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,^[17] kamtschatica by Benedykt Dybowski in 1883,^[18] ambigua by Erwin Stresemann^[19] and mandschurica by Wilhelm Meise in 1934.^[17] Given the uncertainties over the validity of these forms,^[18][20] this article follows the treatment of Turner and Rose.^[7]

• H. r. rustica, the nominate European subspecies, breeds in Europe and Asia, as far north as the Arctic Circle, south to North Africa, the Middle East and Sikkim, and east to the Yenisei River. It migrates on a broad front to winter in Africa, Arabia, and the Indian subcontinent.^[7] The barn swallows wintering in southern Africa are from across Eurasia to at least 91°E,^[21] and have been recorded as covering up to 11,660 km (7,250 mi) on their annual migration.^[22] The nominate European subspecies was the first to have its genome sequenced and published.^[23]
• H. r. transitiva was described by Ernst Hartert in 1910. It breeds in the Middle East from southern Turkey to Israel and is partially resident, though some birds winter in East Africa. It has orange-red underparts and a broken breast band.^[7] The holotype of Chelidon rustica transitiva Hartert (Vog. pal. Fauna, Heft 6, 1910. p. 802), an adult female, is held in the vertebrate zoology collection of National Museums Liverpool at World Museum, with accession number NML-VZ T2057.^[24] The specimen was collected in the Plains of Esdraclon, Palestine on 16 December 1863 by Henry Baker Tristram. The specimen came to the Liverpool national collection through the purchase of Canon Henry Baker Tristram's collection by the museum in 1896.^[25]
• H. r. savignii, the resident Egyptian subspecies, was described by James Stephens in 1817 and named for French zoologist Marie Jules César Savigny.^[26] It resembles transitiva, which also has orange-red underparts, but savignii has a complete broad breast band and deeper red hue to the underparts.^[9]
• H. r. gutturalis, described by Giovanni Antonio Scopoli in 1786,^[17] has whitish underparts and a broken breast band. The breast is chestnut and the lower underparts more pink-buff.^[27] The populations that breed in the central and eastern Himalayas have been included in this subspecies,^[28] although the primary breeding range is Japan and Korea. The east Asian breeders winter across tropical Asia from India and Sri Lanka^[29] east to Indonesia and New Guinea. Increasing numbers are wintering in Australia. It hybridises with H. r. tytleri in the Amur River area. It is thought that the two eastern Asia forms were once geographically separate, but the nest sites provided by expanding human habitation allowed the ranges to overlap.^[7] H. r. gutturalis is a vagrant to Alaska and Washington,^[30] but is easily distinguished from the North American breeding subspecies, H. r. erythrogaster, by the latter's reddish underparts.^[7]
• H. r. tytleri, first described by Thomas Jerdon in 1864, and named for British soldier, naturalist and photographer Robert Christopher Tytler,^[17] has deep orange-red underparts and an incomplete breast band. The tail is also longer.^[27] It breeds in central Siberia south to northern Mongolia and winters from eastern Bengal east to Thailand and Malaysia.^[7]
• H. r. erythrogaster, the North American subspecies described by Pieter Boddaert in 1783,^[17] differs from the European subspecies in having redder underparts and a narrower, often incomplete, blue breast band. It breeds throughout North America, from Alaska to southern Mexico, and migrates to the Lesser Antilles, Costa Rica, Panama and South America to winter.^[10] A few may winter in the southernmost parts of the breeding range. This subspecies funnels through Central America on a narrow front and is therefore abundant on passage in the lowlands of both coasts.^[31] Since the 1980s, small numbers of this subspecies have been found nesting in Argentina.^[32]

  

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.^[33]

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.^[6]

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.^[34] Because it takes around 2 weeks for a pair to build a nest from mud, hair, and other materials, old nests are highly prized.^[35]

This species breeds across the Northern Hemisphere from sea level to 2,700 m (8,900 ft),^[36] but to 3,000 m (9,800 ft) in the Caucasus^[6] and North America,^[37] 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.^[7]

In winter, the barn swallow is cosmopolitan in its choice of habitat, avoiding only dense forests and deserts.^[38] 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.^[10][39][40] In the absence of suitable roost sites, they may sometimes roost on wires where they are more exposed to predators.^[41] Individual birds tend to return to the same wintering locality each year^[42] and congregate from a large area to roost in reed beds.^[39] These roosts can be extremely large; one in Nigeria had an estimated 1.5 million birds.^[43] 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.^[7][44]

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.^[45] 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,^[7] and even Antarctica.^[46]

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 11 m/s (40 km/h), up to 20 m/s (72 km/h) and a wing beat rate of approximately 5, up to 7–9 times each second.^[47][48]

The barn swallow typically feeds in open areas^[49] 7–8 m (23–26 ft) 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.^[6] Swallows have been observed feeding on insects that fly around active white stork nests as well.^[50] 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.^[6] On the wintering grounds, Hymenoptera, especially flying ants, are important food items.^[7] Grasshoppers, crickets, dragonflies, beetles and moths are also preyed upon.^[51] When egg-laying, barn swallows hunt in pairs, but otherwise will form often large flocks.^[7]

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.^[52]

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.^[53] Another study showed that a single population breeding in Denmark actually wintered in two separate areas.^[54]

The barn swallow drinks by skimming low over lakes or rivers and scooping up water with its open mouth.^[37] This bird bathes in a similar fashion, dipping into the water for an instant while in flight.^[42]

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.^[9] 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.^[55]

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.^[56] In all subspecies, this is followed by a short "Q-syllable" and a trilled series of pulses, termed the "rattle."^[57] The rattle is sometimes followed by a terminal "Ω-Note" in some subspecies' populations, and always at the end of H. r. tytleri song.^[56]

Female songs are much shorter than male songs, and are only produced during the early part of the breeding season.^[8] Females sing spontaneously, though infrequently, and will also countersing in response to each other.^[8]

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.^[6] 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,^[58] the breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female.^[6][59] 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.^[60] 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.^[61]

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 damage again demonstrate breeding quality; a positive association exists between spot size and the number of offspring produced each season.^[62]

The breeding season of the barn swallow is variable: in the southern part of the range, the breeding season usually is from February or March to early to mid September, although some late second and third broods finish in October. In the northern part of the range, it usually starts late May to early June and ends the same time as the breeding season of the southernmost birds.^[63]

Both sexes defend the nest, but the male is particularly aggressive and territorial.^[7] Once established, pairs stay together to breed for life, but extra-pair copulation is common, making this species genetically polygamous, despite being socially monogamous.^[64] Males guard females actively to avoid being cuckolded.^[65] Males may use deceptive alarm calls to disrupt extrapair copulation attempts toward their mates.^[66]

As its name implies, the barn swallow typically nests inside accessible buildings such as barns and stables, or under bridges and wharves.^[67] Before man-made sites became common, it nested on cliff faces or in caves, but this is now rare.^[7] The neat cup-shaped nest is placed on a beam or against a suitable vertical projection. It is constructed by both sexes, although more often by the female, with mud pellets collected in their beaks and lined with grasses, feathers, algae^[67] or other soft materials.^[7] The nest building ability of the male is also sexually selected; females will lay more eggs and at an earlier date with males who are better at nest construction, with the opposite being true with males that are not.^[68] After building the nest, barn swallows may nest colonially where sufficient high-quality nest sites are available, and within a colony, each pair defends a territory around the nest which, for the European subspecies, is 4 to 8 m² (40 to 90 sq ft) in size. Colony size tends to be larger in North America.^[37]

In North America at least, barn swallows frequently engage in a mutualist relationship with ospreys. Barn swallows will build their nest below an osprey nest, receiving protection from other birds of prey that are repelled by the exclusively fish-eating ospreys. The ospreys are alerted to the presence of these predators by the alarm calls of the swallows.^[37]

Barn swallows will normally raise two broods, with the original nest being reused for the second brood and being repaired and reused in subsequent years. The female lays two to seven, but typically four or five, reddish-spotted white eggs.^[7] The clutch size is influenced by latitude, with clutch sizes of northern populations being higher on average than southern populations.^[69] The eggs are 20 mm × 14 mm (3⁄4 in × 1⁄2 in) in size, and weigh 1.9 grams (29 grains), of which 5% is shell. In Europe, the female does almost all the incubation, but in North America the male may incubate up to 25% of the time. The incubation period is normally 14–19 days, with another 18–23 days before the altricial chicks fledge. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.^[7] Compared to those from early broods, juvenile barn swallows from late broods have been found to migrate at a younger age, fuel less efficiently during migration and have lower return rates the following year.^[70]

The barn swallow will mob intruders such as cats or accipiters that venture too close to their nest, often flying very close to the threat.^[60] Adult barn swallows have few predators, but some are taken by accipiters, falcons, and owls. Brood parasitism by cowbirds in North America or cuckoos in Eurasia is rare.^[6][37]

Hatching success is 90% and the fledging survival rate is 70–90%. Average mortality is 70–80% in the first year and 40–70% for the adult. Although the record age is more than 11 years, most survive less than four years.^[7] Barn swallow nestlings have prominent red gapes, a feature shown to induce feeding by parent birds. An experiment in manipulating brood size and immune system showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape.^[71]

The barn swallow has been recorded as hybridising with the cliff swallow (Petrochelidon pyrrhonota) and the cave swallow (P. fulva) in North America, and the house martin (Delichon urbicum) in Eurasia, the cross with the latter being one of the most common passerine hybrids.^[60]

In literature

Many literary references are based on the barn swallow's northward migration as a symbol of spring or summer. The proverb about the necessity for more than one piece of evidence goes back at least to Aristotle's Nicomachean Ethics: "For as one swallow or one day does not make a spring, so one day or a short time does not make a fortunate or happy man."^[81]

The barn swallow symbolises the coming of spring and thus love in the Pervigilium Veneris, a late Latin poem. In his poem "The Waste Land", T. S. Eliot quoted the line "Quando fiam uti chelidon [ut tacere desinam]?" ("When will I be like the swallow, so that I can stop being silent?") This refers to the myth of Philomela in which she turns into a nightingale, and her sister Procne into a swallow.^[84]

In culture

Gilbert White studied the barn swallow in detail in his pioneering work The Natural History of Selborne, but even this careful observer was uncertain whether it migrated or hibernated in winter.^[16] Elsewhere, its long journeys were well observed, and a swallow tattoo is traditional among sailors as a symbol of a safe return; the tradition was that a mariner had a tattoo of this fellow wanderer after sailing 5,000 nautical miles (9,300 kilometres). A second swallow would be added after 10,000 nmi (19,000 km) at sea.^[85]

In the past, the tolerance for this beneficial insectivore was reinforced by superstitions regarding damage to the barn swallow's nest. Such an act might lead to cows giving bloody milk, or no milk at all, or to hens ceasing to lay.^[86] This may be a factor in the longevity of swallows' nests. Survival, with suitable annual refurbishment, for 10–15 years is regular, and one nest was reported to have been occupied for 48 years.^[86]

It is depicted as the martlet, merlette or merlot in heraldry, where it represents younger sons who have no lands. It is also represented as lacking feet as this was a common belief at the time.^[87] As a result of a campaign by ornithologists, the barn swallow has been the national bird of Estonia since 23 June 1960, and is also the national bird of Austria.^[88][89]