Now researchers report that parasites, too, are capable of hiding in plain sight. In looking at lice that live in the feathers of birds, researchers found that light-colored lice live on light-colored birds, whereas dark-colored lice live on dark-colored birds.

The study will appear in a forthcoming issue of The American Naturalist. The researchers looked at 26 pairs of related but differently colored birds, like a black swan and a white swan, and a white pelican and a brown pelican. In every case, the color of the lice on the bird’s body matched its feather color. Birds regularly preen their feathers and are more apt to find lice that are contrasting in color to their feathers.

Over time, lice evolved to match their hosts’ coloring, said Sarah Bush, a University of Utah biologist and the study’s lead author.

The researchers found, however, that lice in one area where it’s hard to preen — on the head — did not necessarily match feather color. “Birds can’t look at their own heads,” Dr. Bush said. “There’s no evolutionary selection for lice to be cryptically colored, unless they are removed.”

The lice on all the birds’ heads tended to be dark regardless of feather color, leading Dr. Bush to believe that darker lice may have an evolutionary advantage.

“Nobody’s tested it, but it may protect them from UV radiation,” she said.