The 74 passerine families contain more than half the world's bird species. Most passerines are relatively small and there is a basic similarity of shape throughout the group. Identification may depend on minor details. Legs vary according to lifestyle, for instance whether the bird forages on the ground, or perches in foliage.
Bill shapes vary with the food eaten - most commonly insects, fruit, or seeds.
The shape of body, wings, and tail help too, and song can often be decisive in identifying a passerine species. Nearly half the bird species in the world, including all the larger ones, belong to the 99 non-passerine families. The non-passerines occupy a varied range of habitats, and include birds of freshwater and the sea, birds that are almost permanently airborne, flightless birds, and species dwelling mainly on the ground, and some that are adapted to perching in trees or bushes. Non-passerines often build very simple nests, or the eggs may be laid on bare ground or rock. The young are helpless in some species, but in others they are down-covered, active chicks that can run and feed themselves almost from hatching.