Potentially, pathogenic bacteria are one of the main infective agents againstwhich a battery of chemical and physical barriers has evolved in animals.Among these are the secretions by the exocrine uropygial gland in birds. Theantimicrobial properties of uropygial secretions may prevent colonization andgrowth of microorganisms on feathers, skin and eggshells. However, uropygialgland secretions also favour the proliferation of feather mites that feed onsecretions and microorganisms living on feathers that would otherwise reacheggshells during incubation if not consumed by feather mites. Therefore, at theinterspecific level, uropygial gland size (as an index of volume of uropygialsecretion) should be positively related to eggshell bacterial load (i.e. the risk ofegg infection), whereas eggshell bacterial loads may be negatively related toabundance of feather mites eating bacteria. Here, we explore these previouslyuntested predictions in a comparative framework using information oneggshell bacterial loads, uropygial gland size, diversity and abundance offeather mites and hatching success of 22 species of birds. The size of theuropygial gland was positively related to eggshell bacterial loads (mesophilicbacteria and Enterobacteriaceae), and bird species with higher diversity andabundance of feather mites harboured lower bacterial density on theireggshells (Enterococcus and Staphylococcus), in accordance with the hypothesis.Importantly, eggshell bacterial loads of mesophilic bacteria, Enterococcus andEnterobacteriaceae were negatively associated with hatching success, allowingus to interpret these interspecific relationships in a functional scenario, whereboth uropygial glands and mutualistic feather mites independently reduce thenegative effects of pathogenic bacteria on avian fitness
Soler et al. (Tue,) studied this question.