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%.