The barn swallow (Hirundo rustica) is a widespread migratory bird, known for its iridescent blue-black plumage and deeply forked tail. This species has adapted closely to human environments, using structures like barns, bridges, and eaves as sheltered locations for raising their young. The annual cycle revolves around the breeding season, following a precise timeline for arrival, nesting, and departure.
Arrival and Initial Nest Construction
The nesting season begins with the swallows’ return migration; males typically arrive on the breeding grounds slightly earlier than females. In North America, arrival generally falls between late March in the southernmost regions and mid-May in northern latitudes. Once a pair forms or reunites, they immediately scout for a suitable nest site, usually attached to a surface inside a sheltered area.
Pairs often reuse a well-maintained nest from a previous year, but they will construct a new one if necessary. Both the male and female gather small pellets of wet mud, often mixed with grass stems, from nearby puddles or riverbanks. These materials are applied to form a small, cup-shaped structure, which takes approximately one to two weeks to complete. The final stage involves lining the mud cup with finer materials like grasses, animal hair, and soft feathers, preparing it for the eggs.
The Incubation and Fledging Period
Once the nest is finalized, the female begins laying eggs, typically one per day, until the clutch is complete. The average clutch size is four to five eggs, though clutches can range from two to seven. The incubation period lasts approximately 14 to 16 days.
The female performs most of the incubation, especially at night, though the male may assist during the day. After hatching, the altricial young are blind and helpless, initiating the nestling period. This phase, where the young remain in the nest and are fed continuously by both parents, is the longest part of the cycle, usually lasting between 18 and 24 days.
The parents make hundreds of trips daily to bring insects until the nestlings are strong enough to fledge, or leave the nest. Even after fledging, the young remain dependent on the adults, who continue to feed them for about another week while they develop flight skills. From the first egg laid to independence, a single reproductive cycle takes approximately six to seven weeks.
Regional Timing Shifts and Seasonal Departure
The timing of the barn swallow’s nesting cycle is sensitive to geography and local weather conditions. Swallows in warmer, southern regions, such as the southern United States, can begin nesting in late March or early April. Those in northern areas may not start until late May or June. Adverse weather, such as a late cold snap or prolonged rain, can delay nest building and egg laying, as it affects the availability of flying insects, their sole food source.
Raising multiple broods per year is a major factor extending the nesting season. Barn swallows frequently raise two successful broods, and in warmer climates, they may attempt a third. The second clutch is typically started shortly after the first brood has fledged, extending the active breeding period into mid-to-late summer, often through July and August.
The annual cycle concludes with their seasonal departure and migration back to wintering grounds in Central and South America. Swallows in the far north may begin their southward journey as early as late July. However, the peak fall migration across most of the continent occurs from late August through September. By October, most barn swallows have left their breeding areas.