uropygial

a. [ See Uropygium. ] (Anat.) Of or pertaining to the uropygium, or prominence at the base of the tail feathers, in birds. [ 1913 Webster ]

Uropygial gland, a peculiar sebaceous gland at the base of the tail feathers in most birds. It secretes an oily fluid which is spread over the feathers by preening. [ 1913 Webster ]