Migratory passerine bird, and the most widespread species of swallow
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The barn swallow (Hirundo rustica) is the most widespread species of swallow in the world, occurring on all continents, with vagrants reported even in Antarctica. It is a distinctive passerine bird with blue upperparts and a long, deeply forked tail. In Anglophone Europe, it is just called the swallow; in northern Europe, it is the only member of family Hirundinidae called a "swallow" rather than a "martin".
There are six subspecies of barn swallow, which breed across the Northern Hemisphere. Two subspecies, ("H. r. savignii and H. r. transitiva") have fairly restricted ranges in the Nile valley and eastern Mediterranean, respectively. The other four are more widespread, with winter ranges covering much of the Southern Hemisphere.
The barn swallow is a bird of open country that normally nests in man-made structures and consequently has spread with human expansion. It builds a cup nest from mud pellets in barns or similar structures and feeds on insects caught in flight. This species lives in close association with humans, and its insect-eating habits mean that it is tolerated by humans; this acceptance was reinforced in the past by superstitions regarding the bird and its nest. There are frequent cultural references to the barn swallow in literary and religious works due to both its living in close proximity to humans and its annual migration. The barn swallow is the national bird of Austria and Estonia.
Description.
The adult male barn swallow of the nominate subspecies "H. r. rustica" is long including of elongated outer tail feathers. It has a wingspan of and weighs . It has steel blue upperparts and a rufous forehead, chin and throat, which are separated from the off-white underparts by a broad dark blue breast band. The outer tail feathers are elongated, giving the distinctive deeply forked "swallow tail". There is a line of white spots across the outer end of the upper tail. The female is similar in appearance to the male, but the tail streamers are shorter, the blue of the upperparts and breast band is less glossy, and the underparts paler. The juvenile is browner and has a paler rufous face and whiter underparts. It also lacks the long tail streamers of the adult.
Although both sexes sing, female song was only recently described. (See below for details about song.) Calls include "witt" or "witt-witt" and a loud "splee-plink" when excited or trying to chase intruders away from the nest. The alarm calls include a sharp "siflitt" for predators like cats and a "flitt-flitt" for birds of prey like the hobby. This species is fairly quiet on the wintering grounds.
The distinctive combination of a red face and blue breast band renders the adult barn swallow easy to distinguish from the African "Hirundo" species and from the welcome swallow ("Hirundo neoxena") with which its range overlaps in Australasia. In Africa the short tail streamers of the juvenile barn swallow invite confusion with juvenile red-chested swallow ("Hirundo lucida"), but the latter has a narrower breast band and more white in the tail.
Taxonomy.
The barn swallow was described by Carl Linnaeus in his 1758 10th edition of "Systema Naturae" as "Hirundo rustica", characterised as "H. rectricibus, exceptis duabus intermediis, macula alba notatîs". "Hirundo" is the Latin word for "swallow"; "rusticus" means "of the country". This species is the only one of that genus to have a range extending into the Americas, with the majority of "Hirundo" species being native to Africa. This genus of blue-backed swallows is sometimes called the "barn swallows".
The "Oxford English Dictionary" dates the English common name "barn swallow" to 1851, though an earlier instance of the collocation in an English-language context is in Gilbert White's popular book "The Natural History of Selborne", originally published in 1789:
The swallow, though called the chimney-swallow, by no means builds altogether in chimnies ["sic"], but often within barns and out-houses against the rafters ... In "Sweden" she builds in barns, and is called "ladusvala", the barn-swallow.
This suggests that the English name may be a calque on the Swedish term.
There are few taxonomic problems within the genus, but the red-chested swallow—a resident of West Africa, the Congo Basin, and Ethiopia—was formerly treated as a subspecies of barn swallow. The red-chested swallow is slightly smaller than its migratory relative, has a narrower blue breast-band, and (in the adult) has shorter tail streamers. In flight, it looks paler underneath than barn swallow.
Subspecies.
Six subspecies of barn swallow are generally recognised. In eastern Asia, a number of additional or alternative forms have been proposed, including "saturata" by Robert Ridgway in 1883, "kamtschatica" by Benedykt Dybowski in 1883, "ambigua" by Erwin Stresemann and "mandschurica" by Wilhelm Meise in 1934. Given the uncertainties over the validity of these forms, this article follows the treatment of Turner and Rose.
The short wings, red belly and incomplete breast band of "H. r. tytleri" are also found in "H. r. erythrogaster", and DNA analyses show that barn swallows from North America colonised the Baikal region of Siberia, a dispersal direction opposite to that for most changes in distribution between North America and Eurasia.
Behaviour.
Habitat and range.
The preferred habitat of the barn swallow is open country with low vegetation, such as pasture, meadows and farmland, preferably with nearby water. This swallow avoids heavily wooded or precipitous areas and densely built-up locations. The presence of accessible open structures such as barns, stables, or culverts to provide nesting sites, and exposed locations such as wires, roof ridges or bare branches for perching, are also important in the bird's selection of its breeding range.
Barn swallows are semi-colonial, settling in groups from a single pair to a few dozen pairs, particularly in larger wooden structures housing animals. The same individuals often breed at the same site year after year, although settlement choices have been experimentally shown to be predicted by nest availability rather than any characteristics of available mates. Because it takes around 2 weeks for a pair to build a nest from mud, hair, and other materials, old nests are highly prized.
This species breeds across the Northern Hemisphere from sea level to , but to in the Caucasus and North America, and it is absent only from deserts and the cold northernmost parts of the continents. Over much of its range, it avoids towns, and in Europe is replaced in urban areas by the house martin. However, in Honshū, Japan, the barn swallow is a more urban bird, with the red-rumped swallow ("Cecropis daurica") replacing it as the rural species.
In winter, the barn swallow is cosmopolitan in its choice of habitat, avoiding only dense forests and deserts. It is most common in open, low vegetation habitats, such as savanna and ranch land, and in Venezuela, South Africa and Trinidad and Tobago it is described as being particularly attracted to burnt or harvested sugarcane fields and the waste from the cane. In the absence of suitable roost sites, they may sometimes roost on wires where they are more exposed to predators. Individual birds tend to return to the same wintering locality each year and congregate from a large area to roost in reed beds. These roosts can be extremely large; one in Nigeria had an estimated 1.5 million birds. These roosts are thought to be a protection from predators, and the arrival of roosting birds is synchronised in order to overwhelm predators like African hobbies. The barn swallow has been recorded as breeding in the more temperate parts of its winter range, such as the mountains of Thailand and in central Argentina.
Migration of barn swallows between Britain and South Africa was first established on 23 December 1912 when a bird that had been ringed by James Masefield at a nest in Staffordshire, was found in Natal. As would be expected for a long-distance migrant, this bird has occurred as a vagrant to such distant areas as Hawaii, Bermuda, Greenland, Tristan da Cunha, the Falkland Islands, and even Antarctica.
Feeding.
The barn swallow is similar in its habits to other aerial insectivores, including other swallow species and the unrelated swifts. It is not a particularly fast flier, with a speed estimated at , up to and a wing beat rate of approximately 5, up to 7–9 times each second.
The barn swallow typically feeds in open areas above shallow water or the ground often following animals, humans or farm machinery to catch disturbed insects, but it will occasionally pick prey items from the water surface, walls and plants. Swallows have been observed feeding on insects that fly around active white stork nests as well. In the breeding areas, large flies make up around 70% of the diet, with aphids also a significant component. However, in Europe, the barn swallow consumes fewer aphids than the house or sand martins. On the wintering grounds, Hymenoptera, especially flying ants, are important food items. Grasshoppers, crickets, dragonflies, beetles and moths are also preyed upon. When egg-laying, barn swallows hunt in pairs, but otherwise will form often large flocks.
The amount of food a clutch will get depends on the size of the clutch, with larger clutches getting more food on average. The timing of a clutch also determines the food given; later broods get food that is smaller in size compared to earlier broods. This is because larger insects are too far away from the nest to be profitable in terms of energy expenditure.
Isotope studies have shown that wintering populations may utilise different feeding habitats, with British breeders feeding mostly over grassland, whereas Swiss birds utilised woodland more. Another study showed that a single population breeding in Denmark actually wintered in two separate areas.
The barn swallow drinks by skimming low over lakes or rivers and scooping up water with its open mouth. This bird bathes in a similar fashion, dipping into the water for an instant while in flight.
Swallows gather in communal roosts after breeding, sometimes thousands strong. Reed beds are regularly favoured, with the birds swirling "en masse" before swooping low over the reeds. Reed beds are an important source of food prior to and whilst on migration; although the barn swallow is a diurnal migrant that can feed on the wing whilst it travels low over ground or water, the reed beds enable fat deposits to be established or replenished.
Song.
Males sing to defend small territories (when living in colonies, less so in solitary pairs) and to attract mates. Males sing throughout the breeding season, from late April into August in many parts of the range. Their song is made up of a "twitter warble," followed by a rising "P-syllable" in European "H. r. rustica" and the North American "H. r. erythrogaster." In all subspecies, this is followed by a short "Q-syllable" and a trilled series of pulses, termed the "rattle." The rattle is sometimes followed by a terminal "Ω-Note" in some subspecies' populations, and always at the end of "H. r. tytleri" song.
Female songs are much shorter than male songs, and are only produced during the early part of the breeding season. Females sing spontaneously, though infrequently, and will also countersing in response to each other.
Breeding.
The male barn swallow returns to the breeding grounds before the females and selects a nest site, which is then advertised to females with a circling flight and song. Plumage may be used to advertise: in some populations, like in the subspecies "H. r. gutturalis", darker ventral plumage in males is associated with higher breeding success. In other populations, the breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female. Males with longer tail feathers are generally longer-lived and more disease resistant, females thus gaining an indirect fitness benefit from this form of selection, since longer tail feathers indicate a genetically stronger individual which will produce offspring with enhanced vitality. Males in northern Europe have longer tails than those further south, whereas in Spain the male's tail streamers are only 5% longer than the female's; in Finland, the difference is 20%. In Denmark, the average male tail length increased by 9% between 1984 and 2004, but it is possible that climatic changes may lead in the future to shorter tails if summers become hot and dry.
Males with long streamers also have larger white tail spots, and since feather-eating bird lice prefer white feathers, large white tail spots without parasite