Uropygial gland

It is located dorsally at the base of the tail (between the fourth caudal vertebra and the pygostyle) and is greatly variable in both shape and size.

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 typically two.

[2] These include kiwis (Apterygidae), emu (Dromaiidae), ostriches (Struthionidae), rheas (Rheidae), cassowaries (Casuariidae), mesites (Mesitornithidae), bustards (Otididae), pigeons and doves (Columbidae), amazon parrots (Psittacidae), frogmouths (Podargidae), and woodpeckers (Picidae).

However, some types of diester waxes called uropygiols and containing hydroxyfatty acids and/or alkane-diols exist in the secretions of the uropygial gland of some groups of birds.

[9] Emperor Frederick II, in his thirteenth-century treatise on falconry, was seemingly the first to discuss the function of the uropygial gland of birds.

He believed that its product not only oiled the plumage but also provided a poison which was introduced by the claws of hawks and owls thus bringing quicker death to their prey.

[2][4] Anecdotal reports indicating that the waterproofing effect of the hydrophobic uropygiols might be increased by electrostatic charge to the oiled feather through the mechanical action of preening[12] are not supported by scientific studies.

[5] Similarly, there was no evidence to support a role of the gland secretion in attracting biting midges and black flies in blue tits (Cyanistes caeruleus) and feral pigeons (Columba livia).

During the breeding season, greater flamingos increase the frequency of their spreading uropygial secretions over their feathers and thereby enhance their colour.

The preen oil contained several odd-chain fatty acids, which suggests they may be derived from lipolysis by the uropygial gland and/or its microbiome.