by Whit Gibbons

January 22, 2017

Last week I saw one of the most colorful birds of North America. Swimming around in our beaver pond were a pair of wood ducks, the only species of duck that commonly nests in the Southeast.

A male wood duck is unparalleled among our birds, looking like an elegant hand-painted carving. The female, with her brown body, blue feathers on the wing tips and stylish white eye patch, is attractive, but as with many birds her colors pale in comparison to the brilliant plumage of the male.

Males are more brightly colored than females in many avian species. Cardinals, bluebirds and goldfinches are obvious examples. Such color contrasts are particularly dramatic in most species of tropical parrots, with brightly feathered males and less colorful females.

The biological explanations for why the sexes have different colors are complex. But in the simplest terms, the most brightly colored males are more likely to attract females, and drab-colored females are less likely to attract predators during nesting.

One of the intriguing aspects of ecology is that exceptions occur that require biological explanations different from the norm. A study in Australia examined a type of parrot in which the two sexes differ in color but in a manner opposite 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 color contrasts between the sexes are known among a few other birds, but in such cases the species also typically has sex role reversal. In other words, when females are more colorful than males, females compete with each other for mates, instead of the conventional situation in which males compete with other males. In addition, the males of such species care for the eggs and young.

Enter the exception: the Australian parrot with its brightly colored females and drab males. Despite the disparate color patterns between the sexes, Australian parrot males still compete for mates, and females tend the eggs and young, as with most birds in which the males are the brightly colored ones.

An alluring part of being an ecologist is the prospect of trying to solve such intriguing biological puzzles. In this case, investigators observed a phenomenon of nature that 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. What made them different from all of the other examples in the bird world?

The answer lies in the Australian parrot’s ecology and lifestyle, which are quite different between the sexes. One way in which 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 can retreat from predators.

Equally as important as escaping predators is the need to 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 try to kill the offspring of another female. In defending the hollow and nest from other parrots, the female’s bright plumage gives her a more conspicuous and threatening appearance to a competitor. Natural selection, therefore, has favored bright plumage in the females.

In contrast, male parrots forage in the tree canopy among the leaves. In that situation, their green plumage makes them less conspicuous to predatory raptors. Below the canopy, where the females are during the breeding season, the males’ green plumage makes them highly visible against the tree trunks.

Wood ducks follow the standard color contrasts and behaviors between the sexes we have come to expect among birds. The Australian parrot is one of those refreshing exceptions that make being an ecologist utterly fascinating.

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