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Swallows were traditionally cave nesters, building mud nests on ledges in caves, and even inside hollow trees. Since Neolithic times they used artificial habitats and now most nest around human habitation. The close association with people has allowed the bird to expand its range greatly through the availability of new sites.
They prefer to nest in association with domestic animals, most often in cow and pig sheds and stables. They sometimes nest under bridges, in wells and mineshafts, and even chimney stacks. They occasionally use very odd sites, including inhabited rooms and regularly moving vehicles.
Swallows are not truly colonial, but as nest choice primarily depends on food availability, several pairs can nest close together in loose groups, although single nests are most common.
Swallows return to Britain in April and May. The males return ahead of the females to claim nest sites. The main breeding season runs from April to August, when insects are most abundant, although late broods can be found still in the nest in September. During this time the birds raise two broods, sometimes three.
The nest is a deep bowl of mud mixed with grass and bound together with saliva, built by both birds on top of a rafter or another suitable ledge inside a building or under an overhang. The feather lining is added by the female. Building of a new nest takes about a week, but birds tend to re-use the previous years nest where possible.
The 4-5 eggs, white speckled with reddish brown, are laid at daily intervals, although bad weather can delay laying. The female incubates alone for 14-16 days and broods the young for the first week of their life, while they are naked and unable to maintain their own body temperature. The male shares the feeding duties until the brood is independent. In good conditions the brood will fledge at 21 days, but bad weather can slow down their development and delay the fledging.
From 70 to 90% of eggs laid produce fledged young in a good year, although in years of cold wet weather food shortage can be a major cause of mortality. After fledging, the broods stay in family groups for another week or fortnight, when they are fed by their parents and return to the nest to roost. Fledged young from first broods often help their parents feed a second brood.
The autumn migration takes place in September and October. Juveniles and adults travel together and family groups sometimes stay together on migration.
Nest sites are traditional, and provided the food supply has not changed, the same nests or sites are used from one year to the next, frequently by the same two birds. Swallows breed when they are one year old, many returning to breed within 3 km of the nest they fledged from most birds within 30 km. They are short-lived birds, and most live fewer than four years. The maximum recorded age is 16 years.
