When a bird preens its feathers, it uses a little of nature’s own pomade: an oil made by glands just above the tail. This oil helps clean and protect the bird’s plumage, but also contains a delicate bouquet of scents.
To other birds — potential mates or would-be rivals — these smells carry many messages, not unlike the birdsongs and fancy feathers that are more obvious to human observers. These scents may signal that a bird would be dangerous to encounter or might be ready to mate, or any number of other cues.
However, new research using dark-eyed juncos, a common North American bird, suggests that these odoriferous messages may not be entirely of the bird’s own making. In a study published last month in the Journal of Experimental Biology, biologists reported that microbes living peacefully on the birds’ oil glands may play an important role in making the scent molecules involved. That implies that the birds’ microbiomes may influence both the smell and the behavior it provokes in other birds.
Birds’ scented messages are the focus of the research of Danielle Whittaker, director of the Beacon Center for the Study of Evolution in Action at Michigan State University and an author of the paper. Some years ago, after she gave a talk, Kevin Theis, a colleague who studied scent-producing bacteria living on hyenas and who is a co-author of the new paper, asked her whether she had ever looked at the birds’ microbes.
“I had never thought about bacteria at all,” said Dr. Whittaker. “But all the compounds I was describing were known byproducts of bacterial metabolism.”
Dr. Whittaker took samples of bacteria living on the oil glands of 10 captive dark-eyed juncos and then injected the glands with an antibiotic. When she compared the microbes before and after the treatment, the results seemed to show that two groups of bacteria in particular had taken a hit from the treatment. Furthermore, when she compared the scent molecules in the oil before and after the treatment, there were significant differences.
For a follow-up experiment, she and her colleagues grew bacteria from untreated juncos in Petri dishes and checked to see what aroma molecules they were making.
“Every bacterium we looked at produced at least some of” the aromatic molecules, said Dr. Whittaker. The microbes living in individual juncos, if they worked together, were capable of making all the scent compounds she had been studying.
It isn’t clear whether the bacteria are the only source of the smell, or whether the birds produce some of the molecules as well. But the study opens up the intriguing possibility that birds’ smells may be linked to their microbiomes.
Dr. Whittaker has found in other studies that birds tend to have bacteria and odors in common with the individuals they spend time with, suggesting that the microbes move between hosts. There is some evidence that the same is true for humans. A study of roller derby participants found that after a match, skaters’ skin microbiomes were more similar than they had been before.
If birds’ smells are shaped by their bacteria, then perhaps their preferences for mates and for other choices are related to certain populations of microbes as well. Dr. Whittaker is planning to study a population of urban juncos to see whether they have their own microbes and version of what’s attractive, as far as smell goes, when compared to their woodland brethren.
In just a handful of decades, these city-living juncos no longer interbreed with their country cousins. Dr. Whittaker wonders whether their microbiomes could be contributing to this process.
“I want to see if that is playing a role in the difference between their odors, and see if that might be playing a role in why they’re becoming reproductively isolated,” she said. “Are they choosing to mate with birds that have the same smell as them? Is the microbiome helping make that go faster?”