Two related South American species of birds, difficult to distinguish with the human eye, use ultraviolet light to differentiate between themselves, according to a zoologist at the University of Wisconsin-Madison.
The markings on the two sibling species of mountain tanagers, which live and often flock together in the northern Andes of Colombia and Ecuador, are remarkably similar, with vibrant blue trim on the wings and tail plumage, black on the head, and bright yellow on the crown and underparts. The biggest difference is in the coloring of the plumage on their backs: The black-chinned mountain tanager is olive-colored, whereas the blue-winged mountain tanager is black. But in the field this difference is difficult to see.
"They're similar-looking - even to an experienced birder," says Robert Bleiweiss, associate professor of zoology at UW-Madison. "I hate to admit this, especially as I'm a very experienced birder, but even I had trouble with them."
When he examined the birds under UV light, though, he found that the difference between the two species is clear. The back of the black-chinned mountain tanager displays high plumage reflectance under UV light, whereas the back of the blue-winged mountain tanager has low reflectance. The birds can see this distinction because of a special receptor in their eyes that allows them to see UV light outside the usual visual perception of humans.
"But it's one thing to say that they differ," Bleiweiss says. "The second step is, is it important to interbreeding?"
To answer that question, he examined other populations of blue-winged mountain tanagers in which the plumage color on the back of those birds ranges from black to the olive color. And these birds, despite the variation, interbreed.
Under UV light, he discovered that all of the blue-winged mountain tanager populations had a similar low reflectance, no matter what color the plumage on their back was in visible light.
This would imply that the birds use the visible cue of UV plumage reflection to distinguish between species to mate, Bleiweiss says.
And this difference between the species, he says, brings them up to the standard of differences between other tanagers that live together, which are known for their distinctive and brightly colored plumage. The visual similarities between the two distinct species had long been a curiosity for birders.
"It's the bird standard that matters," Bleiweiss says. "We can look at it with better technology, but we won't see exactly what birds do. That would be the ultimate goal - to get into the birds' brain and see the way that they do."
The next challenge, he says, will be to find out if this pattern of differences in UV reflectance is true in other sibling species.
"We might learn that sibling species are not so similar," he says. "They just differ in ways we find difficult to detect."