Traditionally, the bird order Apodiformes contained three living families: the swifts, Apodidae, the tree swifts, Hemiprocnidae, and the hummingbirds, Trochilidae. In the Sibley-Ahlquist taxonomy, this order is raised to a superorder Apodimorphae in which hummingbirds are separated as a new order, Trochiliformes, but this is refuted by subsequent research. With nearly 450 species identified to date, they are the most diverse order of birds after the passerines.
As their name ("footless") suggests, their legs are small and have limited function aside from perching. The feet are covered with bare skin rather than the scales (scutes) that other birds have. Another shared characteristic is long wings with short, stout humerus bones (Hyman 1992). The evolution of these wing characteristics has provided the hummingbird with ideal wings for hovering (Mayr 2002).
The hummingbirds, swifts and crested swifts share other anatomical similarities with one another. They also have similarities (notably as to the skull) with their probable closest living relatives, the owlet-nightjars (Mayr, 2002). The owlet-nightjars are apparently convergent with the closely related Caprimulgiformes, which form a clade Cypselomorphae with the Apodiformes (Mayr, 2002).
Apodiformes have evolved on the Northern Hemisphere. Eocypselus, a primitive genus known from the Late Paleocene or Early Eocene of north-central Europe, is somewhat difficult to assign; Dyke et al. (2004) consider it a primitive hemiprocnid. but most researchers believe that at present this genus cannot be unequivocally assigned to the Apodiformes or the Caprimulgiformes. The Early Eocene Primapus, found in England, is similar to both a primitive swift and the aegialornithids, which are in some aspects intermediate between swifts and owlet-nightjars. Fossil swifts are known to have existed during that time in Europe, and aegialornithids were possibly present in North America. By the late Eocene (around 35 MYA), primitive hummingbirds started to diverge from the related jungornithids; the Middle Eocene Parargornis (Messel, Germany) and the Late Eocene Argornis, found in today's southernmost Russia, belong to this lineage. Cypselavus (Late Eocene – Early Oligocene of Quercy, France) was either a primitive hemiprocnid or an aegialornithid.
The placement of the Aegialornithidae is not quite clear. Various analyses place them sufficiently close to the Apodiformes to be included here, or into the unique owlet-nightjar lineage in the Cypselomorphae.