Birds have a slightly different way of doing it but it still uses sunlight. https://en.wikipedia.org/wiki/Uropygial_gland#Feather_and_body_integrity https://en.

Birds have a slightly different way of doing it but it still uses sunlight. https://en.wikipedia.org/wiki/Uropygial_gland#Feather_and_body_integrity https://en.wikipedia.org/wiki/Uropygial_gland#Feather_and_bo... > It has also been speculated that in some species, preen oil contains a precursor of vitamin D; this precursor is converted to vitamin D by the action of sunlight and then absorbed through the skin.