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Among the substances produced by birds are the secretions of the uropygial gland, which may be pigmented orange, red or yellow.

The uropygial or preen gland is found in the majority of birds and is situated near the base of the tail.

They also found evidence that the birds transferred carotenoids from their uropygial gland to their feathers by rubbing their head on their neck, breast and back feathers.

They then looked for the pigments that may tinge the plumage both in the secretions of the uropygial gland and on the surface of feathers.