Northern Cardinal (Cardinalis cardinalis)

Dressed for Success?

Nearly every morning since mid-March, a Northern Cardinal has perched itself to sing from the top of a small maple in my neighbor's yard. Dressed in almost fluorescent red against a blue sky, the boisterous cardinal is by far the most conspicuous animal in the neighborhood.

It seems odd. Aren't small animals like cardinals supposed to spend their lives in cardinalfear of predators? Aren't they supposed to be hiding quietly?

What is important enough to make my cardinal performer draw so much attention to himself?

Evolutionary biologists believe the cardinal who sings across my lawn is trying to send a signal. The brilliance of his color and his song advertises to other cardinals, with the avian equivalent of billboards and loudspeakers, that he is dominant, genetically fit, and ready to mate.

A male that doesn't advertise well presumably won't get a good mate, or a good territory, or both. Pretty high stakes.

It's thought that this imperative holds for males of all sexually reproducing animals. The stakes should be highest of all for polygynous species where males have more than one mate. Red-Winged Blackbird males, for example, can have several mates and father dozens of offspring each year. This leaves some males no mate at all.

That makes the evolutionary pressure to be the most attractive male very strong.

That's why it seems logical that the most dichromatic birds (that is the species where males and females are most differently colored) should be polygynous species like the Red-Winged Blackbird, where the roles of males and females are very different.

But cardinals seem to be a glaring refutation of this prediction. They are one of the most highly dichromatic birds in North America. The female mate of that startling crimson male has some muted red on her wings and tail, and a bright orange-red bill, but is otherwise colored with grey, brown, and olive -- classic camouflage colors.cardinal

Yet male and female cardinals pair up one to one, and their roles are more similar than those of most dichromatic birds.

Female cardinals lay the eggs, of course, and they do all of the incubating, but males feed females on the nest, and play a slightly larger role than females in feeding and caring for the young.

Even as singers, the cardinal sexes are far more similar than most birds in North America in that both male and female cardinals sing.

Female cardinals sing songs at least as complex as those of males, but they sing more softly and usually from their nest, not the tops of trees. While males may be singing as advertisement, females seem to be singing primarily to communicate information to their mates such as, "Where are you? I'm hungry." or "No thanks. I'm full."

It may be that every trip to the nest by the bright red male increases the chance of discovery by an egg-eating squirrel, raccoon, snake, or Blue Jay. By giving males information on needs at the nest, females might be limiting the number of exposing trips by males.

We do know that cardinal nests are at great risk. Research has shown that the most precarious nests are those of birds like cardinals that nest a few feet off the ground in shrubs and bushes. Only about one in five cardinal nests produce any surviving young at all.

But if brightly colored males put their nests at risk, it only highlights again the question of why male cardinals are so startlingly red.

Scientists studying this question have found some evidence that the brightest cardinals do get the best territories and have the most offspring. The correlation is not overwhelming, however, and it's not clear whether it's cause or effect.

Maybe the brightest males intimidate other males and are thereby able to get the best territories. Laboratory experiments, however, have not shown that cardinal males are particularly intimidated by the brightness of other males.

It's possible that males with the best territories eat the best food, and that helps to make them more red. We know that the cardinal's red color comes from substances called carotenoids. These pigments, like the orange color found in carrots, are made only by plants. Any animal that uses carotenoids for coloration, including most red, orange, or yellow birds, must get it from what it eats.

Cardinals need carotenoids to be red, but it appears that they don't need much, and carotenoids are fairly common in the fruits and berries they often eat. So it's not clear that territory quality would affect the intensity of their coloration.
Another possibility is that male cardinals are so red because females won't mate with them if they aren't. "Sexual selection" theory says that traits like coloration and singing ability tell females something about a male's suitability as a mate.

If red coloration is somehow a marker of a male's genetic quality, his foraging ability, or his ability to gain the best territory, females might preferentially mate with the reddest males.

Sons and daughters who carry the advantageous traits of their fathers would be more likely to propagate their mother's genes, including a genetic tendency to prefer loud, red males.

After many generations of this reinforcement, you might end up with a gaudy display like that of the male Northern Cardinal.

A variation on these themes is that the bright red coloration in male cardinals might not be for the sake of their mates, but to attract attention from other males' mates.

It was once thought that cardinals and most other songbirds were exclusively monogamous, but DNA studies have shown that a significant proportion of young in most birds' nests are fathered by someone other than the mated male.

In cardinals, that proportion has been measured as being as low as one in ten and as high as one in three. Some bird studies have shown that when females mate with another male, they seem to choose one who is of better quality, or is higher in the dominance ranking than their own mates.

Since there is a limit to how many chicks a male cardinal can father with its primary mate, one way to win the "game" of evolution is to father the chicks in your neighbor's nest.

It may be that the bright red color of males has less to do with attracting a mate than with attracting trysts.

Maybe the monogamous male cardinal is not so different from the polygynous male Red-Winged Blackbird after all.

If it sounds as if there are mostly maybes and possibilities in these theories, it's because that's the reality. Where researchers once looked for a single sexual selection explanation for everything from a peacock's tail to a cardinal's coloration to the outrageous songs of a mockingbird, they are now resigned to the idea that so many factors are involved that no one answer will suffice.

The truth is, we don't yet know why a male cardinal is so shockingly red, or why he sings so loudly.

Still, it's a thrill to watch him every morning, boldly proclaiming something to the world, and trying to attract just as much attention as he can.




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