Feeding
The Northern Hawk-Owl feeds on a variety of prey, which can include small rodents to mammals more robust in size, and a variety of birds, a typical diet for many boreal owls. In Eurasia the Northern Hawk-Owl is known to feed primarily on voles from the Microtus family. These voles usually follow a 3-4 year cycle of abundance. Therefore, S. u. ulula numbers are affected negatively when the numbers of voles are lowest. In North America the subspecies S. u. caparoch also feeds upon microtine voles, but its population is primarily based on the ten year cycling of the Snowshoe Hare (Lepus americanus). In Eurasia the Northern Hawk-Owl’s biomass consists of about 94% microtine voles, whereas in North America the percentage of biomass contributed by voles can be as low as 20%. Juvenile hares are considerably more important at 40-50%.
Other animals that are important prey items for the Northern Hawk-Owl include the red squirrel (taniasciurus handsonicus) which has been documented to contributing as much as 20% to the Hawk-Owl’s biomass. A long list of others include mice, rats, voles, lemmings, the short-tail weasel (Mustela erminea), partridge, Spruce Grouse (Falcipennis canadensis), doves, Pileated Woodpeckers (Dryocopus pileatus), sparrows, jays, robins, starlings, buntings, grackles, and finches. In the winter, feeding strategies change; where in summer the main source of food is mammals, in the winter a bigger portion of the Hawk-Owl’s biomass consists of ground dwelling birds, such as the ptarmigan and the grouse.
The Northern Hawk-Owl is a partially diurnal hunter, although it has been recorded hunting at varying times and does not appear to have a preferred hunting time. Whether the bird resides in Eurasia or North America, the strategy is usually similar. The Northern Hawk-Owl will perch and scour the immediate area for prey. As these owls are considered a search-oriented species they likely do not stay put for long if the site is not producing prey. The Hawk-Owl prefers open, forest-type environments when perching. These environments include sphagnum bogs and partially deforested areas. The preferred perching tree of the Northern Hawk-Owl is the spruce tree. When the Hawk-Owl attacks, it goes from a horizontal posture into a gliding dive. If the prey is further away, the bird will flap its wings a few times during the dive to increase distance. The Hawk Owl has exceptional hearing and can plunge into snow to capture rodents below the surface.
The type of prey the Hawk-Owl catches will determine its' eating strategy. For mammalian prey the ritual is generally the same: the Northern Hawk-Owl will eviscerate its' prey, eats the head first (especially for prey like the red squirrel, whose head is fairly large), and then—when tackling larger prey—it will eat the organs and cache the remains; with smaller prey, the owl will simply swallow the body whole.
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