Environmental factors associated with nesting habits and age shape the composition and connection between skin and uropygial gland microbiomes of birds

Bacterial communities on skin and feathers can act as a critical line of defence against pathogenic infections in birds and may originate from secretions produced by the uropygial gland. These secretions reach the bird integument during preening, with the preening effort possibly determining the connectivity between uropygial gland and integument microbiomes. The risk of pathogen infections depends on a number of variables, including environmental conditions (i.e. temperature and humidity), species identity, life-history traits (i.e. cavity vs. open-cup nesters) and life stage (i.e. age). Bacterial symbionts of the host, particularly those of the uropygial gland, may counter such pathogenic infections. We therefore hypothesise that bacterial communities of the uropygial gland differ among host species, age and nesting habits, with higher bacterial diversity in nestlings due to their immature immune system, and in cavity nesters due to potentially increased pathogen exposure. We examined this using 16S rRNA metabarcoding of bacterial communities of the uropygial secretion (N = 352) and uropygial gland skin (N = 339) of nestlings and adults of 26 bird species from 14 families in southern Spain. In accordance with the hypotheses, we find species-specific differences in bacterial communities of uropygial gland skin and secretion, as well as an effect of age, with nestlings showing a higher bacterial diversity, especially in the uropygial gland skin. Additionally, the microbiotas of cavity-nesting species are more diverse and heterogeneous than those of open nesters, with these effects more pronounced in adult and uropygial secretions. Finally, the uropygial gland is relatively larger in cavity- than in open-nester species, which suggests that cavity nesters preen more often than the open nesters. Moreover, we found a stronger sharing of secretion and skin microbes in cavity nesters and nestlings compared to adults and open nesters. Overall, our findings on the effects of age and nest type on structuring bird uropygial gland skin and secretion microbiota imply that age and pathogen risks related to nest environment could drive the external microbiome assembly in birds.