Delichon

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.

Taxonomy

The four Delichon species are members of the swallow family of birds, and are classed as members of the Hirundininae subfamily which comprises all swallows and martins except the very distinctive river martins. DNA studies suggest that there are three major groupings within the Hirundininae, broadly correlating with the type of nest built.[1] The groups are the "core martins" including burrowing species like the sand martin, the "nest-adopters", which are birds like the tree swallow that utilise natural cavities, and the "mud nest builders". The Delichon species construct a closed mud nest and therefore belong to the latter group; they appear to be intermediate between the Hirundo and Ptyonoprogne species that make open cup nests, and the Cecropis and Petrochelidon swallows, which have retort-like closed nests with an entrance tunnel.[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.[3] The suggested taxonomic sequence of the mud-building swallows has been recommended by at least two European taxonomic committees.[4][5]

The genus Delichon was created by American naturalist Thomas Horsfield and British entomologist Frederic Moore in 1854 to accommodate the Nepal house martin that was first described by Moore in the same year, and is therefore the type species for the genus.[6][7] The two other house martins were moved to Delichon from the genus Chelidon in which they had been placed up to that time.[8] In 2021, the Siberian house martin was split from the common house martin based on morphological and vocal differences.[9][10] The name of the genus, "Delichon", is an anagram of the Ancient Greek term χελιδὡν/chelidôn, meaning swallow.[11]

Description

Delichon martins are 13–15 cm (5–6 in) long, blackish blue above with a contrasting white rump, and with white or grey underparts. They are chunky, bull-headed and short-tailed birds, and have feathering on the toes and tarsi.[17] The common house martin is the largest bird, with an average weight of 18.3 g (0.65 oz), and has the most deeply forked tail; the Nepalese species is the smallest (15 g, 0.53 oz) and has the squarest tail. Distinctive species plumage features are the black chin and black undertail coverts of the Nepal house martin, and the greyish wash to the underparts of the Asian house martin.[12] As with other swallows and martins, the moult is slow and protracted because of the need to maintain efficient flight at all times to enable feeding. Moult normally starts on arrival at the wintering grounds, but overlaps with the breeding season for the non-migratory Nepal house martin.[18]

The Delichon martins have simple flight calls of one to three notes. In the two more widespread species these have a distinctive buzzing quality. The male's song is a short simple ripple, perhaps less musical than that given by other swallows.[12][16]

As a group, the house martins cannot easily be confused with any other swallows. 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.[16]