The swift family remains one of the more complicated groups of birds in taxonomic research, but the swiftlet tribe is a rather well-defined group. Its internal systematics is confusing; the plumage is usually dull, with shades of black, brown, and gray; from their outward appearance, most species are very similar. Swiftlets have four toes, except the Papuan swiftlet which lacks the hallux (back toe). Their legs are very short, preventing the birds from perching, but allowing them to cling to vertical surfaces. Flight is mainly gliding due to very long primary feathers and small breast muscles. The larger Aerodramus swiftlets weigh about 14 grams and are 10 cm long.
Swiftlets are insectivores; hymenopterans and dipterans being the most abundant prey (Lourie & Tompkins, 2000). Typically, they leave the cave during the day to forage and return to their roost at night. Males and females look similar; as usual in such cases, these birds are monogamous and both partners take part in caring for the nestlings. Males perform aerial displays to attract females and mating occurs at the nest. The breeding season overlaps the wet season, which corresponds to an increased insect population. Clutch size depends on the location and the food source, but it is generally not large; Aerodramus swiftlets lay 1-2 eggs. The eggs are a dull white color and are laid every other day. Many if not all species are colonial nesters; some build their nests in high, dark corners on cave walls. Swiftlets in temperate zones do migrate but, most Aerodramus swiftlets live in the tropical Indo-Pacific region and do not migrate. These birds usually remain in one cave or other roosting/nesting site. Some examples of caves include the Niah Caves, Gunung Mulu National Park, and Niah National Park which are all located in Sarawak, Malaysian Borneo.
The genus Aerodramus is of special interest due to its use of echolocation and their intricately constructed saliva nests which in some species contain no other material such as feathers, moss or twigs and are collected, selling at extremely high prices (see Bird's nest soup). Over the past twenty years, the high demand for the nests of some Aerodramus species has had an adverse effect on their populations (Hobbs, 2003; Marcone, 2005).
The use of echolocation was once used to separate Aerodramus from the non-echolocating genera Collocalia and Hydrochous (virtually nothing is known about Schoutedenapus). But recently, the Pygmy Swiftlet Collocalia troglodytes was discovered making similar clicking noises in and outside their cave (Price et al., 2004). Characteristics of behavior, such as what materials apart from saliva the nests contain, can be used to differentiate between certain species of Aerodramus (Lee et al., 1996).