The great horned owl is strictly carnivorous and eats almost every living woodland animal above ground except the largest mammals.
The great horned owl kills weaker owls from the barred owl (Strix varia) down, most of the hawks, and such nocturnal animals as weasels and minks. Game birds of all kinds, (grouse, turkeys, rails, quails, pheasants) wading birds, (geese, phalaropes, woodcocks, wild ducks,) poultry, (domesticated ducks (Anas platyrhynchos domesticus), chicken (Gallus gallus domesticus), guinea hens) a few small birds (woodpeckers), rabbits, (especially bush rabbits,) hares, squirrels, gophers, mice, rats, woodchucks, opossums, porcupines, skunks, snakes, fish, (dace, goldfish (Carassius auratus), bullheads, perch,) eels, crawfish, scorpions, frogs, and insects (crickets, beetles, grasshoppers, katydids,) are all eaten by this rapacious bird. It is particularly destructive to rats.
Ordinarily, when there is good hunting, this owl has a plentiful supply of food, and when there is game enough, it slaughters an abundance and eats only the brains; but in winter when house mice (Mus musculus) keep mostly within the buildings, when woodchucks and skunks have holed up, and when field mice are protected by deep snow, then if rabbits are scarce and starvation is imminent, the owl will attack even the domestic cat (Felis catus), and usually with success.