Preen gland microbiota of songbirds differ across populations but not sexes

The fermentation hypothesis for chemical recognition posits that symbiotic microbes living on vertebrate hosts may provide olfactory cues of identity, sex or other salient features. In birds, preen oil is a major source of body odour that differs between populations and sexes. We hypothesized that population and sex differences in preen oil chemistry reflect underlying differences in preen gland microbiota, predicting that these microbes also differ among populations and between the sexes. We also predicted that pairwise similarity in the community composition of preen gland microbiota would covary with that of preen oil chemical composition. We analyzed preen oil and characterized bacterial communities from preen gland swabs of song sparrows (Melospiza melodia), sampling birds at sites for which population and sex differences in preen oil have been reported, and at a third site that has been less studied. Consistent with prior work, we found population and sex differences in preen oil chemistry. By contrast, we found population differences but no sex differences in preen gland microbiota. Overall similarity in the community composition of preen gland microbiota did not significantly covary with that of preen oil chemistry, but we identified a subset of five microbial genera that maximally correlated with preen oil composition. Although both preen gland microbiota and preen oil composition differ across populations, we did not observe an overall association between them that would implicate symbiotic microbes in mediating variation in olfactory cues associated with preen oil. Instead, certain subsets of microbes may be involved in mediating olfactory cues, but experimental tests are required to better understand how symbiotic microbes may affect avian signaling and social behaviour.