The taxonomy of the 15 to 20 nearly identical species is controversial. Dull brown or gray above (sometimes glossed with blue or green) and paler on the rump and underparts, swiftlets are 9 to 15 cm (3.5 to 6 inches) long. They fly lower, more erratically, and more slowly than most swifts.

The swiftlet is remarkable on two counts: the nest, made chiefly or entirely of saliva, is the basis of bird's-nest soup; and, with the oilbird (q.v.), certain swiftlets are the only birds known to use echolocation to find their way around dark caverns, as do bats. The swiftlet's “sonar” consists of clicking sounds at frequencies of 1,500 to 5,500 hertz—audible to the human ear. They are emitted at the rate of about six per second. The nest is a small bracket, sometimes containing bits of fern or bark, that may be glued to a tree or cliff but usually is made in a mountain or coastal cave. A single colony may contain a million birds.