The smallest species by weight may be the Fanti sawwing, at a mean body mass of 9.4 g (0.33 oz) while the purple martin and southern martin, which both weigh in excess of 50 g (1.8 oz) on average, rival one another as the heaviest swallows. Like the unrelated swifts and nightjars, which hunt in a similar way, they have short bills, but strong jaws and a wide gape. The morphology of the eye in swallows is similar to that of a raptor. The long eyes allow for an increase in visual acuity without competing with the brain for space inside of the head. They also have relatively long eyes, with their length almost equaling their width. Swallows have two foveae in each eye, giving them sharp lateral and frontal vision to help track prey. The bill of the sand martin is typical for the family, being short and wide. Their body shapes allow for very efficient flight the metabolic rate of swallows in flight is 49–72% lower than equivalent passerines of the same size. Swallows have adapted to hunting insects on the wing by developing a slender, streamlined body and long, pointed wings, which allow great maneuverability and endurance, as well as frequent periods of gliding. The Hirundinidae have an evolutionarily conservative body shape, which is similar across the clade, but is unlike that of other passerines. The phylogeny of the swallows is closely related to evolution of nest construction the more basal saw-wings use burrows as nest, the core martins have both burrowing (in the Old World members) and cavity adoption (in New World members) as strategies, and the genus Hirundo and its allies use mud nests. The saw-wings are the most basal of the three, with the other two clades being sister to each other. Some agreement exists that three core groups occur within the Hirundininae, the saw-wings of the genus Psalidoprocne, the core martins, and the swallows of the genus Hirundo and their allies. 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. 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. The two families diverged in the early Miocene around 22 million years ago. Phylogenetic analysis has shown that the family Hirundinidae is sister to the cupwings in the family Pnoepygidae. 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). The family Hirundinidae was introduced (as Hirundia) by the French polymath Constantine Samuel Rafinesque in 1815. (These two systems are responsible for the same species being called sand martin in the Old World and bank swallow in the New World.) In the New World, "martin" is reserved for members of the genus Progne. 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. This family comprises two subfamilies: Pseudochelidoninae (the river martins of the genus Pseudochelidon) and Hirundininae (all other swallows, martins, and saw-wings). A number of European and North American species are long-distance migrants by contrast, the West and South African swallows are nonmigratory. They also occur on a number of oceanic islands. 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. The term "swallow" is used colloquially in Europe as a synonym for the barn swallow. Highly adapted to aerial feeding, they have a distinctive appearance. 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. For other uses, see Swallow (disambiguation). This article is about the family of birds.
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