This article is about the family of birds. For the action, see Swallowing. For other uses, see Swallow (disambiguation).
The swallows and martins, or Hirundinidae, are a family of passerinebirds found around the world on all continents except 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. There are around 83 species in 19 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 non-migratory.
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.)
Taxonomy and systematics
The family Hirundinidae was introduced (as Hirundia) by the French polymathConstantine Samuel Rafinesque in 1815. The swallows and martins are morphologically unique within the passerines, with molecular evidence placing them as a distinctive lineage within the Sylvioidea (Old World warblers and relatives). They have also been linked to the white-eyes and the tits. Under the Sibley-Ahlquist taxonomy they have been placed in the infraorderPasserida.
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.
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; the metabolic rate of swallows in flight is 49–72% lower 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 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 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.
Distribution and habitat
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 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 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 region 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, 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 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 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. Colonial species may mob predators of 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 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 (this is different from incubation as that involves warming the eggs, not just stopping 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.
Status and conservation
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 endangeredwhite-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. As the species hasn't been reliably seen since 1980 it may already be extinct. 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.
Relationship with humans
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 further experimentation stalled. 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 swallow is called the "bird of freedom" because it cannot endure captivity and will only mate in the wild.
- Subfamily: Pseudochelidoninae (river martins)
- Subfamily Hirundininae (all other swallows and martins)
- Genus: Psalidoprocne (saw-wings)
- Genus: Pseudhirundo
- Genus: Cheramoeca
- Genus: Phedina
- Genus: Riparia
- Genus: Tachycineta
- Tree swallow, Tachycineta bicolor
- Mangrove swallow, Tachycineta albilinea
- Tumbes swallow, Tachycineta stolzmanni
- White-winged swallow, Tachycineta albiventer
- White-rumped swallow, Tachycineta leucorrhoa
- Chilean swallow, Tachycineta meyeni
- Golden swallow, Tachycineta euchrysea
- Violet-green swallow, Tachycineta thalassina
- Bahama swallow, Tachycineta cyaneoviridis
- Genus: Progne
- Purple martin, Progne subis
- Cuban martin, Progne cryptoleuca
- Caribbean martin, Progne dominicensis
- Sinaloa martin, Progne sinaloae
- Grey-breasted martin, Progne chalybea
- Galapagos martin, Progne modesta
- Peruvian martin, Progne murphyi
- Southern martin, Progne elegans
- Brown-chested martin, Progne tapera
- Genus: Notiochelidon
- Genus: Haplochelidon
- Genus: Atticora
- Genus: Neochelidon
- Genus: Stelgidopteryx
- Genus: Alopochelidon
- Genus: Hirundo
- Barn swallow, Hirundo rustica
- Red-chested swallow, Hirundo lucida
- Angolan swallow, Hirundo angolensis
- Pacific swallow, Hirundo tahitica
- Hill swallow, Hirundo domicola
- Welcome swallow, Hirundo neoxena
- White-throated swallow, Hirundo albigularis
- Ethiopian swallow, Hirundo aethiopica
- Wire-tailed swallow, Hirundo smithii
- Blue swallow, Hirundo atrocaerulea
- White-bibbed swallow, Hirundo nigrita
- Pied-winged swallow, Hirundo leucosoma
- White-tailed swallow, Hirundo megaensis
- Black-and-rufous swallow, Hirundo nigrorufa
- Pearl-breasted swallow, Hirundo dimidiata
- Genus: Ptyonoprogne
- Genus: Delichon
- Genus: Cecropis
- Greater striped swallow, Cecropis cucullata
- Lesser striped swallow, Cecropis abyssinica
- Red-breasted swallow, Cecropis semirufa
- Mosque swallow, Cecropis senegalensis
- Red-rumped swallow, Cecropis daurica
- Sri Lanka swallow, Cecropis hyperythra
- West African swallow, Cecropis domicella
- Striated swallow, Cecropis striolata
- Rufous-bellied swallow, Cecropis badia
- Genus: Petrochelidon
- Red-throated cliff swallow, Petrochelidon rufigula
- Preuss's cliff swallow, Petrochelidon preussi
- Red Sea cliff swallow, Petrochelidon perdita
- South African cliff swallow, Petrochelidon spilodera
- Forest swallow, Petrochelidon fuliginosa
- Streak-throated swallow, Petrochelidon fluvicola
- Fairy martin, Petrochelidon ariel
- Tree martin, Petrochelidon nigricans
- American cliff swallow, Petrochelidon pyrrhonota
- Cave swallow, Petrochelidon fulva
- Chestnut-collared swallow, Petrochelidon rufocollaris
- ^ abTurner, Angela; Rose, Chris (1989). Swallows and martins: an identification guide and handbook. Houghton-Mifflin. ISBN 0-395-51174-7.
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- ^Bock, Walter J. (1994). History and Nomenclature of Avian Family-Group Names. Bulletin of the American Museum of Natural History. Number 222. New York: American Museum of Natural History. pp. 149, 252.
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- ^ abMayr, E.; Amadon, D (1951). "A Classification of Recent Birds"(PDF). American Museum Novitates. 1496: 16.
- ^Sheldon, Frederick H.; Whittingham, Linda A.; Moyle, Robert G.; Slikas, Beth; Winkler, David W. (April 2005). "Phylogeny of swallows (Aves: Hirundinidae) estimated from nuclear and mitochondrial DNA sequences"(PDF). Molecular Phylogenetics and Evolution. 35 (1): 254–270. doi:10.1016/j.ympev.2004.11.008.
- ^ abcdefghijklmnopTurner, Angela (2004). "Family Hirundinidae (Swallows and Martins)". In Josep del Hoyo; Andrew Elliott; David A. Christie. Handbook of the Birds of the World. Volume 9. Lynx Edicions. pp. 602–638. ISBN 84-87334-69-5.
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- ^Norberg, R. Ake (1994). "Swallow Tail Streamer is a Mechanical Device for Self Deflection of Tail Leading Edge, Enhancing Aerodynamic Efficiency and Flight Manoeuvrability". Proc. R. Soc. Lond. B. 257 (1350): 227–233. doi:10.1098/rspb.1994.0119.
- ^ abBuchanan, Katherine L.; Evans, Matthew R. (2000). "The effect of tail streamer length on aerodynamic performance in the barn swallow". Behavioral Ecology. 11 (2): 228–238. doi:10.1093/beheco/11.2.228.
- ^Møller, Anders pape (1992). "Sexual selection in the monogamous barn swallow (Hirundo rustica). II. Mechanisms of sexual selection". Journal of Evolutionary Biology. 5 (4): 603–624. doi:10.1046/j.1420-9101.1992.5040603.x.
- ^ abGaunt, Abbot (1969). "Myology of the Leg in Swallows". Auk. 86 (1): 41–53. doi:10.2307/4083540. JSTOR 4083540.
- ^Bańbura, Jerzy (1986). "Sexual dimorphism in wing and tail length as shown by the Swallow Hirundo rustica". Journal of Zoology. 201 (1): 131–136. doi:10.1111/j.1469-7998.1986.tb03625.x.
- ^Gill, Frank B. (1995). Ornithology. W. H. Freeman. p. 434. ISBN 978-0-7167-2415-5.
- ^Pratt, H.; Bruner, P; Berrett, D. (1987). The Birds of Hawaii and the Tropical Pacific. Princeton: Princeton University Press. p. 229. ISBN 0-691-08402-5.
- ^Sinclair, Ian; Olivier Langrand (2005). Birds of the Indian Ocean Islands. Struik. p. 118. ISBN 978-1-86872-956-2.
- ^Korczak-Abshire, Małgorzata; Lees, Alexander; Jojczyk, Agata (2001). "First documented record of barn swallow (Hirundo rustica) in the Antarctic". Polish Polar Research. 32 (4): 355–360. doi:10.2478/v10183-011-0021-9.
- ^Tarburton, M.K. (1993). "A Comparison of the Breeding Biology of the Welcome Swallow in Australia and Recently Colonized New Zealand". Emu. 93 (1): 34–43. doi:10.1071/MU9930034.
- ^In 1878 Dr. Elliott Coues, listed titles of 182 papers dealing with the hibernation of swallows ((USGS: Northern Prairie Wildlife Research Center) "Early ideas about migration").
- ^Lariewski, Robert C.; Thompson, Henry J. (1965). "Field Observation of Torpidity in the Violet-Green Swallow"(PDF). Condor. 68 (1): 102–103.
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The tree swallow (Tachycineta bicolor) is a migratorypasserinebird that breeds in North America and winters in Mexico, Central America and the Caribbean. It is a very rare vagrant to western Europe.
Taxonomy and etymology
The tree swallow is usually placed in the genus Tachycineta, although it is sometimes placed in the genus Iridoprocne, along with the mangrove swallow, white-rumped swallow, white-winged swallow, and Chilean swallow.
The generally accepted genus name is from Ancient Greektakhukinetos, "moving quickly", and the specific bicolor is Latin and means "two-coloured". The other genus name, Iridoprocne, comes from the Greekiris, meaning rainbow, and Procne, a figure who supposedly turned into a swallow.
This swallow has a length between about 12 and 14 cm (4.7 and 5.5 in) and a weight of approximately 17 to 25.5 g (0.60 to 0.90 oz). The male has mostly glossy blue-green , with the wings and tail being blackish. The  and the cheek patch are white, although the coverts are grey-brown. The female can be differentiated from the male as the former is duller, and sometimes has a brown forehead. The juveniles can be distinguished be their brown upperparts and grey-brown washed breast.
The tree swallow's alarm call is a "peeh" or "pee-deeh"; this call can serve to silence older nestlings that are begging while a predator is near.
Distribution and habitat
The tree swallow breeds in North America. Its range extends to north-central Alaska and up to the tree limit in Canada. It is found as far south as Tennessee in the eastern part of its range, California and New Mexico in the west, and Kansas in the centre. It occasionally breeds further south in the United States (US). The wintering range is primarily southern US coasts and south, along the Gulf Coast, to Panama and the Northwestern coast of South America, in addition to being found in the West Indies. When a swallow returns to nest, it usually does not change breeding sites.
The habitat of this swallow is primarily in open and wooded areas, especially those near water. The fact that it is able to habitate open areas is due to the construction of nest boxes in such regions.
Being highly social outside of the breeding season, tree swallows may form flocks of several thousand birds near roost sites. Flocks near Vacherie, Louisiana, were estimated to contain well over 1 million birds during December 2009.
The tree swallow usually renests in the same area to breed again; only about 14% of females and 4% of males disperse to breed at a new site per year. This dispersal, although, is influenced by breeding success; about 28% of breeders disperse after they fail to fledge a chick, compared to the 5% that disperse when they are successful. Most do not disperse far, usually breeding at sites less than 10 kilometres (6.2 mi) away from their original grounds. The nest is found in pre-existing holes, usually in trees and artificial structures such as pipes and fence posts, and in nestboxes. The nest hole is, on average, 3.4 metres (11 ft) above ground level, although about 45% of them are less than 2 metres (6.6 ft) above the ground. Nests are usually spaced 10 to 15 metres (33 to 49 ft) apart, and those that are closer in distance are usually further apart in terms of laying date. The nest cup itself is made from grass, moss, pine needles, and aquatic plants, and lined with feathers, all of which are collected mostly by the female.
Eggs are laid from early May to mid-June and chicks fledge between mid-June and July. When the eggs are laid is influenced by latitude, age of the female, and wing length of the female. On average, birds in the southern part of the breeding range breed earlier than those in the northern portion. The female's wing length and age are both inversely correlated with the timing of breeding. It generally lays when the temperature and the abundance of food (that the female can catch) are good enough for females to start laying eggs. These observations support the theory that the tree swallow is an income breeder (breeding based on food conditions during the laying season). This species is generally socially monogamous, but up to 8% of breeding males are polygynous. Polygyny is influenced by territory: males having territories with nestboxes at least 5 metres (16 ft) apart are more likely to be polygynous. It is suggested that this polygyny depends on the conditions during the laying season: better conditions, such as an abundance of food, allow females in polygyny who do not receive help in foraging to lay more eggs.
The tree swallow has high rates of extra-pair paternity, with 38% to 69% of nestlings being a product of extra-pair paternity, and 50% to 87% of broods containing at least one nestling that was the result of an extra-pair copulation. One factor that might contribute to this is that females have control over copulation, making paternity guards ineffective. Extra-pair paternity does not change the level of parental care the male contributes in the tree swallow, contrary to other birds. The tree swallow also differs in terms of the composition of extra-pair fathers; in a study that found the paternity of 35 extra-pair nestlings, it was determined that 25 extra-pair young were from fathers from sites near the nest where the female is, about 3 from sites within 2 kilometres (1.2 mi), and 7 that had fathers that were floaters (those present at breeding grounds that presumably do not breed). In the tree swallow, floating thus helps males in good condition produce more chicks, while allowing males in bad condition to be successful through parental care.
How extra-pair fathers are chosen and why females even breed with other males (because they can control copulation) is controversial. One theory, called the genetic compatibility hypothesis, states that increased offspring fitness results from increased heterozygosity, and thus that female tree swallows would prefer to mate with those that have more different alleles from them. This theory is justified on the basis that young produced from extra-pair mates usually are more heterozygous than within-pair offspring. In additional support of this theory, females are likely able to judge, after copulation, whether or not a male's sperm is of high quality and whether she should allow it to impregnate her. Another theory, called the good genes theory, says that females choose extra-pair males based on if they have good genes that would influence survival and mating success. This is supported on the basis that it is able to explain why some tree swallows do not have any extra-pair young, whereas others do. Although this is true, there is criticism for a lack of phenotypic difference between extra-pair males and pair-bonded males. But, there are results that may be in support of this theory. For example, in a 2007 study, it was found that in increased plumage brightness and increased age in extra-pair males, together, increased the number of extra-pair young.
Courtship for the tree swallow starts with a male attacking an unknown female; this can be stimulated by the female doing a wing-fluttering flight, which is possibly an invitation to court. The male may then take a vertical posture, with a raised tail, slightly spread, and with wings slightly drooped. This stimulates the female to try and land on the male's back, but he flies to prevent this; this is repeated. After the male courts the female, he flies to his chosen nest site, and the female inspects it. The pair bond takes time to develop. During copulation, the male hovers over the female, and then mounts her, giving aggression calls. He then makes with the female while holding her neck feathers in his bill and standing on her slightly outstretched wings. Copulation occurs multiple times.
The tree swallow lays a clutch of two to eight, although usually four to seven, pure white, and translucent at laying, eggs that measure about 19 by 14 centimetres (7.5 by 5.5 in). These eggs are incubated by the female, usually after the second-to-last egg is laid, for 11 to 20 days, although most hatch after 14 to 15 days. They hatch slightly asynchronously, with an average of 28 hours between the time of the first and final egg is laid. The laying order predicts the hatching order, with eggs generally hatching in the order they were laid. When a brood hatches asynchronously, a weight hierarchy is established, with nestlings hatched earlier weighing more than those hatched later. This allows for the female to prioritize which chick to give food to in times of food shortage, although this weight difference is less pronounced about 12 days into nesting, suggesting that this brood-reduction only has a significant effect early in the nestling period. Infanticide of the chicks and eggs sometimes occurs when a male is replaced by another male. Infanticide usually does not occur when the clutch is not complete, as replacement males then have a chance to fertilize at least one egg. When the male arrives during incubation, it sometimes commits infanticide, but other times adopts the eggs, as there is a chance that some eggs were sired from the replacement male. If the replacement male arrives after the chicks hatch, although, infanticide is usually committed, though the female will sometimes prevent this.
The sex ratio of the hatchlings is male biased in females of better condition, and these males produced by the females in better condition are themselves in better condition. This is hypothesized to be because males have more variability in reproductive success, thus meaning a female in better condition will produce a male in better condition that may have better reproductive success (more than that of a female of similar condition).
Nestling tree swallows are able to thermoregulate at a capacity of 75% compared to the adult at an average age of 9.5 days when out of the nest, and from nine to four days when in the nest (depending on the size of the brood).
The tree swallow forages 0 to 50 metres (160 ft) above the ground singly or in groups. Its flight is a mix of flapping and gliding. During the breeding season, this is mostly within 4 kilometres (2.5 mi) of the nest site. When it is foraging for nestlings, although, it usually goes up to 200 metres (660 ft) from the nest, mostly staying in sight of it, and forages at a height up to 12 metres (39 ft). In addition to being caught in flight, insects are sometimes taken from the ground, water, vegetation, and vertical surfaces.
The tree swallow eats mostly insects, with some molluscs, spiders, and fruit. In North America, flies make up about 40% of the diet, with beetles and ants supplementing it. Otherwise, the diet is about 90% flies. The seed and berry food is mainly from the genus Myrica, which is mainly taken in the all four of the Northern Hemisphere seasons except summer. Crustaceans were also found to be important in the wintering diet in a study on Long Island, New York.
Both sexes feed the nestlings, although the male feeds the chicks less than the females. The hatching order affects how much a chick is fed; last-hatched nestlings (in cases where hatching is asynchronous) are likely fed less than those hatched earlier. Nestlings closer to the entrance of the nest are also more likely to be fed, as are those who started begging first. The diet itself is composed mostly of insects, with insects in the orders Diptera, Hemiptera, and Odonata making up most of the diet. These insects are mostly up to 10 millimetres (0.39 in) in size, but sometimes are up to 60 millimetres (2.4 in) in length. In nests near lakes acidified by humans, calcium supplements, primarily fish bones, crayfish exoskeletons, clam shells, and the shells of bird eggs, are more important in the diet of nestlings. To get these calcium supplements, the adult tree swallow travels further than usual; sometimes up to 650 metres (2,130 ft) away from the nest.
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