Temporal range: Early Eocene to present
|Eastern yellow robin (Eopsaltria australis)|
|Song of a chipping sparrow (Spizella passerina)|
A songbird is a bird belonging to the suborder Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin oscen, "songbird". The Passeriformes contains 5,000 or so 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 (~4,000 species), the other being the Tyranni (~1,000 species), which are most diverse in the Neotropics and absent from many parts of the world. 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. 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.
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. 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.
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. However, 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.
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.
Sexual selection can be broken down into several different studies regarding different aspects of a bird's song. As a result, song can vary even within a single species. Many believe that song repertoire and cognition have a direct relationship. However, a study published in 2013 has shown that all cognitive ability may not be directly related to the song repertoire of a songbird. Specifically, spatial learning is said to have an inverse relationship with song repertoire. So for example, this would be an individual who does not migrate as far as others in the species, but has a better song repertoire. This suggests an evolutionary trade-off between possible alleles. With natural selection choosing traits best fit for reproductive success there could be a trade off in either direction depending on which trait would produce a higher fitness at that time period.
Song repertoire can be attributed to male songbirds as it is one of the main mechanisms of courtship. Song repertoires differ from male individual to male individual and species to species. Some species may typically have large repertoires while others may have significantly smaller ones. Mate choice in female songbirds is a significant realm of study as song abilities are continuously evolving. Males often sing to assert their dominance over other males in competition for a female, sometimes in lieu of a combative episode, and to arouse the female by announcing a readiness to mate. Though less frequent, females have also been known to sing and occasionally in duet with a mate as an affirmation of their partnership. While some will sing their song from a familiar perch, other species common to grasslands will sing a familiar song each time they fly.  Currently there have been numerous studies involving songbird repertoires, unfortunately, there has yet been concrete evidence to confirm that every songbird species prefers larger repertoires. A conclusion can be made that it can vary between species on whether a larger repertoire is connected to better fitness. With this conclusion, it can be inferred that evolution via natural selection, or sexual selection, favors the ability to retain larger repertoires for these certain species as it leads to higher reproductive success. During times of courtship, it is said that male songbirds increase their repertoire by mimicking other species songs. The better the mimicking ability, retaining ability, and the quantity of other species mimicked has been proven to have a positive relationship with mating success. Female preferences cause the constant improvement of accuracy and presentation of the copied songs. Another theory known as the "song-sharing hypothesis" suggests that females prefer simpler, more homogenous songs that signal a male of familiar territory. As birdsong can be broken into regional dialects through this process of mimicry, the foreign song of a newcomer suggests the lack of territorial possession. This can be costly in the wake of territorial conflicts between disparate songbird populations and may compel a female to prefer a male spouting a familiar song of the area. 
Sibley and Alquist divided songbirds into two "parvorders", Corvida and Passerida (standard taxonomic practice would rank these as infraorders), distributed in Australo-Papua and Eurasia respectively. Subsequent molecular studies, however, show this treatment to be somewhat erroneous. Passerida is a highly diverse lineage, uniting over one third of all bird species to include (in 2015) 3,885 species). These are divided into three major superfamilies (though not exactly corresponding to the Sibley-Ahlquist arrangement), in addition to some minor lineages.
In contrast, Sibley & Alquist's "Corvida" is a phylogenetic grade, and an artefact of the phenetic methodology. The bulk of the "Corvida" make up the large clade Corvides (812 species as of 2015), which is a sister group to the Passerida. The remaining 15 oscine families (343 species in 2015) form a series of basally branching sister groups to the Corvoid - Passerid clade. All of these groups, which form at least six successively branching basal clades, are found exclusively or predominantly in Australasia. Australian endemics are also prominent among basal lineages in both Corvoids and Passerids, suggesting that songbirds originated and diverged in Australia.
Scrub-birds and lyrebirds, of which there are just two species of each, represent the oldest lineage of songbirds on Earth. The rufous scrubbird, Atrichornis rufescens, is essentially confined to the Gondwana Rainforests of Australia World Heritage Area, occurring in both Queensland and New South Wales sections. It is now only found at elevations above 600 m (2,000 ft).