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Swiftlets are birds from the four genera Aerodramus, Collocalia, Hydrochous and Schoutedenapus, which form the tribe Collocaliini within the swift family Apodidae. The group contains around thirty species mostly confined to southern Asia, south Pacific islands, and northeastern Australia, all within the tropical and subtropical regions. They are in many respects typical members of the Apodidae, having narrow wings for fast flight, with a wide gape and small reduced beak surrounded by bristles for catching insects in flight. What distinguishes many but not all species from other swifts and indeed almost all other birds is their ability to use a simple but effective form of echolocation to navigate in total darkness through the chasms and shafts of the caves where they roost at night and breed. The nests of some species are built entirely from solidified threads of their saliva, which are edible and thus collected for human consumption as the famous Chinese delicacy, the bird's nest soup.