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 as the common name for Hirundo rustica in the United Kingdom and Ireland. Around 90 species of Hirundinidae are known, divided into 21 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 and Hirundininae. 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.
Taxonomy and systematics
The family Hirundinidae was introduced 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. 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. 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. 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 and cavity adoption as strategies, and the genus Hirundo and its allies use mud nests.
The genus level cladogram shown below is based on a molecular phylogenetic study by Drew Schield and collaborators that was published in 2024. The choice of genera and the number of species is taken from the list of birds maintained by Frank Gill, Pamela C. Rasmussen and David Donsker on behalf of the International Ornithological Committee.
Fossil record
The oldest known fossil swallow is Miochelidon eschata from the Early Miocene of Siberia; it is the only record of Hirundinidae from the Miocene. It is likely a basal member of the family.Description
The Hirundinidae have an evolutionarily conservative body shape, which is similar across the clade, but is unlike that of other passerines. 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. 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 two 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.
Like the unrelated swifts and nightjars, which hunt in a similar way, they have short bills, but strong jaws and a wide gape. Their body lengths range from about and their weight from about. The smallest species by weight may be the Fanti sawwing, at a mean body mass of while the purple martin and southern martin, which both weigh in excess of on average, rival one another as the heaviest swallows. The wings are long, pointed, and have nine primary feathers. The tail has 12 feathers and may be deeply forked, somewhat indented, or square-ended. A long tail increases maneuverability, and may also function as a sexual adornment, since the tail is frequently longer in males. In barn swallows, the tail of the male is 18% longer than those of the female, and females select mates on the basis of tail length.
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. The leg muscles of the river martins are stronger and more robust than those of other swallows. 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; 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.
The most common hirundine plumage is glossy dark blue or green above and plain or streaked underparts, often white or rufous. Species that burrow or live in dry or mountainous areas are often matte brown above. The sexes show limited or no sexual dimorphism, with longer outer tail feathers in the adult male probably being the most common distinction.
The chicks hatch naked and with closed eyes. Fledged juveniles usually appear as duller versions of the adult.
Distribution and habitat
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, the Mascarene martin breeds on Reunion and Mauritius in the Indian Ocean, and a number of migratory species are common vagrants to other isolated islands and even to some sub-Antarctic islands and Antarctica. 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. 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.
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. 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. There were several reports of suspected torpor in swallows from 1947, such as a 1970 report that white-backed swallows in Australia may conserve energy this way, but the first confirmed study that they or any passerine entered torpor was a 1988 study on house martins.
Behaviour and ecology
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 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. These roosts can be enormous; one winter-roosting site of barn swallows in Nigeria attracted 1.5 million individuals. 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.Diet and feeding
For the most part, swallows are insectivorous, taking flying insects on the wing. 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. In addition, the ease of capture of different insect types affects their rate of predation by swallows. 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.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. Similar separation occurs where feeding overlaps with swifts. Niche separation may also occur with the size of prey chosen.