Complex sentence: The birds called Swiftlets or Cave Swiftlets are contained within the genus "Aerodramus" (formerly "Collocalia") and it contains around thirty species mostly confined to southern Asia and south Pacific islands and north eastern Australia all within the tropical and subtropical regions.
Split into simpler sentences: The birds called Swiftlets or Cave Swiftlets are contained within the genus "Aerodramus" (formerly "Collocalia"). It contains around thirty species mostly confined to southern Asia and south Pacific islands and north eastern Australia all within the tropical and subtropical regions.