|Eastern Yellow Robin (Eopsaltria australis)|
|Song of a Chipping Sparrow (Spizella passerina)|
Many, see text
A songbird is a bird belonging to the clade Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as scientific or vernacular name is Oscines, from Latin oscen, "a songbird". This group contains some 4,000 species 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, the other being the Tyranni which are most diverse in the Neotropics and absent from many parts of the world. These 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.
There is evidence to suggest that songbirds evolved 50 million years ago in the part of Gondwana that later became Australia, New Zealand, New Guinea and Antarctica, before spreading around the world.
The song in this clade is essentially territorial in that it communicates the identity and whereabouts of an individual to other birds and also signals sexual intentions. It is not to be confused with bird calls which 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.
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 which 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 which 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.
Taxonomy and systematics 
Under the Sibley-Ahlquist taxonomy this suborder is divided into two "parvorders", Corvida and Passerida (standard taxonomic practice would rank these as infraorders). The families of suborder Passeri are listed below as being in either Corvida or Passerida.
However, this treatment has since turned out to be very much in error. In reality, there are three major superfamilies (though not exactly corresponding to the Sibley-Ahlquist arrangement) of Passerida, in addition to some minor lineages thereof. The "Corvida" are fictional, an artefact of the phenetic methodology. Instead the bulk of these form the large superfamily Corvoidea and the smaller Meliphagoidea, and there are a number of small basal groups.