A BRIGHT BILL MEANS A BETTER BIRD

by Whit Gibbons

September 29, 2003

"Eat your carrots." Parents have been saying that for years. Turns out the admonition may have scientific merit. Two reports suggest that carotenoids, the pigment that makes carrots orange and maple leaves turn orange in autumn, may be important for some animals to have. The two research programs independently reached similar conclusions about the importance of carotenoids in mating systems of birds. Birds cannot produce carotenoids the way plants do, and they don't eat carrots, but eating other types of plant material high in carotenoids significantly impacts their mating success.

The orange pigment is responsible for the color of the male's bill in some birds that use bill color to advertise that they are highly fit individuals and would be worthwhile mates. The color of a bird's bill in this situation is known as a secondary sexual trait, which means that it is a feature that distinguishes males from females. A deer's antlers, a peacock's tail, and the bright colors of a red-breasted sunfish are all secondary sexual characteristics. Such traits can sometimes seem to be of superficial significance, yet they can be quite important in determining which individuals are more successful in the mating game, often influencing which individuals are chosen as mates. However, determining why the bird with the brightest orange or red bill is most likely to be successful with females of the species has been something of a mystery.

In one of the studies, Bruno Faivre of the Université de Bourgogne in Dijon, France, and colleagues conducted experiments with a type of blackbird in which males with higher carotenoid levels have brighter orange bills. These are also the males who are most likely to become paired with females that are in the best body condition. Carotenoids, which produce brighter bills in the blackbirds, are also used in immune responses. In other words, carotenoids are used to fight disease and infection, which is probably one reason carrots are good for us.

The scientists wanted to test how the carotenoids in a blackbird became partitioned between the display of a colorful bill and the body's need to fight illness. To do this, they injected half of the birds with a substance that produces an immune response. As they had predicted, they discovered that the brightness of bill color was decreased in the birds that were injected, although bill color stayed the same in the healthy birds that were not fighting an ailment. They concluded that the signal of bill color is indeed an indicator of an individual blackbird's health.

Meanwhile, Jonathan D. Blount at the University of Glasgow in Scotland and colleagues confirmed the same phenomenon in experiments conducted with zebra finches. They took 10
pairs of males that were brothers and fed one of each pair water with carotenoids and the other distilled water. The ones fed distilled water were the controls in the experiment. As they anticipated, the bills of birds receiving the carotenoids turned significantly redder than those of the controls.

The next part of the test was to determine if this had any measurable effect on mating success and, sure enough, females spent significantly more time perched next to the males with brighter bills, indicating a preference for them. The next step, like the experiment with the blackbirds, involved injecting the finches with a substance that causes an immune response in birds. The carotenoid-supplemented and control males received the same doses.

All birds responded to the foreign substance in the body, but the carotenoid-supplemented birds showed a type of immune response that has been shown to result in higher survival rates. The significant finding of the two studies is that seemingly trivial or even frivolous secondary sexual traits that are used in mate choice decisions by females can indeed be true indicators of health and presumably higher fitness of males.

Next time you have cardinals around your yard, see if some of the males have brighter red bills than the others, and then see if they are the dominant ones. And don't forget to eat your carrots.

If you have an environmental question or comment, email