Number and colour composition of nest lining feathers predict eggshell bacterial community in barn swallow nests: an experimental study

P>1. The use of feathers as nest lining material has traditionally been explained by the thermoregulatory properties of feathers. Feather nest lining could additionally affect nest detectability by predators, or play a role in a sexually selected context. Furthermore, feather nest lining harbours microorganisms that may influence environmental conditions where eggs and nestlings develop. 2. Microorganisms growing on nest lining feathers could affect the bacterial load of eggshells because they occupy space and/or produce antimicrobial substances against other bacteria, including egg pathogens. Feathers of different colours are known to differ in their bacterial community (i.e. feather degrading bacteria) and, thus, colour composition of nest lining feather could also affect the bacterial environment of avian nests. 3. Here we tested this hypothesis in the barn shallow (Hirundo rustica) by exploring the relationship between eggshell bacterial loads and number of feathers, and the effect of experimentally modified colour composition of nest lining feathers on eggshell bacterial load. 4. In agreement with the hypothesis we found that, before treatment, the number of nest lining feathers (mainly that of unpigmented-white colour) predicted eggshell bacterial load, and that, at the end of the incubation period, eggshells of experimental nests with white feathers had a lower bacterial density than those in experimental nests with black feathers. 5. We failed to detect a relationship between bacterial load and hatching success. However, since evidence of that relationship exists for other species, these results would explain the previously detected experimental effect of colour composition of nest lining feathers on hatching success of swallows. 6. Nest design in general, and the use of nest-lining white feathers in particular, may therefore have important consequences for reproductive success of birds. The reduced eggshell bacterial loads of experimental white nests would explain preferences by barn swallows for feathers of white colour.