Waterfowl Sex Ratios

A variety of factors influence the proportion of males and females in waterfowl populations.
By Dale Humburg    

A variety of factors influence the proportion of males and females in waterfowl populations

Waterfowl hunters in the United States shoot two to three drake mallards for every hen harvested. Yet, males continue to outnumber females, accounting for about 52.5 percent of the midcontinent population in spring. And a sex ratio imbalance favoring males is true for most other waterfowl species.

But it doesn’t start out that way. In fact, the sex ratio of waterfowl at fertilization is essentially 50:50 for males and females. The same holds true at hatch. And, for the most part, the sex composition of first-year birds remains equal when they first return to the breeding grounds eight to 10 months later.

Because there are essentially no differences in the ratio of males to females from hatch to the first breeding season, the imbalance has to result from differences in survival among adult birds. So, if mortality factors affect drakes and hens about equally before arriving on their breeding grounds, what happens after that?

Biological Explanations

Part of the explanation for disparate sex ratios in many waterfowl species lies in the nature and timing of pairing. Most hen mallards are already paired by the time they arrive on breeding areas. The same is not true for many inland diving ducks like canvasbacks, which primarily establish pairs after they arrive on the breeding grounds. Sea ducks including buffleheads, goldeneyes, and scoters do not establish pairs and reproduce until their second year or later. Some subspecies of Canada geese may delay breeding until four or five years of age.

There are also differences in the length of pair bonds among species. In canvasbacks, for example, the duration of pairing is relatively fleeting. Many dabbling duck pairs last only about halfway through the incubation period. In contrast, Canada geese and whistling ducks establish permanent pair bonds. (These birds will re-pair if their mate dies.)

The upshot is that the timing and length of pair bonds correlate with the disparity in sex ratios among species. In species with delayed or short pair bonds, the male wards off harassing drakes, inseminates the hen, and defends a loafing and feeding location—but only for a short period of time. The female is then left alone to face the stresses of nesting, brood rearing, and molting. At these times, the hen is less likely to survive than the male. Thus, greater mortality of hens and relatively higher survival of drakes result in a sex ratio favoring males.

At the opposite end of the biological gradient are species with longer lasting pair bonds. Here, both sexes are exposed to many of the same sources of mortality. Males provide long-term defense of the female, share in brood rearing, and even help incubate the eggs in rare instances. The result is similar mortality and survival among males and females and relatively balanced sex ratios.

Measuring Sex Ratios

Sex ratios are most disparate during the breeding season, increasingly favoring drakes as hen mortality occurs throughout the nesting period. After the young hatch, first-year birds—comprised of an even number of males and females—bring the sex ratio in the total population closer to a 50:50 balance. During the waterfowl hunting season, more males than females of a number of species are harvested, and the sex ratio trends even closer to parity.

Detecting the change in sex ratio over time, however, is not as easy as it might appear. It depends on how and when sex ratios are measured and the reliability of the methods. Sex ratios derived from trapping and banding efforts, hunter bag checks, collection of dead birds killed by disease outbreaks, and field observations all are different, and each presents its own biases.

Considerable differences in the number of males and females tallied can also occur at different times of year (even week to week), locations, and habitat types. Males and females of some species tend to migrate at different times. In the fall, adult males, which complete the molt earlier than females and fledging young, may migrate sooner. During winter, a greater proportion of males of many species such as common mergansers, northern pintails, common goldeneyes, mallards, and canvasbacks stay farther north than females. Smaller females are more susceptible to severe conditions and competition by males for food resources in northern areas, which may force a greater proportion of females to migrate farther south. Males of some species may gain a breeding advantage by wintering farther north, thus ensuring earlier arrival on the breeding grounds.

Field observations of sex ratios can also differ by habitat type. On the breeding grounds, for example, sex ratios are more even on small wetlands with established pairs than in open habitats where remaining unpaired ducks vie for later pairing opportunities. In winter, established pairs opt for more secluded habitats than remaining unpaired birds. Obviously, observed sex ratios will differ considerably in these different habitats.

Implications for Waterfowl Management

In waterfowling circles, there is a long-standing contention that “excess drakes” could provide additional hunting opportunity. But this is true only if additional drakes are not biologically important. If an imbalance of drakes in breeding populations is important to renesting hens or in social interactions among breeders, then increasing harvest pressure on males may not be appropriate.

Initial experiments in the 1960s and early 1970s and more than 30 years of operational use of differential sex regulations for mallards suggest that sex ratios have not changed much. Several studies have shown that regulations increasing harvest opportunity for drake mallards are effective in shifting harvest in favor of drakes while also reducing the harvest of hens. But these differential bag limits may not be biologically significant or appropriate for species other than mallards.

Dramatic changes in sex ratios are of concern to biologists if they coincide with declines in populations. Essentially, this can be an indicator of changes in adult female mortality possibly resulting from increased stress during the breeding season and landscape changes affecting nesting success. Arnold Erickson, after conducting studies during the late 1930s in Minnesota, stated, “In general, the female that nests in a greatly disturbed environment is more subject to predation and the activities of man than the one that nests in undisturbed habitat.” This is why Ducks Unlimited has for nearly 75 years stressed the need for landscape-level habitat conservation.

Dale Humburg is chief biologist at DU national headquarters in Memphis.

Can We Take More Drakes of All Species?

While bag limits for male and female mallards and, for a time, northern pintails have been implemented over the years, there has been limited use of sex-specific regulations for other species. For sex-specific regulations to be appropriate, the following conditions must be met:
  • No difference in sexual maturity exists between males and females.

  • Surplus drakes are not important in providing mates for renesting hens.

  • An increase in drake harvest (due to differential regulations) does not also increase hen harvest.

  • Hunters can identify both drakes and hens and actively select males in the field.

Look More Closely

Differentiating drakes and hens in the field is complicated by plumage variations among species, sexes, and ages. For geese, black ducks, and mottled ducks, the plumage of males and females is quite similar, and in many instances, behavioral differences are the only way to tell the sexes apart at a distance. For sea ducks, which sexually mature in their second year, it’s difficult—if not impossible—to confidently differentiate first-year males from young and adult females. And even among species where drakes and hens have obvious plumage differences, observers have to be careful that light conditions and other visibility biases don’t affect their data collection. Drakes show up a whole lot better than drab-colored females.