Uropygium

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 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.

Pygostyles probably began to evolve very early in the Cretaceous period, perhaps 140-130 million years ago. The earliest known species to have evolved a pygostyle were members of the Confuciusornithidae. The structure provided an evolutionary advantage, as a completely mobile tail as found in species such as Archaeopteryx is detrimental to its use for flight control. Modern birds still develop longer caudal vertebrae in their embryonic state, which later fuse to form a pygostyle.

There are two main types of pygostyle: one, found in Confuciusornithidae, Enantiornithes, and some other Mesozoic birds, as well as in some oviraptorosaurs like Nomingia, is long and rod- or dagger blade-like. None of the known fossils with such pygostyles show traces of well-developed rectrices. The tail feathers in these animals consisted of downy fuzz and sometimes 2-4 central "streamers" such as those found in some specimens of Confuciusornis or in Paraprotopteryx.

...
Wikipedia