The Caprimulgiformes were a traditional order of birds, which are nocturnal. Here they are found, not to be most closely related to one another, but to other birds. The relationships of the frogmouths and the oilbirds are not resolved, but the owlet-nigntjars are closest to swifts, crested swifts and hummingbirds. Nocturnal behaviour seems to have evolved several times.