Experimental old nest material predicts hoopoe (Upupa epops) eggshell and uropygial gland microbiota

Nest re-use in birds has the potential cost of infection by parasites and pathogens but may also be a source of beneficial symbiotic bacteria transmitted horizontally. Eurasian hoopoes (Upupa epops) host antibiotic-producing bacteria in their uropygial gland, but only while breeding, which suggests that the nest-hole may be a source of those symbionts. Interestingly, hoopoes do not build nest, thus might prefer for reproduction nest holes with soft materials from previous reproductions. Here, we tested experimentally this preference by installing in the field new nest boxes that were left empty or filled with either sawdust or a mixture of sawdust and hoopoe´s nest material from the previous year. We explored the experimental effect on the composition of the uropygial secretion bacterial community, on eggshell bacteria loads, and on several proxies of reproductive success. Hoopoes bred significantly more often in nest boxes with nest material than in empty ones, but the type of nest mat...