Sources of variation in uropygial gland size in European birds

Defence mechanisms against parasites and pathogens are some of the most elaborate biological systems in animals. The oily secretion of the avian uropygial gland has been suggested to serve as a chemical defence against feather and eggshell bacteria. Yet, the traits associated with uropygial gland oil production are not well understood. We conducted a phylogenetic analysis comprising 132 European bird species aiming to test: (1) whether life-history and ecological traits drive gland size evolution by potentially promoting microbial infestation and (2) how these traits affects change in the gland size throughout the annual cycle. We show that the size of the uropygial gland is dynamic (i.e. increasing from the nonbreeding to the breeding season, independent of sex). Furthermore, we found that the year-round size of the gland was similar between sexes and was correlated with different ecological and life-history traits promoting microbial infection throughout the annual cycle. During the breeding season, the total eggshell surface area in a clutch correlated significantly and positively with the gland size, suggesting the importance of oil in protecting eggs from microbes. Social species exhibited a larger gland size increase during the breeding season compared to nonsocials; a change that was also predicted by the total eggshell surface area. Aquatic, riparian and non-migratory species had larger glands than terrestrials and migrants, respectively. The findings of the present study suggest that aquatic environments may promote the production of gland oil, through either the need of waterproofing the plumage and/or defending it against the intensified feather degradation in these moist conditions. Finally, we found a negative effect of the incubation period on uropygial gland size, which may suggest an energetic constraint imposed by other development-connected costly activities. Our results show that the role of the uropygial gland dynamically varies during the annual cycle, potentially in response to seasonal variation in parasitic infection risk.(c) 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 543-563.