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 bypredators, or play a role in a sexually selected context. Furthermore, feather nest lining harboursmicroorganisms that may influence environmental conditions where eggs and nestlings develop.2. Microorganisms growing on nest lining feathers could affect the bacterial load of eggshellsbecause 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 feathercould 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 experimentallymodified 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 liningfeathers (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 alower bacterial density than those in experimental nests with black feathers.5. We failed to detect a relationship between bacterial load and hatching success. However, sinceevidence of that relationship exists for other species, these results would explain the previouslydetected experimental effect of colour composition of nest lining feathers on hatching success ofswallows.6. Nest design in general, and the use of nest-lining white feathers in particular, may thereforehave important consequences for reproductive success of birds. The reduced eggshell bacterialloads of experimental white nests would explain preferences by barn swallows for feathers ofwhite colour.
Peralta-Sánchez et al. (Tue,) studied this question.