made up of birds, predominantly water birds with the primary prey species being the crested auklet ("Aethia cristatella" ) (26.9%). In the Russian Far East, similarly, birds occupy up to 57.6% of the diet, a lion's share of which are water birds. In this study, the grey red-backed vole and reed vole ("Microtus fortis") were the most numerous identified individual species but in some years the extremely large Japanese cormorant ("Phalacrocorax capillatus"), at , were more numerous as prey than either vole. Water birds were found to be even more important in the diet in Korea, as in wetland habitat, with birds in general comprising 68.9% by number and 85.3% by biomass there, but in adjacent upland areas birds were slightly secondary to mammals, which made up 38.7% by number and 64.7% by biomass, led by the brown rat. In Korea, the mallard and the similar spot-billed duck ("Anas poecilorhyncha") made up 38.1% of the biomass. Other recorded bird species in the Eurasian eagle-owl's diet includes all type of birds found in their range, including bustards, sandgrouse, parrots, cuckoos, swifts, the hoopoe ("Upupa epops"), the European bee-eater ("Merops apiaster"), the common kingfisher ("Alcedo atthis"), the European roller ("Coracias garrulus"), at least seven species of woodpecker (from the smallest to the largest European species) and more than 80 species of passerine. Among passerines, the only family reported widely as prey besides corvids are thrushes. In the Italian Alps, "Turdus" species were the fourth most frequently recorded type of prey. In particular, the abundant Eurasian blackbird ("Turdus merula") is widely reported to be taken by eagle-owls. As thrushes are relatively small, at among the species known to be predated, pairs in the Italian Alps who depended more so on thrushes usually showed lower productivity during nesting attempts.