Passerines are perching birds with four toes, one of which always points backwards and allows individuals to firmly grasp slender perches. More than half of all the birds in the world are passerines; there are nearly six thousand species worldwide, almost 400 in North America. Most families of passerines are represented in Yellowstone and include species of flycatchers, ravens and jays, swallows, nuthatches, wrens, warblers, thrushes, waxwings, tanagers, finches, blackbirds and sparrows. Their feeding strategies include insectivores, fruit eaters, and scavengers. Plumage ranges from bright multi-colored individuals like warblers and tanagers to solid black like the raven. They will nest on the ground like the vesper sparrow, and in trees and bushes at various heights, the most common nesting choice of most passerines, in the spray at the bases of waterfalls like the dipper, and in globular structures made from pellets of mud plastered to cliffs like the swallow.