House Sparrows Passer domesticus with larger uropygial glands show reduced feather wear

Preen oil is an oleaginous secretion that birds spread onto their plumage when preening. It is secreted by the uropygial (or preen) gland, a holocrine complex that is unique to birds and located in the integument of the rump (Clark 2004). The functions of the uropygial gland are still disputed: it may be involved in sexual communication, through pheromone production (Hirao et al. 2009), and through coloration of the feathers and skin (review in Delhey et al. 2007), as well as in protection against bacteria on the shell surface of eggs (Soler et al. 2008). However, the most widely accepted function is maintenance of the plumage. Several studies have shown that feathers deteriorate when the uropygial gland is experimentally removed (Elder 1954, Jacob & Ziswiler 1982, Moyer et al. 2003). In addition, preen oil has an antimicrobial activity that inhibits the growth of featherdegrading bacteria (Shawkey et al. 2003, Reneerkens et al. 2008, Ruiz-Rodríguez et al. 2009), as well as having insecticidal effects against chewing lice (order Phthiraptera; Moyer et al. 2003, Moreno-Rueda 2010). Moreover, the uropygial gland favours the establishment of feather mites (Acari, suborder Astigmata), which probably improve feather condition by feeding on microbes and dirt trapped in the uropygial secretions (Galván et al. 2008). A protective function against feather degradation caused by ultraviolet radiation has also been proposed (Reneerkens & Korsten 2004), but not demonstrated (Surmacki 2008). Recent studies have emphasized intraspecific variation in uropygial gland size and the associated consequences for bird fitness. The size of the uropygial gland is positively correlated with the quantity of secretion produced (Elder 1954, Martín-Vivaldi et al. 2009). Therefore, individuals investing more in the uropygial gland may gain a number of benefits, such as reduced loads of chewing lice and feather-degrading bacteria (Møller et al. 2009), increased feather-mite load (Galván et al. 2008), or more attractive sexually selected traits, such as larger white wing-bars in House Sparrows Passer domesticus (Moreno-Rueda 2010). Moreover, if uropygial oil improves feather flexibility, individuals with larger uropygial glands would suffer less feather wear by abrasion. To test whether birds investing more in uropygial gland size have plumage in better condition, I assessed whether the size of the uropygial gland is correlated with feather wear in the House Sparrow. In a previous study (Moreno-Rueda 2010) I found that uropygial gland size is positively correlated with body condition, and Sparrows with larger uropygial glands showed fewer feather holes (presumably caused by chewing lice). However, the relationship between uropygial gland size and feather wear has not been investigated previously.