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Impact of Red Fox Predation on the Sex Ratio of Prairie Mallards

Introduction


Interest in disparate sex ratios among North American waterfowl dates from an early analysis of sex ratios of banded ducks by Lincoln (1932), who reported a preponderance of males in 10 principal species. Prominent among these was the mallard (Anas platyrhyrnchos); a sample of about 22,000 mallards contained 56.4% males. Smaller samples of the other species also showed disparate sex ratios. Leopold (1933:111) noted, "Lincoln's evidence points toward the existence of a seriously deranged sex ratio," and he further stated, "How long it has existed, or what causes it, remains unknown." He acknowledged that a preponderance of males may have always existed and could represent a normal condition, but he thought this improbable.

Lincoln's (1932) observations were soon supported by other studies conducted in various geographic areas, which began to raise important questions as to the significance of sex ratio counts made at particular times and places because of differential migration by sex, visibility of hens during the nesting season, and segregation in wintering areas. Yet, in spite of these questions, it was apparent that a preponderance of males existed in most duck populations. Then, as today, the principal unknowns were how disparate the sex ratios were for given species, the causes of the disparities, and the effects of the disparities on productivity.

The subject of disparate sex ratios among North American ducks was first treated in detail by Bellrose et al. (1961), who summarized the available literature and discussed principal mortality factors with respect to their possible effect on sex composition. They extended Mayr's (1939) age classification and considered sex ratios within four age classes: primary- at fertilization; secondary- at hatching; tertiary- between hatching and adulthood; and quaternary- during adulthood. These authors noted distorted quaternary sex ratios in most populations although primary, secondary, and tertiary sex ratios were nearly even. Thus, the disparity arose sometime during adulthood, but the authors were unable to isolate the specific cause. They concluded that hunters and disease take relatively more drakes than hens, and that predators, agricultural operations, and stress take more hens than drakes.

Interest in disparate sex ratios has mounted in recent years, partly because of apparent further distortions in the sex ratios of some species but mostly because of the notion that, possibly, "excess" drakes could be "harvested" without serious impact on the breeding potential of the populations (e.g., Grieb et al. 1970). The mallard became a focal point for such thought because the species is well known to hunters and the sexes are easily distinguished.

While thoughts of expanded hunting of male mallards were tempting, some biologists urged restraint. In 1970 the Central Flyway Technical Committee established a subcommittee to investigate the function of "excess" drakes in various populations of ducks. Dzubin (1970), in a report to that subcommittee, raised several important questions about the origin of disparate sex ratios, the biological importance of the supernumerary drakes, and the potential effects of a reduced male population. Later, Nelson (in Aldrich 1973) suggested further questions for research, including the potential for harvesting "excess" drakes in view of the limited success of some of the experimental hunting seasons that had been tried. Thus, as is often true, a thoughtful examination of the subject exposed the need for additional research before sound management decisions were possible.

In all of the discussions the question of the cause of the disparate sex ratios was raised, especially because sex ratios at hatching are nearly equal and hunters generally take more males than females (Bellrose et al. 1961). More recently, Anderson (1975) analyzed recoveries of mallards banded throughout the North American breeding range and found survival to be higher for males than females, despite heightened vulnerability of males to hunting. He concluded that adult female mallards sustain very significant nonhunting mortality, probably during the nesting and brood-rearing period. These findings reinforced the suggestion by Bellrose et al. (1961) that hen mortality during spring and summer is the principal cause of disparate sex ratios.

Concurrent with the recent interest in sex ratios, a study of predation by red fox (Vulpes vulpes) on waterfowl generated some insightful findings. The study (Sergeant 1972) involved red foxes in eastern North Dakota, and yielded two facts that could bear heavily on the etiology of the disparate sex ratio. First, red fox predation on dabbling ducks was substantial; an average of 4.9 ducks (all dabblers) per fox rearing den was documented from food remains found at the dens. Second, the predation was much more pronounced on females than on males; females made up 84% of all ducks whose sex could be determined. This figure was fairly uniform for all species found at dens.

These findings led us to formulate the hypothesis that predation by red foxes was sufficient to cause a distorted sex ratio of the magnitude observed in prairie dabbling ducks. Although foxes take more females than males of all common prairie-nesting dabbling ducks, we chose the mallard as our subject because of its status as a game species and because its population dynamics are better known than those of any other dabbling duck.

It was impractical to test the hypothesis by direct experimentation. Yet the available information was considerable: data on the population dynamics of mallards; information on numbers of foxes, their habitats, and behavior; and the recently acquired knowledge about fox predation on mallards. To integrate the data for testing the hypothesis, we used mathematical modeling.

To treat the subject of red fox impact on mallard sex ratios, we divided this report into four parts. Part One describes our model, the data used in it, and the results of the simulations. In Part Two we present a modified model for use as a predictive tool. Part Three provides a historical and geographical perspective of fox predation in the Prairie Pothole Region of central North America. In Part Four we discuss the biological and management implications of a distorted sex ratio in mallards. The Conclusion offers some cautionary advice and discusses overall aspects of our findings.


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