<P> The common house martin was originally a cliff and cave nester, and some cliff - nesting colonies still exist, with the nests built below an overhanging rock . It now largely uses human structures such as bridges and houses . Unlike the barn swallow, it uses the outside of inhabited buildings, rather than the inside of barns or stables . The nests are built at the junction of a vertical surface and an overhang, such as on house eaves, so that they may be strengthened by attachment to both planes . </P> <P> Breeding birds return to Europe between April and May, and nest building starts between late March in North Africa and mid-June in Lapland . The nest is a neat closed convex cup fixed below a suitable ledge, with a narrow opening at the top . It is constructed by both sexes with mud pellets collected in their beaks, and lined with grasses, hair or other soft materials . The mud, added in successive layers, is collected from ponds, streams or puddles . House sparrows frequently attempt to take over the nest during construction, with the house martins rebuilding elsewhere if they are successful . The entrance at the top of the cup is so small once it is complete that sparrows cannot take over the nest . </P> <P> The common house martin tends to breed colonially, and nests may be built in contact with each other . A colony size of less than 10 nests is typical, but there are records of colonies with thousands of nests . Four or five white eggs are usually laid, which average 1.9 cm × 1.33 cm (0.75 in × 0.52 in) in size, and weigh 1.7 g (0.060 oz). The female does most of the incubation, which normally lasts 14--16 days . The newly hatched chicks are altricial, and after a further 22--32 days, depending on weather, the chicks leave the nest . The fledged young stay with, and are fed by, the parents for about a week after leaving the nest . Occasionally, first - year birds from the first brood will assist in feeding the second brood . </P> <P> There are normally two broods each year, the nest being reused for the second brood, and repaired and used again in subsequent years . Hatching success is 90%, and fledging survival 60--80% . Third broods are not uncommon, though late nestlings are often left to starve . The average annual mortality for adults nesting in the Western Palaearctic is 40--60%, with most deaths outside the breeding season . A study of British breeders gave an average adult survival rate of just under 40%, but ranging from 25% to 70% . Rainfall in the African wintering grounds is a major factor in adult survival, although wet weather in the breeding areas has very little effect . Although individuals aged 10 and 14 years have been recorded, most survive less than five years . For weeks after leaving the nest the young congregate in ever - increasing flocks which, as the season advances, may be seen gathering in trees or on housetops, or on the wires with swallows . By the end of October, most martins have left their breeding areas in western and central Europe, though late birds in November and December are not uncommon, and further south migration finishes later anyway . </P>

When do house martin chicks leave the nest
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