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.