A songbird is a bird belonging to the clade Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin oscen, "a songbird". The Passeriformes contains 5000 or so species[1][2] found all over the world, in which the vocal organ typically is developed in such a way as to produce a diverse and elaborate bird song.
Songbirds form one of the two major lineages of extant perching birds (~4000 species), the other being the Tyranni (~1000 species), which are most diverse in the Neotropics and absent from many parts of the world.[2] The Tyranni have a simpler syrinx musculature, and while their vocalizations are often just as complex and striking as those of songbirds, they are altogether more mechanical sounding. There is a third perching bird lineage, the Acanthisitti from New Zealand, of which only two species remain alive today.[3]Some evidence suggests that songbirds evolved 50 million years ago in the part of Gondwana that later became India, Sri Lanka, Australia, New Zealand, New Guinea and Antarctica, before spreading around the world.[4][5]
The song in this clade is essentially territorial, because it communicates the identity and whereabouts of an individual to other birds, and also signals sexual intentions. Sexual selection among songbirds is highly based on mimetic vocalization. Female preference has shown in some populations to be based on the extent of a male's song repertoire. The larger a male's repertoire, the more females a male individual attracts.[6] It is not to be confused with bird calls that are used for alarms and contact and are especially important in birds that feed or migrate in flocks. While almost all living birds give calls of some sort, well-developed songs are only given by a few lineages outside the songbirds. And still, not all songbirds proffer a call that is distinctly melodious. Songbirds do, however, possess a highly developed vocal organ, the syrinx, that enables their sonorous activity. This organ, also known as a song box, can be found where the windpipe meets diverging bronchial tubes which lead to the lungs. The organ is a solid, bony structure lined with a film of membranes which air passes through as the songbird calls. While the song boxes of songbirds vary in size and intricacy, this does not necessarily determine the songbird's ability to voice their song. Researchers believe this has more to do with the length of the windpipe.[7]
Other birds (especially non-passeriforms) sometimes have songs to attract mates or hold territory, but these are usually simple and repetitive, lacking the variety of many oscine songs. The monotonous repetition of the common cuckoo or little crake can be contrasted with the variety of a nightingale or marsh warbler. On the other hand, although many songbirds have songs that are pleasant to the human ear, this is not invariably the case. Many members of the crow family (Corvidae) communicate with croaks or screeches, which sound harsh to humans. Even these, however, have a song of sorts, a softer twitter that is given between courting partners. And even though some parrots (which are not songbirds) can be taught to repeat human speech, vocal mimicry among birds is almost completely restricted to songbirds, some of which (such as the lyrebirds or the aptly-named mockingbirds) excel in imitating the sounds of other birds or even environmental noises.[8]
The birds from higher altitudes have evolved thicker downs (also known as jackets) to protect themselves from the changes in climate. Their feathers have outer and inner portions, with the lower down being fluffier and warmer to provide increased warmth.[9]