Barn swallows are socially monogamous. However, extra-pair copulations are common, making this species genetically polygamous. Breeding pairs form each spring after arrival on the breeding grounds. Pairs re-form each spring, though pairs that have nested together successfully may mate together for several years. Males try to attract females by spreading their tails to display them and singing.
Several studies have researched sexual selection in barn swallows. Moller (1994) documented female barn swallows selecting for symmetrical wings and tails in potential mates. Males exhibiting greater symmetry acquired mates more quickly than did asymmetric males. Asymmetry can result from genetic factors such as inbreeding or mutations as well as from environmental stress such as food deficiency, parasite infestation, or the presence of pathogens. Moller observed that individuals affected by these factors not only exhibited asymmetry, but also decreased strength and longevity. Therefor, females that selected symmetrical mates would presumably be selecting superior mates. In addition to selecting for symmetry, females also tend to select males with longer tail feathers. Moller observed a connection between the tail length of male barn swallows and their offspring’s vitality and longevity. Males with longer tail feathers exhibit traits of greater longevity which is passed on to their offspring. Females thus gain an indirect fitness benefit from this form of selection, as longer tail feathers indicate a genetically stronger individual who will produce offspring with enhanced vitality. Individuals with longer tails have also been observed to demonstrate greater disease resistance than their short-tailed counterparts. There is also evidence that males select female mates with long tails.
Unmated adults often associate with a breeding pair for up to an entire season. Though these "helpers" do not usually feed the young, they may help with nest defense, nest building, incubation and brooding. "Helpers" are predominantly male, and may succeed in mating with the resident female, leading to polygyny.
Mating System: monogamous ; polygynandrous (promiscuous) ; cooperative breeder
Barn swallows usually breed between May and August, but this varies greatly with location. They usually raise two broods of chicks each summer. Both birds of a pair make the nest. They build the shell of mud, and line it with grass and feathers. The female lays 3 to 7 eggs (average 5). Both parents incubate the eggs, which hatch in 13 to 15 days. The chicks are naked and helpless when they hatch. Both parents feed and protect the chicks, as well as removing fecal sacs from the nest. The nestlings remain in the nest for about 20 days before fledging. When barn swallows are handled by humans they tend to attempt to fledge at least a day too early. The parents continue to care for the chicks for up to a week after fledging, feeding them and leading them back to the nest to sleep. By two weeks after fledging, the barn swallow chicks have dispersed and often travel widely to other barn swallow colonies. Young barn swallows are able to breed in the first breeding season after they have hatched. Generally, young barn swallows do not produce as many eggs as do older birds.
Breeding interval: Barn Swallows usually produce 2 clutches per season, breeding seasons occur once each year.
Breeding season: Barn Swallows breed from May to August.
Range eggs per season: 3 to 7.
Range time to hatching: 13 to 15 days.
Average fledging age: 20.50 days.
Range time to independence: 2 (high) weeks.
Average age at sexual or reproductive maturity (female): 1 years.
Average age at sexual or reproductive maturity (male): 1 years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Average eggs per season: 4.
In North America, both barn swallow parents incubate the eggs and feed the nestlings. However, females provide more parental care than do males. During the nestling period, barn swallow parents may feed their nestlings up to 400 times per day. Barn swallows feed their chicks insects compressed into a pellet, which is transported to the nest in the parent’s throat. Although all swallows are socially monogamous, barn swallows differ from most swallow species in the sharing of parental care. Juveniles from the first brood of the season have even been observed assisting their parents in feeding a second brood.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); post-independence association with parents
- Bolzern, A., A. Moller, N. Saino. 1997. Immunocompetence, ornamentation, and viability of male Barn Swallows. Proceedings of the National Academy of Sciences, 94: 54-552.
- De Lope, F., A. Moller. 1993. Female reproductive effort depends on the degree of ornamentation. Evolution, 47: 1152-1161.
- McWilliams, G. 2000. The Birds of Pennsylvania. New York: Cornell University Press.
- Moller, A. 1994. Male ornament size as a reliable cue to enhanced offspring viability. Proceedings of the National Academy of Sciences, 91: 6929-6932.
- Moller, A. 1994. Patterns of fluctuating asymmetry and selection against asymmetry. Evolution, 48: 658-671.
- Moller, A. 1993. Sexual selection in the Barn Swallow *Hirundo rustica*: female tail ornaments. Evolution, 47: 417-432.
- Perrins, C. 1989. Encyclopedia of Birds. England: Equinox Ltd..
- Terres, J. 1980. The Audubon Society Encyclopedia of North American Birds. New York: Alfred A. Knopf, Inc.
- Brown, C., B. Brown. 1999. Barn swallow (Hirundo rustica). Pp. 1-32 in A Poole, F Gill, eds. The Birds of North America, Vol. 452. Philadelphia, PA: The Birds of North America.