There are over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers adapted for silent flight. Exceptions include the diurnal northern hawk-owl and the gregarious burrowing owl. Owls hunt mostly small mammals, insects, and other birds, although a few species specialize in hunting fish. They are found in all regions of the Earth except the polar ice caps and some remote islands. Owls are divided into two families: the true (or typical) owl family, Strigidae and the barn-owl family, Tytonidae. A group of owls is called a parliament.