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.
The common house martin is the largest bird of the three, 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.
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.

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.

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.
The variable plumages of the South Asian species and a confused taxonomic history has left their distribution ranges in doubt.
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. The common house martin is the largest bird of the three, 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. 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.
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.
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. The variable plumages of the South Asian species and a confused taxonomic history has left their distribution ranges in doubt.
Delichon