The
uropygial gland, informally known as the
preen gland or the
oil gland, is a bilobate
sebaceous gland possessed by the majority of birds. It is located dorsally at the base of the tail (between the fourth
caudal vertebrae and the
pygostyle) and is greatly variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a for the preen oil (see below). It is a holocrine gland enclosed in a connective tissue capsule made up of glandular
acini that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most usually two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity. The gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla (nipple-like structure).