Delichon is a small genus of passerine birds that belongs to the swallow family and contains four species called house martins.

None of the species are considered threatened, although widespread reductions in common house martin numbers have been reported from central and northern Europe.

DNA studies suggest that there are three major groupings within the Hirundininae, broadly correlating with the type of nest built.

[2] The genetic evidence suggests a close relationship between Hirundo and Delichon, which is further supported by the frequency of interbreeding between two widespread species, the barn swallow and the common house martin, despite being their being in different genera.

The common house martin is a widespread migrant breeder across Europe, north Africa and all northern temperate Asia to Kamchatka.

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,[12] but the races breeding in the Himalayas and Taiwan may just move from the high mountains to lower altitudes.

[14] 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.

Moult normally starts on arrival at the wintering grounds, but overlaps with the breeding season for the non-migratory Nepal house martin.

Four species of the genus Tachycineta have white rumps and underparts, but they have bright metallic green or blue-green upperparts, longer tails, and are restricted to Central and South America.

[19] The variable plumages of the South Asian species and a confused taxonomic history has left their distribution ranges in doubt.

[20][21][22] David Winkler and Frederick Sheldon believe that evolutionary development in the mud-building swallows, and individual species follow this order of construction.

A retort builder like red-rumped swallow starts with an open cup, closes it, and then builds the entrance tunnel.

Winkler and Sheldon propose that the development of closed nests reduced competition between males for copulations with the females.

It is believed that this reduces inter-specific food competition, particularly with the barn swallow which shares much of the breeding and wintering range of the martins.

[34] The genus also hosts endoparasites such as Haemoproteus prognei (avian malaria), which are transmitted by blood-sucking insects including mosquitoes.

A species is assessed as subject to varying levels of threat if it has a small, fragmented or declining range, or if the total population is less than 10,000 mature individuals, or if numbers have dropped rapidly (by more than 10% in ten years or three generations).

[12][40] The lowland breeding common house martin has greatly benefited from forest clearance, creating the open habitats it prefers, and from human habitation which has given it an abundance of safe man-made nest sites,[12] although widespread declines in its numbers have been reported from central and northern Europe since 1970.

Despite this, the huge geographical range and large numbers of the two northern house martins mean that their global status is secure.