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,[1] divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters.[2] 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.[3] 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.)
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Within the family, a clear division exists between the two subfamilies, the Pseudochelidoninae, which are composed of the two species of river martins,[8][9] and the Hirundininae, into which the remaining 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. 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.[10] The saw-wings are the most basal of the three, with the other two clades being sister to each other. 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.[11]
Swallows have two foveae in each eye, giving them sharp lateral and frontal vision to help track prey. They also have relatively long eyes, with their length almost equaling their width. The long eyes allow for an increase in visual acuity without competing with the brain for space inside of the head. The morphology of the eye in swallows is similar to that of a raptor.[13]
Their legs are short, and their feet are adapted for perching rather than walking, as the front toes are partially joined at the base. Swallows are capable of walking and even running, but they do so with a shuffling, waddling gait.[18] The leg muscles of the river martins (Pseudochelidon) are stronger and more robust than those of other swallows.[10][18] The river martins have other characteristics that separate them from the other swallows. The structure of the syrinx is substantially different between the two subfamilies;[8] and in most swallows, the bill, legs, and feet are dark brown or black, but in the river martins, the bill is orange-red and the legs and feet are pink.[10]
The family has a worldwide cosmopolitan distribution, breeding on every continent except Antarctica. One species, the Pacific swallow, occurs as a breeding bird on a number of oceanic islands in the Pacific Ocean,[21] the Mascarene martin breeds on Reunion and Mauritius in the Indian Ocean,[22] and a number of migratory species are common vagrants to other isolated islands and even to some sub-Antarctic islands and Antarctica.[23] Many species have enormous worldwide ranges, particularly the barn swallow, which breeds over most of the Northern Hemisphere and winters over most of the Southern Hemisphere.
The family uses a wide range of habitats. They are dependent on flying insects, and as these are common over waterways and lakes, they frequently feed over these, but they can be found in any open habitat, including grasslands, open woodland, savanna, marshes, mangroves, and scrubland, from sea level to high alpine areas.[10] Many species inhabit human-altered landscapes, including agricultural land and even urban areas. Land-use changes have also caused some species to expand their range, most impressively the welcome swallow, which began to colonise New Zealand in the 1920s, started breeding in the 1950s, and is now a common landbird there.[24]
Species breeding in temperate regions migrate during the winter when their insect prey populations collapse. Species breeding in more tropical areas are often more sedentary, although several tropical species are partial migrants or make shorter migrations. In antiquity, swallows were thought to have hibernated in a state of torpor, or even that they withdrew for the winter under water. Aristotle ascribed hibernation not only to swallows, but also to storks and kites. Hibernation of swallows was considered a possibility even by as acute an observer as Rev. Gilbert White, in his The Natural History and Antiquities of Selborne (1789, based on decades of observations).[25] This idea may have been supported by the habit of some species to roost in some numbers in dovecotes, nests and other forms of shelter during harsh weather, and some species even entering torpor.[10] There were several reports of suspected torpor in swallows from 1947,[26] such as a 1970 report that white-backed swallows in Australia may conserve energy this way,[27] but the first confirmed study that they or any passerine entered torpor was a 1988 study on house martins.[28]
Swallows are excellent flyers and use these skills to feed and attract mates. Some species, such as the mangrove swallow, are territorial, whereas others are not and simply defend their nesting sites. In general, the male selects a nest site, and then attracts a female using song and flight and (dependent on the species) guards his territory. The size of the territory varies depending on the species of swallow; in colonial-nesting species, it tends to be small, but it may be much larger for solitary nesters. Outside the breeding season, some species may form large flocks, and species may also roost communally. This is thought to provide protection from predators, such as sparrowhawks and hobbies.[10] These roosts can be enormous; one winter-roosting site of barn swallows in Nigeria attracted 1.5 million individuals.[29] Nonsocial species do not form flocks, but recently fledged chicks may remain with their parents for a while after the breeding season. If a human being gets too close to their territory, swallows attack them within the perimeter of the nest. Colonial species may mob predators and humans that are too close to the colony.[30]
For the most part, swallows are insectivorous, taking flying insects on the wing.[10] Across the whole family, a wide range of insects is taken from most insect groups, but the composition of any one prey type in the diet varies by species and with the time of year. Individual species may be selective; they do not scoop up every insect around them, but instead select larger prey items than would be expected by random sampling.[31] In addition, the ease of capture of different insect types affects their rate of predation by swallows.[32] They also avoid certain prey types; in particular, stinging insects such as bees and wasps are generally avoided. In addition to insect prey, a number of species occasionally consume fruits and other plant matter. Species in Africa have been recorded eating the seeds of Acacia trees, and these are even fed to the young of the greater striped swallow.[10][33]
The swallows generally forage for prey on the wing, but they on occasion snap prey off branches or on the ground. The flight may be fast and involve a rapid succession of turns and banks when actively chasing fast-moving prey; less agile prey may be caught with a slower, more leisurely flight that includes flying in circles and bursts of flapping mixed with gliding. Where several species of swallows feed together, they separate into different niches based on height off the ground, some species feeding closer to the ground and others feeding at higher levels.[34] Similar separation occurs where feeding overlaps with swifts. Niche separation may also occur with the size of prey chosen.[34]
The more primitive species nest in existing cavities, for example in an old woodpecker nest, while other species excavate burrows in soft substrate such as sand banks.[10] Swallows in the genera Hirundo, Ptyonoprogne, Cecropis, Petrochelidon, and Delichon build mud nests close to overhead shelter in locations that are protected from both the weather and predators. The mud-nesters are most common in the Old World, particularly Africa, whereas cavity-nesters are more common in the New World. Mud-nesting species in particular are limited in areas of high humidity, which causes the mud nests to crumble. Many cave-, bank-, and cliff-dwelling species of swallows nest in large colonies. Mud nests are constructed by both males and females, and amongst the tunnel diggers, the excavation duties are shared, as well. In historical times, the introduction of man-made stone structures such as barns and bridges, together with forest clearance, has led to an abundance of colony sites around the globe, significantly increasing the breeding ranges of some species. Birds living in large colonies typically have to contend with both ectoparasites and conspecific nest parasitism.[35][36] In barn swallows, old mated males and young unmated males benefit from colonial behaviour, whereas females and mated young males likely benefit more from nesting by themselves.[37] be457b7860
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