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19, see text.
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The swallows and martins are a group of passerine birds in the family Hirundinidae which are characterised by their adaptation to aerial feeding. Swallow is used colloquially in Europe as a synonym for the barn swallow.
This family comprises two subfamilies: Pseudochelidoninae (the river martins of the genus Pseudochelidon) and Hirundininae (all other swallows and martins). Within 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, there is no scientific distinction between these two groups. Within 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.) The entire family contains around 83 species in 19 genera.
The swallows have a cosmopolitan distribution across the world and breed on all the continents except Antarctica. It is believed that this family originated in Africa as hole-nesters; Africa still has the greatest diversity of species. 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 non-migratory.
The swallows and martins 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 shape allows for very efficient flight, which costs 50–75% less for swallows than equivalent passerines of the same size.
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 length ranges from about 10–24 cm (3.9–9.4 in) and their weight from about 10–60 g (0.35–2.12 oz). 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 will select mates on the basis of tail length.
The 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 (Pseudochelidon) are stronger and more robust than those of other swallows.
The most common hirundine plumage is glossy dark blue or green above and plain or streaked underparts, often white or rufous. Species which burrow or live in dry or mountainous areas are often matte brown above (e.g. sand martin and crag martin). 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.
The swallows and martins have a worldwide cosmopolitan distribution, occurring 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. 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 will 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 it was thought that swallows hibernated in a state of torpor, 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). 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, even apparently entering torpor.
Swallows are excellent flyers, and use these skills to feed and attract a mate. Some species, like the mangrove swallow, are territorial, whereas others are not and simply defend their nesting site. In general, the males select a nest site, and then attract a female using song and flight, and (dependent on the species) guard their 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 of 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. Non-social 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 will attack them within the perimeter of the nest.
For the most part swallows are insectivorous, taking flying insects on the wing. Across the whole family a wide range of insects are 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 will 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 that is on the wing, but they will 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 swallow feed together they will be separated 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.
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. Swallows in the genera Hirundo, Ptyonoproggne, 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 the rule 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 swallow 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. Old males benefit most from coloniality, since they are able to maintain their own nests and benefit from frequent extra-pair copulations.
Pairs of mated swallows are monogamous, and pairs of non-migratory species often stay near their breeding area all year, though the nest site is defended most vigorously during the breeding season. Migratory species often return to the same breeding area each year, and may select the same nest site if they were previously successful in that location. First-year breeders generally select a nesting site close to where they were born and raised. The breeding of temperate species is seasonal, whereas that of subtropical or tropical species can either be continuous throughout the year or seasonal. Seasonal species in the subtropics or tropics are usually timed to coincide with the peaks in insect activity, which is usually the wet season, but some species like the white-bibbed swallow nest in the dry season to avoid flooding in their riverbank nesting habitat. All swallows will defend their nests from egg predators, although solitary species are more aggressive towards predators than colonial species. Overall the contribution of male swallows towards parental care is the highest of any passerine bird.
The eggs of swallows tend to be white, although those of some mud-nesters are speckled. The average clutch size is around four to five eggs in temperate areas and two to three eggs in the tropics. The incubation duties are shared in some species, in others the eggs are incubated solely by the females. Amongst the species where the male helps with incubation the contribution varies amongst species, with some species like the cliff swallow sharing the duties equally and the female doing most of the work in others. Amongst the barn swallows the male of the American subspecies helps (to a small extent) whereas the European subspecies does not. Even in species where the male does not incubate the eggs the male may sit on them when the female is away to reduce heat loss. Incubation stints last for 5–15 minutes and are followed by bursts of feeding activity. From laying, swallow eggs take between 10–21 days to hatch, with 14–18 days being more typical.
The chicks of swallows hatch naked, generally with only a few tufts of down. The eyes are closed and do not fully open for up to 10 days. The feathers take a few days to begin to sprout, and the chicks are brooded by the parents until they are able to thermoregulate. On the whole they develop slowly compared to other passerine birds. The parents do not usually feed the chicks individual insects but instead a bolus of food comprising ten to a hundred insects. Regardless of whether the species has males that incubate or brood the chicks the males of all swallows and martins will help feed the chicks. It is difficult to judge when swallows and martins fledge, as they will be enticed out of the nest after three weeks by parents but frequently return to the nest afterwards in order to roost.
Swallows are able to produce many different calls or songs, which are used to express excitement, to communicate with others of the same species, during courtship, or as an alarm when a predator is in the area. The songs of males are related to the body condition of the bird and are presumably used by females to judge the physical condition and suitability for mating of males. Begging calls are used by the young when soliciting food from their parents. The typical song of swallows is a simple, sometimes musical twittering.
Species of swallow and martin that are threatened with extinction are generally endangered due to habitat loss. This is presumed to be the reason behind the decline of the critically endangered white-eyed river martin, a species that is only known from a few specimens collected in Thailand. The species presumably breeds in riverbanks, a much diminished habitat in SE Asia. Two insular species, the Bahama swallow and golden swallow, have declined due to forest loss and also competition with introduced species such as starlings and sparrows, which compete with these swallows for nesting sites. The golden swallow formerly bred on the island of Jamaica, but was last seen there in 1989 and is now restricted to the island of Hispaniola.
Swallows are tolerated by humans because of their beneficial role as insect-eaters, and some species have readily adapted to nesting in and around human habitation. The barn swallow and house martin now rarely use natural sites. the purple martin is also actively encouraged by people to nest around humans and elaborate nest boxes are erected. Enough artificial nesting sites have been created that the purple martin now seldom nests in natural cavities in the eastern part of its range.
Because of the long human experience with these conspicuous species, many myths and legends have arisen as a consequence, particularly relating to the barn swallow. The Roman historian Pliny the Elder described a use of painted swallows to deliver a report of the winning horses at a race. During the nineteenth century, Jean Desbouvrie attempted to tame swallows and train them for use as messenger birds, as an alternative to war pigeons. He succeeded in curbing the migratory instinct in young birds and persuaded the government of France to conduct initial testing, but stalled further experimentation. Subsequent attempts to train homing behaviour into swallows and other passerines had difficulty establishing a statistically significant success rate, although the birds have been known to trap themselves repeatedly in order to obtain bait from traps.
According to a sailing superstition, swallows are a good omen to those at sea. This probably arose from the fact that swallows are land-based birds, so their appearance informs a sailor that he is close to shore. An old term of venery for swallows is a "flight" or "sweep."
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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