Other animals that are important prey items for the northern hawk-owl include the red squirrel ("Sciurus vulgaris") which has been documented to contribute 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.