Delichon is a small genus of passerine birds that belongs to the swallow family and contains four species called house martins. These are chunky, bull-headed and short-tailed birds, blackish-blue above with a contrasting white rump, and with white or grey underparts. They have feathering on the toes and tarsi that is characteristic of this genus. The house martins are closely related to other swallows that build mud nests, particularly the Hirundo barn swallows. They breed only in Europe, Asia and the mountains of North Africa. Three species, the common, Siberian and Asian house martins, migrate south in winter, while the Nepal house martin is resident in the Himalayas year-round.
The house martins nest in colonies on cliffs or buildings, constructing feather- or grass-lined mud nests. The typical clutch is two or three white eggs; both parents build the nest, incubate the eggs and feed the chicks. These martins are aerial hunters of small insects such as flies and aphids. Despite their flying skills the Delichon martins are sometimes caught by fast-flying birds of prey. They may carry fleas or internal parasites. None of the species are considered threatened, although widespread reductions in common house martin numbers have been reported from central and northern Europe. This decline is due to factors including poor weather, poisoning by agricultural pesticides, lack of mud for nest building and competition with house sparrows for nest sites.
Delichon is an Old World genus with all four species breeding only in the Northern Hemisphere. The common house martin is a widespread migrant breeder across Europe, north Africa and all northern temperate Asia to Kamchatka. It winters in tropical Africa. The Siberian house martin breeds in northeast Russia and winters in southern Asia. The Asian house martin breeds further south than the Siberian house martin in the mountains of central and eastern Asia; its nominate subspecies winters in Southeast Asia, but the races breeding in the Himalayas and Taiwan may just move from the high mountains to lower altitudes. The Nepal house martin is resident in the mountains of southern Asia.
The preferred habitat of the common and Siberian house martins is open country with low vegetation, such as pasture, meadows and farmland, and preferably near water, although it is also found in mountains up to at least 2,200 metres (7,200 ft) altitude. As the name implies, they readily nest on man-made buildings, and will breed even in city centres if the air is clean enough. The other two species favour mountainous country (and sea cliffs in the case of Asian house martin); they use buildings as nest sites less frequently than their northern relative. The wintering grounds of the two migrant species include a range of open country and hilly habitats.