Cohabitation with farm animals rather than breeding effort increases the infection with feather‐associated bacteria in the barn swallow Hirundo rustica

Feather‐associated bacteria are widespread inhabitants of avian plumage. However, the determinants of the between‐individual variation in plumage bacterial loads are less well understood. Infection intensities can be determined by ecological factors, such as breeding habitat, and can be actively regulated by hosts via preening. Preening, yet, is a resource intensive activity, and thus might be traded‐off against reproductive investment in breeding birds. Here, we studied barn swallows Hirundo rustica to assess the bacterial cost of reproduction in relation to nesting site micro‐habitats. Barn swallows prefer to breed in the company of large‐sized farm animals, although the presence of mammalian livestock in barns assures a warm and humid micro‐climate that favours bacterial proliferation. Thus, we experimentally manipulated brood sizes of birds breeding in barns with, or without, farm animals and measured total cultivable bacteria (TCB) and feather‐degrading bacteria (FDB) from the plumage. We found that the abundance of feather‐associated bacteria (i.e. both TCB and FDB) in females, but not males, breeding in barns with livestock were significantly higher than in conspecifics breeding in empty barns. Plumage bacterial loads, however, were not affected by brood size manipulations in either sex. In addition, we report a negative relationship between both TCB and FDB and hatching date in females, and several sex and seasonal differences in plumage bacterial abundances. Our study is the first to show that breeding micro‐habitat (i.e. livestock co‐tenancy) has consequences for the abundance of feather‐associated bacteria.