One way birds stay clean is by spreading around oil produced by the preen, or uropygial, gland. It’s thought that the oil is primarily used to preserve feathers, but previous studies of bird olfaction have also shown that some species communicate using the oil. Researchers have shown that some birds show a preference for certain oily smells, with some songbirds ditching the father of their chicks if a better smelling stud comes along.

— Read more at Birds Sniff Each Other’s Bacteria to Help Choose a Mate at Smithsonian.