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

Part Three: Historical and Geographical Perspectives

Searching questions have been raised about the history and causes of disparate sex ratios in ducks (Dzubin 1970; Aldrich 1973) because, as pointed out by Bellrose et al. (1961:427), "It seems reasonable to question the value of those drakes in excess of the number needed to provide mates for the hens in waterfowl populations. In the event such drakes do not play an essential role in species survival, an effort should be made to provide for their utilization." The implication of our findings is that "extra" drakes are indeed present in prairie mallard populations, largely due to differential adult mortality rates from red fox predation and hunting. Our findings will draw greater attention to these "extra" drakes and, therefore, it is imperative to understand how present sex ratios came to be. Before the supernumerary drakes are harvested we must determine if disparate sex ratios of the magnitude observed today are a result of natural evolutionary mechanisms that enhance the survival of the species or if they are largely an artifact of modern man's activities.

Our North Dakota reference area is part of the Prairie Pothole Region of central North America, which includes the aspen parkland transition area to the north. Although the Prairie Pothole Region is a relatively small part (12%) of the North American mallard breeding range (Fig.9), it is the nesting ground for about half of the continental mallards (Pospahala et al. 1974). Because of the large number of mallards nesting in the Prairie Pothole Region, factors that significantly affect mallards there are of consequence to the continental population.

The Prairie Pothole Region was settled by Europeans during the past 100 years; since settlement began there have been considerable changes in mallard habitats, human interactions with mallards, and predator compositions and densities. Our purpose in Part Three is to examine the changes in our North Dakota reference area since pristine times that relate to mallard sex ratios and to place them in perspective relative to the entire Prairie Pothole Region. Emphasis is on factors affecting densities and distributions of mallards and of canids. Finally, we conclude by employing the predictive model of Part Two to reconstruct probable mallard sex ratios during pristine times in that region.

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