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

Mallard Sex Ratios in Pristine Times

The historical evidence which suggests that canid predation was less intense during pristine times than at present leads us to suspect that sex ratios were less disparate then. Not to be forgotten, of course, are the effects of other mortality factors. Most significant among these is drake-selective hunting, which is a strong mortality force counterbalancing hen-selective predation. Also, many other mortality factors were different during pristine times. There were more species of predators and certainly the effects of disease and natural calamities operated. In general we believe that these other mortality factors, in total, slightly favored survival of the drake.

We employed our predictive model from Part Two to explore presettlement mallard sex ratios by making the following assumptions: (1) hunting rates for both male and female mallards were zero, (2) mallard densities were as great or greater than at present, (3) canid predation was no more and probably less intense than at present, and (4) the sum of "other" mortality was comparable in intensity and selectivity to that observed today. We analyzed both Case I and Case II apportionment of the kill and scavenging of mallards by foxes. Male mallard populations were set averaging 200,000 (approximately the current level), 400,000, 800,000, and 1,600,000; standard deviations were increased proportionally. Fox densities (meaning all canid densities) were taken to be approximately half of the current level (generated by a = 1.50 and b = -0.00010 in Equation 5), approximately one-fourth the current level (a = 1.50, b = -0.00020), and about one-eighth the current level (a = 1.50, b = -0.00040). Sex specificity of canid predation was kept at 80% females.

Plots of the sex ratio resulting from the above combinations are shown in Fig. 14. Case II apportionments of kill and scavenging are shown; Case I resulted in somewhat more distorted sex ratios at all settings. All situations examined resulted in a preponderance of males, even with extremely few predators and abundant mallards, partially because the rates of "other" mortality, as calculated earlier, are higher for females than males.

The results of this exercise suggest that during pristine times there was a sex disparity favoring males, but that the disparity was much less pronounced than today, despite a higher proportion of older birds in the population. As a conjecture, we might suppose that the presettlement sex ratio was in the magnitude of about 110 males:100 females or less.

Even in pristine times, however, sex ratios were probably unstable and varied in response to environmental changes, which affected the mallard age structure. Bellrose et al. (1961:426) stated, "The age composition of a duck population is reflected in its sex ratios." This occurs because the sex ratio of juvenile recruitment is nearly balanced, which tends to moderate any imbalances in adult cohorts; the greater the proportion of old birds, the greater is apt to be the disparity. Thus, during periods of low productivity, such as would accompany droughts, populations would consist of increasing proportions of adult birds and sex imbalances would become more pronounced.

We infer that the rate of recruitment, on a long-term average, was just sufficient to compensate for adult mortality. If, by the absence of hunting and reduced predation, adult mallards survived longer than today, then the reproductive rate must have been lower. Mallards have a very high reproductive potential: they breed as yearlings, clutch size averages 8-10 eggs, and they renest readily if their clutch is destroyed (Johnsgard 1975). This capacity enables them to expand rapidly, following a drought, for example. Yet, after a series of years in which the population restored itself, either the death rate increased or the reproductive rate declined. We suggest that, although the first factor may have been operative, the second contributed most to stabilizing the population. Thus, the average recruitment was lower than today, which in many years resulted in populations made up more fully of older birds.

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