The Cook's swift (Apus cooki), is a small bird, superficially similar to a house martin. It is, however, completely unrelated to those passerine species, since swifts are in the order Apodiformes. The resemblances between the groups are due to convergent evolution reflecting similar life styles. Cook's swifts breed in limestone caves of Thailand, Myanmar and Indochina. The species has a green iridescence, a shallow tail fork and is a short distance migrant. A 2011 study has many taxonomists splitting this species from the fork-tailed swift complex.