The evolution of size of the uropygial gland: mutualistic feather mites and uropygial secretion reduce bacterial loads of eggshells and hatching failures of European birds

Potentially, pathogenic bacteria are one of the main infective agents against
which a battery of chemical and physical barriers has evolved in animals.
Among these are the secretions by the exocrine uropygial gland in birds. The
antimicrobial properties of uropygial secretions may prevent colonization and
growth of microorganisms on feathers, skin and eggshells. However, uropygial
gland secretions also favour the proliferation of feather mites that feed on
secretions and microorganisms living on feathers that would otherwise reach
eggshells during incubation if not consumed by feather mites. Therefore, at the
interspecific level, uropygial gland size (as an index of volume of uropygial
secretion) should be positively related to eggshell bacterial load (i.e. the risk of
egg infection), whereas eggshell bacterial loads may be negatively related to
abundance of feather mites eating bacteria. Here, we explore these previously
untested predictions in a comparative framework using information on
eggshell bacterial loads, uropygial gland size, diversity and abundance of
feather mites and hatching success of 22 species of birds. The size of the
uropygial gland was positively related to eggshell bacterial loads (mesophilic
bacteria and Enterobacteriaceae), and bird species with higher diversity and
abundance of feather mites harboured lower bacterial density on their
eggshells (Enterococcus and Staphylococcus), in accordance with the hypothesis.
Importantly, eggshell bacterial loads of mesophilic bacteria, Enterococcus and
Enterobacteriaceae were negatively associated with hatching success, allowing
us to interpret these interspecific relationships in a functional scenario, where
both uropygial glands and mutualistic feather mites independently reduce the
negative effects of pathogenic bacteria on avian fitness