By Tina Yerkes, Ph.D.
Somehow it just doesn't seem right that drakes are decked out in attractive colors and eye-catching patterns while hens are dull brown and boring. A theory called sexual selection accounts for this arrangement in many birds. In waterfowl, this is common among species—primarily dabbling ducks—that select new mates each year.
The basic idea behind sexual selection is that the sex that chooses a mate is the one that drives the development of characteristics that make the species successful. Female ducks choose their mates, and that results in drakes having to compete for hens. According to this theory, males with the right stuff win, and those traits are passed on to their offspring. The signals that indicate the right stuff may include large body size, fancy plumage, intricate songs, or striking displays. The tail of a male peacock is a good example of an exaggerated difference between males and females.
Dabbling ducks are seasonally monogamous, which means they choose new mates each year, generally on the wintering grounds. These pair bonds break once the female begins incubation. Males neither help incubate the eggs nor raise ducklings. They do, however, escort and guard females while they are paired, allowing hens to spend most of their time feeding. This behavior likely allows females to increase their foraging efficiency and improves their chances of nesting successfully when they reach the breeding grounds.
Although being choosey appears to provide few obvious advantages for hens, the benefits may be hidden. Hens may indeed be choosing good genes. The "good genes" hypothesis states that exaggerated male plumage or courtship displays truthfully signal genetic or physiological superiority. Another aspect of this concept is the "sexy son" hypothesis, which proposes that female choice is based on the genetic advantage passed on to her male offspring. For example, females that prefer to pair with larger males should produce large sons as well as daughters that show a preference for larger males. This process can escalate over time, resulting in increasingly exaggerated male traits and stronger female preference for those traits.
How do females determine which male has the best genes? According to this theory, females assess characteristics such as color, ornamentation, and vigorous behavior that may be related to better genes. In ducks, males perform courtship displays for females in courting parties, and females choose among displaying drakes. Studies have shown females choose males that are more active in courtship displays and are the fashion icons of the group. They likely choose these flashy looking males because they appear stronger and healthier and likewise will be more capable of protecting them while they feed on wintering areas and during spring migration.
Several studies have been conducted on birds to test these theories. Male house finches, which have brightly colored plumage, were altered to appear brighter or duller than normal. Those dyed brighter were chosen most often, and all obtained mates. Only 30 percent of those dyed a duller color obtained mates. Similar findings were observed in swallows that had their tail feathers artificially elongated or shortened. Apparently, female swallows like longer tails. All the males with extended tails were chosen as mates, and those males actually produced more offspring. All animals are not the same, as each species tends to key in on different male traits.
What makes a drake attractive? In the dabblers that have been studied, courtship displays and plumage characteristics have been shown to be among the most important determinants in mate choice. In both mallards and pintails, males with more vigorous courtship displays are chosen more often than less active males.
Male mallard plumage characteristics important for female choice included a full green head and normal, brown chest. When researchers cosmetically degraded the appearance of drakes by plucking breast and head feathers to create a "beat up" look, hens chose the unaltered, "good-looking" males. Hen pintails preferred males with clean white breasts and more colorful scapulars, but surprisingly tail length didn't appear to be a factor.
As for the hens, there is an advantage to being dull and boring—camouflage. Because female ducks basically go it alone while nesting, they suffer a higher mortality rate than males. As all hunters know, there is great advantage to blending into your environment. Female ducks are dull and brown so that they blend into the vegetation on their nesting areas, such as native grass or brush. It just doesn't pay to attract unwanted attention when trying to incubate a nest full of eggs.
All of this raises the question: Why do male and female geese and swans look similar? Both geese and swans pair for life, and according to theory, the pressure for selection is therefore reduced.
Dr. Tina Yerkes is director of conservation planning at DU's Great Lakes/Atlantic Office in Ann Arbor, Michigan.