Editor, Earth News
Eastern bluebirds can lose their brilliance
Brightly coloured birds can become infected with bacteria that eat their feathers.
That in turn can affect the health of the birds and dull their plumage.
The discovery comes from a study that found that 99% of all Eastern bluebirds surveyed in Virginia, US were infected with feather-degrading bacteria.
Such bacteria were first discovered a decade ago, but the latest research is the best evidence yet that the bugs affect the colour and health of birds.
"Feather-degrading bacteria are relatively new to ornithologists," says Alex Gunderson of Duke University in Durham, North Carolina, US.
"The first report of their occurrence on wild birds was published only ten years ago."
Since then, scientists have found that most species of wild bird probably harbour some feather-degrading bacteria in their plumage, sometimes of more than one species.
Feather-degrading bacteria work by hydrolysing the protein beta-keratin, which constitutes over 90% of a feather's mass.
But these bugs are usually found in a minority of birds sampled, and it has not been clear what impact they have on their hosts.
So Gunderson and colleagues Mark Forsyth and John Swaddle of the College of William and Mary in Williamsburg, Virginia, US surveyed a population of Eastern bluebirds (Sialia sialis) living in Virginia.
They found that 99% of all the birds surveyed carried feather-eating bugs, they report in the Journal of Avian Biology.
What's more, they found a correlation between the bacteria and the brightness of female birds' feathers, with more bacteria causing duller feathers.
"This is some of the best evidence that bacteria are active on the feathers of live birds," says Gunderson.
"The evidence is correlational, so there is a great deal more work that needs to be done to verify it."
"But it does suggest that feather-degrading bacteria could be an important force influencing the ecology and evolution of birds."
However, the bacteria didn't seem to have a significant impact on the feather colour of male birds, a rare example of a parasite appearing to harm one sex while not the other.
"I was surprised that the relationship with feather-degrading bacteria was different for males and females," explains Gunderson.
Alex Gunderson swabs an Eastern bluebird for feather-eating bugs
"It is possible that, because males and females differ somewhat in where they spend their time, they could acquire different species of bacteria that have different effects. It is also possible that physiological differences between males and females result in different effects of bacteria."
"This is complete speculation and at present we do not know the answer to this question."
Another important result was that the bacterial load also correlated with the birds' body condition, which is directly related to the bird's health, and also their reproductive success.
Overall, the results suggest that feather-degrading bacteria may have a significant impact on the birds' ecology.
Birds use feather colours to advertise their health, attract mates and for camouflage, so that means the bacteria could also affect the evolution of bird colour.
"If bacteria detrimentally influence feather colouration, they may place selective pressure on birds to evolve defences against them," says Gunderson.
"There is evidence that certain avian traits are defences against feather-degrading bacteria. For instance, we know that feathers coloured by melanin pigments are resistant to bacterial degradation, and that the preen oil that birds apply to their plumage inhibits the growth of some feather-degrading bacteria."
"In general, an understanding of the influence of feather-degrading bacteria on birds could, to some degree, help explain the evolution of these and other avian traits," he says.