Song birds are any birds that sing musically, almost all of which are in the suborder Oscines of the order Passeriformes, or perching birds. Passeriform birds have feet adapted for gripping branches, plant stems, and similar perches, and they comprise about one-third of living bird families, and one-half of the species.
A major function of singing in birds is to proclaim the location and limits of a breeding territory, that is, an area of habitat that is defended against other birds of the same species, and sometimes against other species as well. Usually, only the male of the species sings. By singing loudly and in a manner that is specific to the species, individual song birds advertise their presence, and their ownership of the local habitat. Usually, a vigorous song by a resident bird is sufficient to deter would-be competitors, but sometimes it is not. In such cases, the conflict can intensify into visual displays at close range, and sometimes into out-and-out fighting, until a winner emerges.
The frequency and intensity of the songs is usually greatest at the beginning of the breeding season, while territories are actively being established. Once a territory is well ensconced, the frequency and loudness of the song often decrease, because all that is required at that stage is occasional reminders to neighbors that the territory remains occupied by the same individual. However, another important function of singing is to attract a mate, and if a territorial male bird has not been successful in achieving this goal, he will continue to sing often and loudly well into the breeding season, until a female finds and chooses him, or he gives up.
With a bit of effort and concentration, it is not too difficult to learn to identify bird species on the basis of their song. In fact, this is the basis of the most common method by which song birds are censused in forests, where it can be very difficult to see these small, cryptic animals because of dense foliage. During surveys of song birds, observations are made on different dates of the places at which singing by various species occurs in the forest. Clusters of observations of singing by a particular species at about the same place but on different dates are ascribed to a particular male bird, and are used to designate his territory.
There are great differences in the songs of various bird species, which can range from the faint, high-pitched twittering of the cedar waxwing (Bombycilla cedrorum), to the loud and raucous jays of the blue jay (Cyanocitta cristata); the enormously varied and twice-repeated phrases of the brown thrasher (Toxostoma rufum); the whistled deea-deee of the black-capped chickadee (Parus atricapillus); the aerial twinklings of the horned lark (Eremophila alpestris); the witchety wichy of the common yellowthroat (Geothlypis trichas), a species of warbler; and the euphonious flutings and trills of the wood thrush (Hylocichla mustelina).