uropygial gland vs preen gland

uropygial gland and preen gland both are nouns.

As nouns, uropygial gland and preen gland are synonyms defined as:

uropygial gland and preen gland: oil-secreting gland situated at the base of the tail in most birds

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