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Sex appeal. Females adore blackbird males with bright bills, which signal vigorous immune systems.

A Flashy Bill of Health

All across the wild kingdom, males often show off to tell potential mates, “Pick me!” Typically a female bird chooses the one with the brightest body parts or the most melodic song. Now evolutionary biologists have shown that this can be a good move on her part--some displays are true boasts of good health.

Only in the past 20 years have researchers begun to glean how sexual traits can reflect a male's vitality. But until now, they could only speculate about the link between health and showiness. One possible connection is red and yellow pigments called caretenoids, which brighten body parts and strengthen the immune response. Now two research teams have shown that this link does exist in some birds and is reflected in the color of the birds' bills. They report their results in the 4 April issue of Science.

In one study, Jonathan Blount, an evolutionary biologist at the University of Glasgow, U.K., and colleagues fed zebra finches water fortified with carotenoids, while the birds' brothers drank plain water. The added carotenoids made the bird's healthier, their bills redder, and the sex appeal greater--females preferred these birds to their drab siblings. Conversely, a team led by evolutionary biologist Bruno Faivre of the University of Burgundy in Dijon, France, found that blackbirds whose immune systems are under stress have duller bills.

“The studies are important and complementary,” says Joseph Waas of the University of Waikato, New Zealand. Together they provide experimental evidence that bill color is a true indicator of the male's fitness because carotenoids help boost the immune system. As a bonus for females, bill color changes to indicate the male's current vigor. “The characteristics that females really pay attention to are things that reflect the [male's] day-to-day well being,” explains Marlene Zuk, an evolutionary biologist at the University of California, Riverside.

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