About Aerodramus brevirostris (Horsfield, 1840)
This 13 to 14 centimeter long swiftlet has swept-back wings shaped like a crescent or a boomerang. It has a slender body and a forked tail. Like most typical swifts, it has narrow wings built for fast flight, plus a wide gape, a small beak surrounded by bristles, all adapted to catch insects mid-flight. Its legs are very short: this prevents the bird from perching, but lets it cling to vertical surfaces. Its upperparts are mainly grey-brown, while its underparts are paler brown. It has a pale grey rump and a pale patch above and behind its bill. The sexes look similar, but juveniles have a less distinct pale rump. There are five subspecies, which differ mostly in the tone of their rumps. The nominate subspecies, A. b. brevirostris, breeds across the Himalayas eastward to Bangladesh, Myanmar, and Thailand. It is an altitudinal migrant: it breeds at elevations up to above 4,500 meters, and winters at elevations between 900 and 2750 meters. A. b. innominata breeds in central China and winters in southwestern Thailand and the Malay Peninsula; its rump is slightly darker grey than that of the nominate A. b. brevirostris. A. b. inopina breeds in southwestern China, and it is the darkest-rumped subspecies. A. b. rogersi, also called the Indochinese swiftlet, breeds in eastern Myanmar, western Thailand, and Laos; it is a small subspecies with a pale rump. A. b. vulcanorum, also called the volcano swiftlet, breeds on volcanic peaks in Java, Indonesia. It has dark underparts and an indistinct pale grey rump. Across most of this species' range, it is the only swift species present. However, in the southern part of its breeding range and across most of its wintering range, it can be very hard to tell this species apart from other Collocalia swiftlets.
