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The level of the fox population exhibited a marked influence on the mallard spring sex ratios. A reduced fox population caused nearly even sex ratios; in effect, the hen-selective fox predation balanced the drake-selective hunting mortality. Higher fox populations resulted in sex ratios more distorted than in the standard case.

The level of the mallard population was not proportionally as important a determinant of the sex ratio as were the other two variables, although a doubling of the population resulted in more even sex ratios. In this situation the fox predation was dispersed over more birds, so the rate dropped to a point where it counteracted hunting in its sex-selective mortality.

Hunting rates exerted a very powerful effect on the sex ratio. For example, a change in the average rate of hunting mortality on males from 16% to 20% caused a nearly even sex ratio. Hunting, of course, affects not only the sex ratio; it partially determines the population size. Because of the potent effects of hunting on population dynamics, hunting should be very carefully employed as a management tool.

We discuss in Part Four the implications of a distorted sex ratio, as they relate to management of mallards. The results of the predictive modeling will be useful in examining the efficacy of various management strategies involving the sex ratio in a population.