The yellow bill colors of many hornbills are produced by preen gland secretions.
From the en.wikipedia.org
They have dense plumage with a large preen gland for waterproofing their feathers.
From the en.wikipedia.org
Birds protect their feathers by preening them with the secretions of the preen gland.
From the sciencedaily.com
All cormorants have preen gland secretions that are used ostensibly to keep the feathers waterproof.
From the en.wikipedia.org
The uropygial or preen gland is found in the majority of birds and is situated near the base of the tail.
From the sciencedaily.com
Unlike other ratites, they have a preen gland.
From the en.wikipedia.org
Like most other ratites, they have no preen gland.
From the en.wikipedia.org
The preen oil gland secretions, by which all aquatic birds make their feathers waterproof, support a natural mechanism that concentrates AIVs from water onto birds'bodies.
From the sciencedaily.com
An IU Bloomington researcher collects preen oil from a junco's uropygial gland into a small glass tube.