Swallow

The swallows, martins, and saw-wings, or Hirundinidae, are a family of passerine songbirds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The term "swallow" is used colloquially in Europe as a synonym for the barn swallow. Around 90 species of Hirundinidae are known, divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are nonmigratory. This family comprises two subfamilies: Pseudochelidoninae (the river martins of the genus Pseudochelidon) and Hirundininae (all other swallows, martins, and saw-wings). In the Old World, the name "martin" tends to be used for the squarer-tailed species, and the name "swallow" for the more fork-tailed species; however, this distinction does not represent a real evolutionary separation. In the New World, "martin" is reserved for members of the genus Progne. (These two systems are responsible for the same species being called sand martin in the Old World and bank swallow in the New World.) The family Hirundinidae was introduced (as Hirundia) by the French polymath Constantine Samuel Rafinesque in 1815. The Hirundinidae are morphologically unique within the passerines, with molecular evidence placing them as a distinctive lineage within the Sylvioidea (Old World warblers and relatives). Phylogenetic analysis has shown that the family Hirundinidae is sister to the cupwings in the family Pnoepygidae. The two families diverged in the early Miocene around 22 million years ago. Within the family, a clear division exists between the two subfamilies, the Pseudochelidoninae, which are composed of the two species of river martins, and the Hirundininae, into which the remaining species are placed.