A bird's view of romantic lighting.

Someone with a weakness for redheads might not fall for Lucille Ball viewed on a black-and-white television set. Give that viewer a color set, however, and he would probably love Lucy.

Female birds face a similar situation. They prefer mates with particular plumage colors, among other characteristics. However, they require ultraviolet light to detect some alluring colors, researchers have now demonstrated. Unlike people, birds see the hues produced when ultraviolet light bounces off objects, such as feathers. When researchers screened out ultraviolet light, female birds failed to see those special colors-or the charm of nearby males, report Andrew T.D. Bennett of the University of Bristol in England and his colleagues.

"Although previous studies have postulated a role for ultraviolet light in avian (and reptilian) mate choice, we believe this to be the first study experimentally demonstrating such effects in any vertebrate," the authors assert in the April 4 Nature.

In a series of experiments, the team studied 32 male and 8 female zebra finches (Taeniopygia guttata). Each of four males had its own room in a special cage. A female could view the males, sometimes directly and sometimes through filters that screened out ultraviolet light.

The more a female hops in front of a male, the greater her interest in him, other studies have shown.

In their first experiment, the researchers found that the females hopped more when viewing a male directly. To make sure that these birds weren't just keen on the lighting conditions, the scientists tested the females' response to the rooms after removing the males. The females showed no preference for rooms they viewed in unfiltered light.

To rule out the possibility that ultraviolet light simply enabled female zebra finches to recognize a potential mate more readily, the researchers tested the birds' ability to detect another desirable trait that would show up only under ultraviolet light.

Female zebra finches prefer males with symmetrical plumage patterns or even symmetrical arrangements of ornaments that researchers attach to the birds. So the team put four legbands on each male, two on each leg. Only two of the bands reflected ultraviolet light. The bands appeared identical to the scientists.

Because the female birds could see ultraviolet light, they could tell which males had one light-reflecting band on each leg and which had two on one leg. They showed more interest in the males with the symmetrical bands, the team reports.