uropygial gland vs oil gland

uropygial gland and oil gland both are nouns.

As nouns, oil gland is a hypernym of uropygial gland; that is, oil gland is a word with a broader meaning than uropygial gland:

uropygial gland: oil-secreting gland situated at the base of the tail in most birds
oil gland: a gland that secretes oil

uropygial gland (noun)	oil gland (noun)
oil-secreting gland situated at the base of the tail in most birds	a gland that secretes oil