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

Management Implications


The conclusion that adult mallard populations in the Prairie Pothole Region are sexually imbalanced raises several questions of importance to the management of mallards and, very probably, other species of ducks: (1) What are the implications to surveys and censuses of mallards, in which each male (singly or in small groups) is presumed to represent a pair? (2) Can the "excess" drakes be harvested without detriment to the species? (3) Should a goal of management be the reduction of red foxes to increase the survival of hens during the nesting season?

Effect of a Sex Disparity on Surveys and Censuses

Because mallards initiate nesting very soon after arrival in spring, surveys and censuses ordinarily involve birds in various stages of nesting activities. Hen mallards in the egglaying phase and in incubation are separated from the drakes for periods of time, during which they are often undetectable in a search of ponds. Drakes, however, are generally visible on ponds throughout the nesting season. Each single drake, or each drake in a small group, is assumed to have a mate (Hammond 1969). If the hen is not seen, she is presumed to be at the nest.

This presumption is tenable if all drakes so counted are paired. If males outnumber females, however, misinterpretation of the counts is possible, depending upon the behavior of the unmated drakes. If they congregate in large flocks (of more than five), most workers do not assume they represent pairs (Dzubin 1969), so the count is interpreted properly. If the unmated drakes are isolated or in small groups, however, standard survey techniques cannot distinguish them from mated drakes waiting for their hens, and they are presumed to represent breeding pairs. This situation could lead to a biased (upwards) estimate of the resident breeding population, the extent of the bias depending upon the fraction of unmated drakes that do not congregate in large flocks. As an example, suppose that the population in a surveyed area at the beginning of the nesting season contains 120 males and 100 females (55% males), all females are monogamously mated, and half the unmated males are alone or in small groups at the time of a survey. A thorough search would turn up 110 males in singles or small groups, 10 males in large groups, and some females, the exact number being irrelevant to the count if all are with their mates. Standard interpretation would suggest a population of 110 pairs plus 10 drakes, for a total of 230 birds. The correct figure, of course, is 100 pairs plus 20 drakes, a total of 220 birds. While the total population count is inflated by only 4.5%, the count of breeding pairs is 10% high. The potential for overestimating the breeding population is considerable and has been discussed by Dzubin (1969).

Hunting Considerations

Waterfowl managers are intrigued by the possibility of harvesting the presumably "excess" drakes if this feat can be accomplished without deleterious effect upon the productivity of the population. Also, a hunter ethic has emerged that promotes sparing the hen. This thinking has been embodied in current waterfowl hunting regulations using a "point" system, under which a hunter is allowed to shoot until the point value of the ducks in his bag reaches or exceeds an established maximum (e.g., 100 points). For example, in the 1974 Central Flyway regulations, hunters in some states were encouraged to take drake mallards in preference to hen mallards by the assignment of points: 20 for the drake versus 70 for the hen. Under more conventional regulations in other parts of the Flyway, the taking of hen mallards was discouraged by a segregated bag limit: three mallards of which only one could be a hen. Although such regulations were established to encourage hunters to take more drakes than hens, it is not known precisely how effectively they operate.

In striving to achieve any particular sex ratio, that goal must be placed in proper perspective relative to other population variables, because of other consequences that may result from the action taken. For example, our predictive model showed that by increasing hunting of drakes, the likely result would be a more balanced sex ratio. Regulations to accomplish this might also have the unintended results of an increased harvest of hens and an overall lowering of the population, which may not be acceptable consequences.

Predator Management

The third question of importance concerns the management of predators, specifically reducing red fox populations. This notion is not new and has been tried occasionally (e.g., Balser et al. 1968; Chesness et al. 1968; Trautman et al. 1973; Duebbert and Kantrud 1974) with various degrees of success. It has some appeal because of the beneficial effect on the productivity of ducks as well as some other game animals consumed by foxes. Certainly the present study implicates the red fox as having a much greater impact on adult mallards than had been previously recognized.

Of the many political, moral, biological, and practical ramifications of predator control, we treat only biological concepts and practical considerations. Because both mallards and red foxes are widely distributed throughout the Prairie Pothole Region, only a widespread reduction of the fox population would have any significant effect on mallard sex ratios. The change that would result from lowering of the fox population is suggested by our predictive simulations (Table 21). An approximate halving (equivalent to maintaining the population at low levels) of the average densities of foxes in our reference area during 1963-73 resulted in a mallard sex ratio near parity.

Like other aspects of game management (e.g., habitat manipulation), predator management is a question of alternatives that are becoming increasingly pertinent as demands for game resources increase. In highly disturbed environments such as those throughout much of the prime mallard breeding range, a predator can have a significant influence on the densities of some game species. Yet, predation is part of the natural mechanism that regulates prey densities and from an evolutionary standpoint, the fox and other predators have probably had a beneficial effect on the survival of the mallard as a species. As one of many mortality forces, predation tends to keep the genetic characteristics of the prey in concert with a changing environment. Conflicts arise when prey species are also game, because predators effectively compete with man for population surpluses.

From what has been learned about the population dynamics of red foxes, it is clear that the task of controlling foxes over a large area is one of considerable proportion. Being territorial, foxes space themselves out and voids are quickly filled by neighboring or transient animals. Foxes disperse widely, individuals frequently traveling over 80 km (Storm et al. 1976). Also, a recent computer simulation study of fox population dynamics (Zarnoch et al. 1974) suggested that an equilibrium population will be sustained with an annual harvest as high as 55% of the population. Thus, even if the tools for achieving control were available, the economics of control were justified, and the biological and recreational ramifications of an extensive fox control program were deemed acceptable, actual control of foxes would not be easily achieved.


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