by Whit Gibbons

September 25, 2005

Earlier this year, the rediscovery of the ivory-billed woodpecker, which was thought to be extinct, surprised ornithologists everywhere. But an equally important finding from a scientific perspective was research that provided a new explanation for why male and female birds may differ dramatically in plumage color.

Birds have always been noted for sexual dichromatism, with males characteristically being the more brightly colored. Sexual dichromatism is an easy scientific term to remember. “Di” means “double,” and “chromatism” refers to color, so a species with sexual dichromatism is one in which the two sexes differ in color. Sexual dichromatism is particularly apparent in birds. Male goldfinches turn bright yellow in the spring, whereas females maintain a subdued, olive coloration. Male cardinals are much brighter red than females. And even in the common house sparrow the male’s suit of brown is more vivid than that of the drab female.

Color contrasts are particularly dramatic in most species of tropical parrots, with brightly plumaged males and less colorful females. A study this year examined an Australian parrot in which the opposite is true. The investigators discovered why the two sexes differ in color but in a manner different from that of other parrots.

The brightly colored red and blue females of the Australian parrot are so distinctly different in appearance from the duller green males that early ornithologists classified them as separate species. Reversed sexual dichromatism is known among birds, but when females are more colorful than males, the species typically has sex role reversal also. In other words, with all previously known cases, females compete with each other for male mates, instead of the conventional situation in which males compete with males. In addition, the males care for the eggs and young. Not so with the Australian parrot. Despite the disparate color patterns between the sexes, Autralian parrot males still compete for mates and females tend the eggs and young, just like with most birds in which the males are the brightly colored ones.

An alluring part of the field of ecology is that you are faced with intriguing biological puzzles every day. In this case, the investigators had a dilemma in which an observed phenomenon of nature did not fit the traditional model. These male parrots, which were not nearly as colorful as the females, should have been assuming a more motherly role. So what made them different form all the other examples in the bird world?

Not unexpectedly, the answer lies in the evolutionary history of the species, and in the pressures of natural selection related to ecology and lifestyle, which operate independently in the two sexes of the parrot. One way the two sexes differ is that the females live most of the year in the forest in tree hollows where they also nest. They forage near the hollows, to which they retreat from predators. Equally as important as escaping predators is that they must protect their young. And it turns out that one of the greatest threats to babies in the nest is other female parrots, which will actually come into the nest hollow and kill the offspring. In defending the hollow and nest from other parrots, the female’s bright plumage makes her more conspicuous and a more threatening competitor. So, natural selection has favored bright plumage in the females.

In contrast, males have been selected for green plumage, which makes them less conspicuous to aerial predators against leaves in the tree canopy where they spend much of their time foraging, with no tree hollows to retreat to for escape from raptors. However, the green color makes them highly visible to other males against tree trunks below the canopy where the females are to be found during the breeding season and where competition for mates occurs between males. The study demonstrates how evolution can lead to exceedingly complex scenarios in nature and specifically how plumage color can evolve independently in two sexes of a species.

By the way, in case you ever have a chance to observe one, ivory-billed woodpeckers show sexual dichromatism. Both sexes are contrasting black and white, and both have a large crest on the head, but the crest of the female is black and that of the male is bright red.

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