The findings, published in the journal Current Biology, suggest that female birds may use UV light to communicate with each other and to attract mates.
"I was really excited when I saw the UV patterns on the female specimens," says Mary Caswell Stoddard, a research scientist at the University of California, Berkeley, who led the study. "It was such a surprise, and it suggests that there is a lot more to bird communication than we know about."
Stoddard and her colleagues studied the plumage of 48 species of birds, including songbirds, parrots, and hummingbirds. They found that 27 of the species had UV patterns that were visible to birds but not to humans.
The UV patterns were often located on the females' wings, tails, and heads. This suggests that they may be used for signalling purposes, says Stoddard.
"The UV patterns could be used to communicate with other females, or they could be used to attract males," she says.
Further research is needed to determine how female birds use UV light to communicate. But the findings of Stoddard and her colleagues suggest that there may be a lot more to bird communication than we know about.
"It's amazing how much we're still learning about these animals," says Stoddard. "Every time we look at them, we find something new."
