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 fear
of predators? Aren't they supposed to be hiding quietly?
What is important enough to make my cardinal performer draw so much attention
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
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.
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
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
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
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
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.