Many birds migrate here in winter including swiftlets, and alpine swift. The names refer to the main type of nests produced by swiftlets in each cave. Seychelles swiftlets, Seychelles bulbuls and Seychelles sunbirds. The nests of "Aerodramus" swiftlets are constructed with saliva as a major component. The genus "Aerodramus" was thought to be the only echolocating swiftlets. Empty shop lots were later converted to become shelter for swiftlets. These swiftlets nest in the buildings and their nests are then harvested for sale. The famous marble and limestone cliffs represents the nestling ground of the swiftlets. Some animals use echolocation and these include swiftlets, bats and river dolphins. The more than 100 caves also hold several species of bats and swiftlets. Most "Aerodramus" swiftlets live in the tropical Indo-Pacific region and do not migrate. Collocalia is a genus of swifts, containing some of the smaller species termed "swiftlets". The Moluccan swiftlets ("Aerodramus infuscatus") is a swift in the family Apodidae. "Aerodramus" swiftlets are aerial insectivores, which take prey like flies on the wing. These swiftlets can pose major identification problems where several species occur.