Robert Heinsohn, professor of evolutionary biology at the Australian National University, explains.
Males are more colorful or ornamented than females in most, but not all, bird species. Understanding this phenomenon requires a basic grasp of the evolutionary forces that shape the behavior and morphology of individuals and species. Charles Darwin developed much of the theory that helps explain this. He proposed that traits promoting survival in individuals are favored by the process of natural selection, whereas traits that help the individuals of just one sex (usually the males) compete for mates are favored by sexual selection. Sexual selection is responsible for many of the features unique to one sex in a given species. These features can be divided into two general categories: those acting as weapons that allow males to fight for access to females (antlers on deer, for example) and those acting as ornaments that attract the attention of females, such as long tails on birds.
Darwin concluded that color differences between sexes in birds (also known as sexual dichromatism) result largely from female preference for bright colors in males. This general rule has received much support since Darwin's time, but other influences have also been noted. For example, females of species that are exposed to predators while incubating tend to have dull colors, although both sexes may be brightly colored in species that nest in tree hollows because the females are less visible to predators. Color can also aid individuals in recognizing members of their own species. And in species that are not good to eat, colors can provide a warning to potential predators.
Color is also used in contests between males over mates or resources such as territory. Conspicuous colors can help show that an area is already occupied and that the occupant is in good condition and prepared to fight. The red shoulder patch on red-winged blackbirds provides an excellent example. The patch is coverable and is shown to males and females of the same species but never to predators. Males who had their patch experimentally covered tended to lose their territories more often than did uncovered birds. Similar results have been shown in other species such as scarlet-tufted malachite sunbirds, confirming that the brilliant badges function primarily in male-male competition over territories.
Some studies have shown that females use the brightness of a male's color as an important indicator of his health and vitality. House finches provide one of the best examples of this tactic. This species is monogamous and males exhibit orange or red in their crowns and elsewhere in their plumage. The extent and brightness of the color in individuals is directly related to carotenoid pigments that are picked up from high quality seed. Extensive field studies have shown that artificially brightened males were much preferred by females and that naturally brighter males were better at providing food to the female and her chicks. Not all plumage colors derive from diet, however. Blues and greens consist of structural pigments that are manufactured by the birds themselves. They, too, may provide good indicators of a bird's health and abilities, but this has not yet been clearly demonstrated.
Researchers realized only quite recently that birds see a much wider range of color than people do. They even have colors in their plumage that are invisible to the human eye. Birds have four color cones in their eyes (compared to three in humans), which allow them to see the ultraviolet part of the color spectrum. Scientists using spectroradiometers to measure the extent of ultraviolet coloration have found that males in many apparently monochromatic species (those with similarly colored sexes, such as European starlings) in fact sport bright ultraviolet colors that females use extensively in their choice of mate.
Males are usually the most colorful sex because females are more likely to be in short supply due to the extra work involved in incubation and chick rearing. Males must thus compete for the chance to mate with them. In an interesting twist, a handful of species are known to have reversed sex roles in which males incubate the eggs and females defend territories and fight amongst themselves for access to the males. These species provide ¿the exceptions that prove the rule,¿ because they demonstrate that the competitive sex is the one most likely to have bright colors. Phalaropes, sandpipers and button quail are good examples of species in which the females are more colorful.