Microbiome research is a hot topic. Exciting new studies have found links between the symbiotic microbes (bacteria, fungi, archaea and protists) that live naturally on animals’ bodies and animal health, fitness and even behaviour. Microbes can influence animal behaviour by contributing to or even altering animal body odour. In mammals, specialized scent glands harbour diverse bacterial communities, and these glands produce odorous substances used in territorial interactions like scent marking. Scent gland secretions can also communicate information about an animal’s sex, health, reproductive status and group membership.
In birds, it was long believed that their sense of smell was limited or nonexistent, and thus relatively unimportant. But a growing number of researchers have been working hard to disprove this myth. All bird species tested to date have a fully functional sense of smell, and they use smell in a variety of ways, from finding food and avoiding predators to communicating with members of their own and even other species.
Most birds secrete preen oil – a major contributor to bird body odour – from a specialized gland called the uropygial or preen gland. What’s more, birds pay attention to the odour cues available in preen oil. Studies show that birds can distinguish species, sex, individual identity and genetic quality and compatibility just by smelling preen oil!
Like mammalian scent glands, the preen gland harbours diverse bacterial communities that could play a role in shaping preen oil odour and, consequently, avian behaviour. From our prior work, we know that there are sex and population differences in the chemical composition of song sparrow (Melospiza melodia) preen oil. We hypothesized that these sex and population differences in preen oil reflect underlying differences in preen gland bacteria.
To explore this, we swabbed the preen glands of approximately 150 free-living adult song sparrows from three populations in Ontario, Canada for which we had preen oil chemical data. Using next-generation techniques, we sequenced an evolutionarily conserved region of the bacterial genome, the V4 region of the bacterial 16S rRNA gene. This allowed us to characterize the preen gland microbial communities of song sparrows to the genus level. Then, we compared and contrasted the microbial communities of male and female birds sampled from the three populations. Finally, we tested whether the chemical composition of song sparrow preen oil was significantly correlated with the community composition of their preen gland bacteria.
So what did we find? Consistent with other studies in birds, the preen gland of song sparrows harboured diverse bacterial communities. We found 44 unique sequence variants from at least 27 bacterial genera. As with our preen oil chemical data, there were significant population differences in preen gland bacteria. However, we did not find sex differences in preen gland bacteria like we did for preen oil chemicals. Overall, we did not find any correlation between the chemical composition of preen oil and preen gland bacterial communities.
Most of the genera we identified from song sparrows’ preen glands have been previously identified from birds, but some species host environmentally- or even species-specific bacteria. Five of the genera we identified in song sparrows have not been previously reported from the preen gland or feathers of other bird species. As avian microbiome data continue to build, we will ultimately get a better picture of the true diversity of their symbiotic microbes, and the potential importance of specialized or host-specific symbionts.
The population differences we found were driven by certain subsets of bacteria that were more common in some populations than others. The most urban population we sampled differed from the more rural populations along one axis of variation (based on a principal component analysis). This suggests that symbiotic bacteria could vary along an urban/rural gradient, but more studies are needed to test this.
Previous research on sex differences in symbiotic bacteria is mixed – some studies find evidence of sex differences and others do not. Microbes can be transferred socially (horizontal transmission) between individuals who interact closely, so it is possible that mated pairs of birds share similar bacteria, leading to a lack of sex differences. However, it is also possible that differences in life history, such as male/female differences in territory maintenance and parental care, could maintain sex differences. In our view, understanding the factors underlying sex differences, or lack thereof, in symbiotic microbial communities is an interesting area for future research.
Our main hypothesis was that sex and population differences in preen oil reflect underlying differences in preen gland bacteria. So, we expected to find a significant correlation between preen oil chemicals and preen gland bacteria. Contrary to our expectation, we did not find an overall correlation. Preen oil serves multiple functions in birds, including waterproofing and feather maintenance, and producing odour cues used in social communication and mate choice. Given the multiple functions of preen oil, it may be that bacteria associated with the preen gland also serve multiple and diverse functions. If true, this could explain the lack of an overall correlation between preen oil and gland bacteria. Intriguingly, we did identify a subset of bacteria that maximized correlations between preen oil and preen gland bacteria. This subset included two genera – Ralstonia and Bacillus – that contain odour-producing species. This means that these genera are promising candidates for future research investigating whether subsets of preen gland bacteria are involved in shaping preen oil composition, thereby affecting bird odour, and potentially behaviour.