Ultraviolet (UV) reflectance of the plumage is common in birds and plays an important role in sexual signalling. Recently, it has been proposed that birds are able to modify plumage UV reflectance by the application of uropygial gland secretion.
Delhey, K., Peters, A., Biedermann, P., & Kempenaers, B. (2008). Optical properties of the uropygial gland secretion: no evidence for UV cosmetics in birds. Naturwissenschaften, 95(10), 939 - 946. https://doi.org/10.1007/s00114-008-0406-8