House martins traditionally built their mud nests on cliff faces. By the 19th century they started making use of buildings, allowing them to expand their range. The traditional nest sites had been all but abandoned by early 1900s in favour of close association with people, which allowed the birds to exploit even urban areas.
House martins build nests on outer walls of buildings under the eaves. Exceptionally they can be found inside roofs or in sheds. They are colonial nesters, with an average group size of four to five nests, although large colonies with groups of tens or even hundreds of nests are sometimes reported. Town colonies tend to be smaller than countryside ones.
The breeding season runs from May to August when insects are abundant, but some chicks are still in the nest in September. The nest is made of pellets of mud mixed with grass, lined with feathers and vegetable fibre. A new nest is completed in one to two weeks, while repairs to an existing one normally take only a few days.
Four or five white eggs are laid at daily intervals, sometimes delayed by bad weather. Both sexes incubate for 14-16 days, and the chicks hatch together. The female broods them for a week, while they are naked and unable to maintain their own body temperature. Both parents feed them.
Being totally dependent on flying insects, extended periods of bad weather seriously affect nestling growth and survival. In years of cold wet weather food shortage is a major cause of mortality. However, nestlings can survive for a few days without food by drawing on their large fat reserves and by going into torpor, a kind of hibernation, which reduces their energy requirement to a minimum.
The chicks leave the nest at 22-23 days, depending on the brood size and weather. Once fledged, the young return to the nest to roost and to be fed for several days and can remain in the colony for several more weeks before they disperse to join pre-migratory flocks.
House martins are frequently double brooded and three broods are not uncommon. Fledged young from first broods often help their parents feed a second brood.
Colonies are traditional and nests are usually occupied from one year to the next, although rarely by the same two birds. House martins breed when one year old. Males often return to the colony they fledged from or close by, while females tend to settle several kilometres away.
They are short-lived, and most birds only breed for one year, though a few can have five or six breeding seasons. The oldest known wild bird was 14 years 6 months old.