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Sometimes it’s the tiniest culprits that cause the biggest controversies.

Researchers have finally honed in on the microbes that are attacking bird tails around North America: They consist of three closely related species of Bacillus, also known as the “feather eaters.” The bacteria come from a family of generalists that are capable of breaking down a number of different materials, but likely mutated at some point to absorb keratin from feathers.

Other researchers had suspected the bacteria's role in rampant feather loss across the continent. But outside of controlled lab tests, nobody was able to prove it, says Cody Kent, a PhD student at Tulane University and lead author on a new study, published yesterday in . By studying the effects in wilder settings, Kent was able to build a trove of evidence that solidifies the connection between microbes and degraded tail feathers.

Between 1996 and 2005, Kent and a team from Ohio Wesleyan University collected data from nearly 3,500 avians and 150 different species. The subjects were caught along their migratory routes and breeding grounds, which spanned six U.S. states and Canadian provinces, from the Bay of Fundy to the swarthy bayous of Louisiana. After swabbing the birds’ backs, tail feathers, wings, and breasts, the researchers disinfected the nets and other equipment to keep the bacteria from spreading.

“We found that the bacteria were quite common in all the birds we sampled,” Kent says, adding that 39 percent of the wild individuals they caught tested positive. Yet not all species were affected equally. Ground-foraging birds, such as House and Song Sparrows, showed higher levels of the microbe. On the other hand, some species of hummingbirds and woodpeckers came up clean. Kent says finding out why there were variations in the level of contamination is probably the next step in the research.

So far, there isn’t any reason to panic, Kent says. The microbes have likely been around as long as birds themselves, given their dependence on keratin—perhaps even longer. The impacts may also not all be bad. One potential benefit is that they cause bluebird feathers to shine brighter, allowing males that are covered in bacteria to sire more offspring.

The bacteria might increase the chance of some birds getting attacked and eaten by hawks, as the damage on their tails could ultimately affect their speed and turning abilities during escapes. There may also be evolutionary implications, Kent says, as the microbe could have some consequence on molting frequency or preening behavior. Feathers typically degrade over time, whether it’s due to the friction from drilling holes in tree trunks or natural causes such as weather. But adding the bacterial element could further speed up that process.

“Somebody’s finally really showed that these bacteria are making a difference to the wear and tear of feathers of birds,” says Wesley Hochachka, an assistant director of the Bird Population Studies Program at the Cornell Lab of Ornithology. Hochachka was not involved in the study.

“There’s a relationship where the bacteria are doing damage to the birds’ feathers and hence the birds’ potential livelihood, but it’s not severe enough to really make an impact,” he says. “This is a nice study that finally put it into context so we know that it's not a major issue.”