From The Art and Popular Culture Encyclopedia
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. With nearly 450 species identified to date, they are the most diverse order of birds after the passerines.
As their name ("footless" in Latin) 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 as well as 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).