Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs are the bird sounds that are melodious to the human ear. In ornithology and birding, (relatively complex) songs are distinguished by function from (relatively simple) calls.
The distinction between songs and calls is based upon complexity, length, and context. Songs are longer and more complex and are associated with courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact. Other authorities such as Howell and Webb (1995) make the distinction based on function, so that short vocalizations, such as those of pigeons, and even non-vocal sounds, such as the drumming of woodpeckers and the "winnowing" of snipes' wings in display flight, are considered songs. Still others require song to have syllabic diversity and temporal regularity akin to the repetitive and transformative patterns that define music. It is generally agreed upon in birding and ornithology which sounds are songs and which are calls, and a good field guide will differentiate between the two.
Bird song is best developed in the order Passeriformes. Some groups are nearly voiceless, producing only percussive and rhythmic sounds, such as the storks, which clatter their bills. In some manakins (Pipridae), the males have evolved several mechanisms for mechanical sound production, including mechanisms for stridulation not unlike those found in some insects.
Song is usually delivered from prominent perches, although some species may sing when flying. The production of sounds by mechanical means as opposed to the use of the syrinx has been termed variously instrumental music by Charles Darwin, mechanical sounds and more recently sonation. The term sonate has been defined as the act of producing non-vocal sounds that are intentionally modulated communicative signals, produced using non-syringeal structures such as the bill, wings, tail, feet and body feathers.
In extratropical Eurasia and the Americas almost all song is produced by male birds; however in the tropics and to a greater extent the desert belts of Australia and Africa it is more typical for females to sing as much as males. These differences have been known for a long time and are generally attributed to the much less regular and seasonal climate of Australian and African arid zones requiring that birds breed at any time when conditions are favourable, although they cannot breed in many years because food supply never increases above a minimum level. With aseasonal irregular breeding, both sexes must be brought into breeding condition and vocalisation, especially duetting, serves this purpose. The high frequency of female vocalisations in the tropics, Australia and Southern Africa may also relate to very low mortality rates producing much stronger pair-bonding and territoriality.