EPOW - Ecology Picture of the Week

The Tiniest Uropygial

Anna's Hummingbird (Calypte anna), Family Trochilidae
Bodega Bay, California

Explanation:  The hummingbird has the tiniest uropygial of all animals.

Umm, it's not what you think.  A uropygial is a gland found at the base of the tail in birds.  It secretes bodily oils that birds pinch out with their beaks -- such as being demonstrated in this week's main photo, above, by an Anna's Hummingbird.  

The bird then coats its feathers with the oil.  But why?

Technically, the uropygial is a bilobed holocrinc gland and its secretions serve, in some bird species, to waterproof the feathers.  Some sources also suggest that it manufactures Vitamin D precursors; it keeps the skin, bill, and feathers supple and healthy; and even has antibacterial properties.  

Research also suggests that the oil of the gland wards off feather mites in house sparrows (Pap et al. 2010).

Some birds may see in ultraviolet light, and the oils from the uropygial gland might help birds to identify the sex of others of their species or to indentify individuals.  However, some studies have suggested, at best, the oils from the gland may have evolved to not interfere with ultraviolet signals of avian plumages, rather than to enhance them (Delhey et al. 2008).  

Regardless, when you next see a bird preening, watch carefully for when it dabs its bill with uropygial oil from the gland at the base of its tail.  Now you know what it is doing.
   
 

Information:
     Delhey, K., A. Peters, P.H. Biedermann, and B. Kampenaers.  2008.  Optical properties of the uropygial gland secretion: no evidence for UV cosmetics in birds.  Naturwissenschaften 95(10):939-946.
     Pap, P.L., C.I. Vágási, G. Osváth, C. Muresan, and Z. Barta.  Seasonality in the uropygial gland size and feather mite abundance in house sparrows Passer domesticus: natural covariation and an experiment.  Journal of Avian Biology 41(6):653-661.
     Soler, J.J., M. Martin-Vivaldi, M. Ruiz-Rodríguez, E. Valdivia, A.M. Martin-Platero, M. Martínez-Bueno, J. M. Peralta-Sánchez, and M. Méndez.  2008.  Symbiotic association between hoopoes and antibiotic-producing bacteria that live in their uropygial gland.  Functional Ecology 22(5):864-871.