structural:reformatting, information:classification
Taxonomy

Taxonomists have long classified swifts and treeswifts as relatives of the hummingbirds, a judgment corroborated by the discovery of the Jungornithidae (apparently swift-like hummingbird-relatives) and of primitive hummingbirds such as Eurotrochilus. Traditional taxonomies place the hummingbird family (Trochilidae) in the same order as the swifts and treeswifts (and no other birds); the Sibley-Ahlquist taxonomy treated this group as a superorder in which the swift order was called Trochiliformes.

The taxonomy of the swifts is complicated, with genus and species boundaries widely disputed, especially amongst the swiftlets. Analysis of behavior and vocalizations is complicated by common parallel evolution, while analyses of different morphological traits and of various DNA sequences have yielded equivocal and partly contradictory results.

The Apodiformes diversified during the Eocene, at the end of which the extant families were present; fossil genera are known from all over temperate Europe, between today's Denmark and France, such as the primitive swift-like Scaniacypselus (Early–Middle Eocene) and the more modern Procypseloides (Late Eocene/Early Oligocene – Early Miocene). A prehistoric genus sometimes assigned to the swifts, Primapus (Early Eocene of England), might also be a more distant ancestor.