Dietary biology of the Eurasian eagle-owl
Equally or even more so than the great horned owl, the Eurasian eagle-owl is a threat to any smaller type of raptor it encounters, whether other owls or diurnal birds of prey. All told, more than 20 species of accipitrid, 15 species of owl and 9 species of falcon have been found amongst their prey. The Eurasian eagle-owl is the primary predator of other birds of prey throughout Eurasia, not even the prolific raptor-killing northern goshawk ("Accipiter gentilis") equals the sheer number of raptors taken. Up to 6% of the overall food by number and 36% by prey biomass for Eurasian eagle-owls can be comprised by other raptorial birds. The primary raptorial prey taken by Eurasian eagle-owls in Europe are two species: the long-eared owl and the common buzzard. Both are taken in such large numbers that they rank in the top five most regularly hunted bird species in Europe. In Luxembourg, the long-eared owl and common buzzard were the fourth and fifth most regularly hunted prey species. Other high rankings have included the long-eared owl being the fourth most regular prey species in Bavaria and the buzzard being the third most regular prey species in a study from the Czech Republic. While migrating, long-eared owls appear to select areas to move through partially based on whether or not eagle-owls are detected, thus eagle-owls have a very serious influence on the behaviour of this prey. Despite the large numbers taken of both of these, the more scarce eagle-owl does not seem to have a serious deleterious effect on their overall population, especially compared to anthropogenic factors.
Other than these two species, a large share of the raptorial prey for eagle-owls is made up of other owls. Given that all European owls are to some extent nocturnal, they may be encountered and killed upon detection by the Eurasian eagle-owl. In Europe, it has killed every other species of owl, from the tiny pygmy owl and scops owl to the large great grey owl and the snowy owl. The Eurasian eagle-owl is the only raptor that has been known to prey on snowy owl on multiple occasions. However, the threat Eurasian eagle-owls poses to other raptors can be exaggerated. Occasionally, with adequate mammal prey populations, they can nest as close as a few dozen meters of other raptors and never harass them. When nesting in the same groves as the long-legged buzzards ("Buteo rufinus") in southeastern Bulgaria, Eurasian eagle-owls did not predate the buzzards at any point. Long-legged buzzards have been killed however in Kazakhstan. Even in those occasional cases where they pick off one or several raptors, they do not effect the overall population, unless the raptorial prey is already heavily diminished due to some other (often human-based) cause. For example, despite both being a known predator of both, they did not significantly depress numbers of either northern goshawks or tawny owls in southern Finland. One study showed that many raptors engage in spatial avoidance during potential encounters with Eurasian eagle-owls and while the predation by eagle-owls did decrease raptor population densities, they seldom caused declines in breeding success or altered habitat occupancy.
Northern goshawk
Outside of the accipitrid group, heavy predation on different varieties of raptorial birds by northern goshawks can continue unabated. Many types of owl are taken and in Europe, the northern goshawk is the second most prolific predator of owls behind the Eurasian eagle owl ("Bubo bubo"). In Bavaria, Germany, the long-eared owl ("Asio otus") was the second most common prey species for nesting goshawks. In the Białowieża Forest of Poland, fairly high numbers of the tawny owl ("Strix alucco") were taken. In all, some 18 species of owl have been recorded in the diet, ranging in size from the Eurasian ("Glaucidium passerinum") and northern pygmy owls ("Glaucidium gnoma") at and , respectively, to all the large northern "Strix" owls including adults and even the great horned owl. Whether adults have ever been killed as prey though is unknown but goshawks have been known to kill great horned owls that they've found near their nests. In addition, about eight species of falcon have been identified in the foods of goshawks. Adult falcons of small species such as kestrels and merlins ("Falco columbarius") can be overpowered quite easily if they can manage to surprise the prey. Larger falcons have turned up in the diet as well, including the prairie falcon ("Falco mexicanus") and the saker falcon ("Falco cherrug"), although possible only nestlings of these species. Brief aerial skirmishes between goshawks and peregrine falcons ("Falco peregrinus") have been described but neither species is known to have killed one another in the wild. In Schleswig-Holstein, Germany, at least four small passerines species were recorded as nesting close to active goshawk nest, due to the incidental shelter that the fierce goshawks inadvertently provides from smaller raptors which are their main predators. Such raptors, including Eurasian kestrels, Eurasian sparrowhawks and long-eared owls, not only avoid goshawk activity where possible but also were found to have lower nest productivity any time they nested relatively close to goshawks per the study. A similar phenomenon, with goshawks inadvertently providing shelter to small passerines, has been recorded in North America as well.
Competition for northern goshawks can also come from mammalian carnivores. Martens, and to a lesser extent other weasels, are presumably one of their more major competitors as their diet often consists of similar prey primarily during spring and summer, tree squirrels and woodland birds, but little has been studied in terms of how the two types of predator effect each other. Most recorded interactions have been predatory, as the goshawk has been recorded preying on a dozen species, from the least weasel ("Mustela nivalis") to the stone marten ("Martes foina"). Northern goshawks have also been recorded as feeding on much bigger predators such as the red fox ("Vulpes vulpes"), the raccoon dog ("Nyctereutes procyonoides") and the striped skunk ("Mephitis mephitis"), but it is not clear whether these were actual kills, as many may be encountered as already dead carrion. Domestic carnivores are sometimes eaten, including dogs and cats, the latter of which has reportedly been taken alive by goshawks. The red fox is a surprisingly considerable competitor for resources with northern goshawks. It was found in Norway that goshawk numbers were higher when voles were at peak numbers, not due to voles as a food source but because foxes were more likely to eat the rodents and ignore grouse, whereas during low vole numbers the foxes are more likely to compete with goshawks over grouse as prey. A decrease of the fox population of Norway due to sarcoptic mange was found result in an increase of grouse numbers and, in turn, northern goshawks. In some areas, red foxes have been found to steal up to half of the goshawks’ kills.