Pygostyle describes a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component of the uropygium, a structure colloquially known as the bishop's nose, parson's nose, pope's nose, or sultan's nose. This is the fleshy protuberance visible at the posterior end of a bird (most commonly a chicken or turkey) that has been dressed for cooking. It has a swollen appearance because it also contains the uropygial gland that produces preen oil.

Yixianornis grabaui, one of the oldest known species with a pygostyle like that in living birds

The other pygostyle type is plowshare-shaped. It is found in Ornithurae (living birds and their closest relatives), and in almost all flying species is associated with an array of well-developed rectrices used in maneuvering. The central pair of these attach directly to the pygostyle, just as in Confuciusornis. The other rectrices of Ornithurae are held in place and moved by structures called bulbi rectricium (rectricial bulbs), a complex feature of fat and muscles located on either side of the pygostyle. The oldest known species with such a pygostyle is Hongshanornis longicresta.[3]

As evidenced by the oviraptorosaurian cases, the pygostyle evolved at least twice, and rod-shaped pygostyles seem to have evolved several times, in association with shortening of the tail but not necessarily with a retractable fan of tail feathers. In other words, the pygostyles of oviraptorosaurs and Confuciusornis were likely weight-saving measures, and the specialized "true" pygostyles of ornithurans were adapted from these later to improve flight performance.[1]

The bird clade Pygostylia was named in 1996, by Luis Chiappe, for the presence of this feature and roughly corresponds to its appearance in the bird family tree, though the feature itself is not included in its definition.[4] In 2001, Jacques Gauthier and Kevin de Queiroz (2001) re-defined Pygostylia to refer specifically to the apomorphy of a short tail bearing an avian pygostyle.