Wikipedia
Aerodramus is a genus of small, dark, cave-nesting birds in the Collocaliini tribe of the swift family. Its members are confined to tropical and subtropical regions in southern Asia, Oceania and northeastern Australia. Many of its members were formerly classified in Collocalia, but were first placed in a separate genus by American ornithologist Harry Church Oberholser in 1906.
This is a taxonomically difficult group of very similar species. Echolocation, DNA sequencing and parasitic lice have all been used to establish relationships, but some problems, such as the placement of the Papuan swiftlet are not fully resolved. These swiftlets can pose major identification problems where several species occur.
What distinguishes Aerodramus swiftlets from other swifts, and indeed almost all other birds, is their ability to use a simple but effective form of echolocation. This enables them to navigate within the breeding and roosting caves.
The nests of Aerodramus swiftlets are constructed with saliva as a major component. In two species, saliva is the only material used, and the nests are collected for the famous Chinese delicacy ' bird's nest soup', the over-collection of which puts pressure on the swiftlet populations.