Taxonomy and Systematics
The swallows and martins are morphologically unique within the passerines, but the use of DNA-DNA hybridization studies has suggested relationships with the Old World warblers (a large wastebin taxon that has recently been split into several new families), the white-eyes and the tits. Under the Sibley-Ahlquist taxonomy they have been placed in the infraorder Passerida.
Within the family there is a clear division between the two subfamilies, the Pseudochelidoninae which is composed of the two species of river martins, and the Hirundininae, into which the remaining 81 species are placed. The division of the Hirundininae has been the source of much discussion, with various taxonomists variously splitting them into as many as 24 genera and lumping them into just 12. There is some agreement that there are three core groups within then Hirundininae, the saw-wings of the genus Psalidoprocne, the core martins and the swallows of the genus Hirundo and their allies.
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