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

Abstract


Preponderances of males in many populations of wild ducks have been reported since 1932, but the cause of the sex disparity has only been speculated upon. This report examines the role that red fox (Vulpes vulpes) predation may play in the etiology of unbalanced sex ratios among dabbling ducks. A simple model of mallard (Anas platyrhyrnchos) population dynamics, as affected by fox predation, hunting, and other mortality, was developed for the Prairie Pothole Region of North Dakota during 1963-73.

Annual estimates of red fox densities, obtained from aerial censuses, sightings by rural mail carriers, and numbers of foxes killed at strychnine-baited draw stations, averaged 0.1 family per square kilometer. Spring mallard densities were adapted from annual Waterfowl Breeding Pair Surveys conducted by the U.S. Fish and Wildlife Service. An index to the rate of fox predation (including scavenging) on mallards was the average number of mallards discernible in food remains found above ground at fox rearing dens. The index was converted to a measure of fox predation by accounting for mallards at dens but not detectable in food remains found above ground, mallards taken by foxes but not brought to dens, and the number of dens used by foxes to rear their pups. Most (79.8%) of the mallards found at fox dens were female. The average annual predation rates were 0.959 male and 3.601 female mallards taken per fox family. Survival rates of adult mallards, as estimated from preseason banding data for 1963-67, averaged 67.8% for males and 61.7% for females. Hunting mortality rates, estimated by adjusting recovery rates to account for birds that were not retrieved or not reported by hunters, averaged 15.8% and 10.2% for adult males and adult females, respectively. The model was constructed so that the actual numbers of female mallards need not be known. That quantity was calculated within the model, by the use of generalized recruitment rates, which incorporated both the addition of young into the population and the net effect of immigration and emigration.

The output of primary interest was the simulated spring sex ratio in the final (11th) year of the reference period, which averaged 126 males per 100 females.

The validity of the model was assessed by mathematical means and by comparing its results to those from field studies. Recorded spring sex ratios of prenesting mallards in the Prairie Pothole Region varied from 108 to 129 males per 100 females, as compared to the simulated 11th-year value of 126:100. Available fall sex ratios of adult mallards were also in agreement with the results of the simulations. Mortality rates for spring through summer, as calculated from the model, averaged 16.4% for males and 28.5% for females, both figures comparable to those obtained in other studies. One conclusion drawn from the model was that mortality rates for factors other than fox predation and hunting were slightly higher for females than for males. An examination of the sex specificity of other mortality factors suggested that: accidents represent a small but significant source of mortality, somewhat higher in females than males; losses attributed to weather and disease appear to affect males and females in proportion to their occurrence in exposed populations; and among predators other than foxes in the reference area, only the mink (Mustela vison), which takes mostly drakes, has been identified as an important predator of adult mallards. Mortality factors other than fox predation and hunting tend to apply at slightly higher rates in females than males, which agrees with the results of the model.

After being modified to make predictive inferences, the model yielded an average male:female sex ratio of about 118:100, a value somewhat lower than those obtained in the initial model, but more reflective of the situation throughout the 11-year period. The simulated effects on the sex ratio of a sustained high fox population were further distortions, to about 131 males per 100 females. A sustained low fox population resulted in nearly even sex ratios. Halving of the mallard population heightened the disparity to 128 males per 100 females, while doubling yielded more balanced sex ratios. The sex specificity and intensity of hunting had a profound effect on the sex ratio; the simulated removal of hunting resulted in a sex ratio of about 155 males per 100 females, while increasing only the male rate yielded a more nearly balanced ratio.

The Prairie Pothole Region is the nesting ground for about half of the continental mallard population. During the past 100 years the character of the region has changed from nearly pristine wilderness to intensive agriculture. This transition altered the relationship between mallards and their environment, including man and predators. Human settlement changed the prairie canid composition from a multi-species, wolf (Canis lupus)-dominated complex during pristine times toward a single-species red fox population. This change increased the vulnerability of breeding mallards to canid predation. The predictive model was used to explore presettlement sex ratios, and suggested that in pristine times the mallard population was less imbalanced than today, the sex ratio probably being 110 or fewer males per 100 females.

Although the existence of "excess" drakes has been recognized in many populations, their role is poorly understood. Three possible advantages, and one potential disadvantage, that a surplus of males may confer on a population are described. The fragmentary evidence suggests that the presence of slightly more males than females offers some advantage to the species. Some differences between dabbling and diving ducks are pointed out by referring to the canvasback (Aythya valisineria), a species with very disproportionate sex ratios. An analysis of the mortality factors of this species suggests a reason for the more distorted sex ratio than in the mallard, even though predation losses may be less. Unlike the mallard, predation and hunting have compounding rather than compensatory effects on the sex ratio.

One management implication of an imbalanced sex ratio is that the usual methods of estimating duck populations from sample surveys may be biased upwards if unmated drakes are not distinguished from mated drakes waiting for their hens. Also, hunting regulations are a potent agent for altering the sex ratio. Finally, predator management directly affects certain populations of prey species. Sorting out the exact relationship between predator and prey populations is complex and the task of controlling foxes to reduce predation on hen mallards is difficult. The overall conclusions of the report are: an imbalanced sex ratio in prairie mallards can result from reduced female survival attributed largely to predation by red foxes; hunting has only partially restored sexual balance; the sex ratio of prairie mallards has become increasingly a function of human-influenced mortality, and red foxes are now the major single source of nonhunting mortality of hen mallards in North Dakota.


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