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?
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.