Swifts and swiftlets are among the most aerial of birds, feeding on flying insects and landing only to nest. Although superficially similar to swallows and martins (Hirundinidae) they are not closely related: swifts are not passerines (perching birds) and are more closely related to hummingbirds (Trochilidae). Swifts have long, curved wings and small weak legs and feet (the scientific name of the family is derived from the Greek meaning "footless"). They nest in elevated positions on cliffs and buildings; swiftlets build edible nests of saliva that are harvested in parts of Asia for "bird's nest soup". Representatives of the family are found throughout the world.