The Swallows, martins,
and saw-wings, are a family of passerine birds 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.
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, no scientific distinction between these two groups is made. In
the New World, "martin" is reserved for members of the genus Progne.
(These two systems are responsible for the sand martin being called bank swallow in the New World.)