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

The Role of Supernumerary Drakes

In a brief summary of mallard social and sexual behavior, Johnsgard (1975) noted that the species is monogamous and pair formation takes place primarily in winter, so that most hens arrive on the breeding ground with mates. During the breeding season, mallard exhibit nest tenacity and aggression, which McKinney (1965) believed serve as a mechanism to disperse pairs and nests. Pair bonds gradually relax during incubation; the drake spends progressively less time with his mate until he finally abandons her. Unmated drakes intermixed within the breeding population have been reported for several species by Hochbaum (1944) and Bezzel (1959). Both authors observed pairs of mallards that were attended at various times during the breeding season by one or more mallard drakes.

Although the existence of supernumerary drakes has been recognized, there is almost no concrete evidence on their role in the breeding biology. Most discussion on the topic has been in the form of speculation and was summarized by Aldrich (1973), who cited three hypothetical advantages and one potential disadvantage an excess of males.

The first hypothesis was that an excess of drakes in group courtship is necessary to stimulate behavior-endocrine reactions leading to pair formation and synchrony of breeding condition in pairs. This conjecture was earlier presented by Dzubin (1970), who noted that in other avian species there was evidence that behavioral interactions are required to stimulate the female to breed. Dzubin (1970:11) stated, "Lack of the drake may have an inhibitory effect on the completion of the nesting cycle." If this is true, it would presumably be beneficial to have a slight surplus of drakes in order to maximize the number of females breeding.

The second hypothesis given by Aldrich was that an excess of drakes is necessary to ensure a mate for renesting of hens that lose their first clutches of eggs at a time after their first mates have departed. Dzubin mentioned the possibility that excess drakes may provide renesting hens with drakes that are either more aggressive and capable of maintaining a waiting area, or more "virile," in the sense that their testes have not regressed, thus ensuring viable eggs for replacement clutches.

Aldrich's third hypothesis was that "excess" drakes ensure that there will be at least one available male to pair with each female in species the males of which mature at an older age than females. Lack (1968) cited two seabirds in which the males initiate breeding an average of 1 year later in life than the females. Male mallards and other dabbling ducks, however, are capable of breeding as yearlings, but it is possible that older males are more virile and better able to protect the hen and her loafing and feeding sites. It would then be advantageous to have enough older drakes to accommodate all hens, leaving younger drakes unmated.

According to Aldrich, the potential disadvantage of excess drakes was that they may be damaging to production because they harass the hens, either causing them to fail to nest or to disperse too far from water to nest during drought periods, with resulting hazard to ducklings. This idea is similar to that originally offered by Bellrose et al. (1961) and expanded upon by Crissey (1969), who proposed a theory of mallard population dynamics in which production is more dependent upon habitat conditions (specifically, the number of July ponds) than upon the size of the breeding population. Crissey further suggested that the available habitat could support more breeding hens if the number of males were reduced so as to lessen the spacing and territorial requirements caused by male activity. He proposed a ratio of 50 or fewer males per 100 females. His theory and consequent management recommendations have found little favor with waterfowl biologists. Dzubin (1970:9) cautioned that a reduction in the number of males to half that of females "is biologically untenable (for the mallard)," and went on to cite various supportive arguments.

The fragmentary evidence suggests that the presence of slightly more males than females in mallard populations confers some survival advantage to the species. Prairie mallards have evolved a reproductive strategy that reflects the mean population response to selective pressures. "The behavior of each species has evolved as a compromise between many competing selection pressures" (McKinney 1973:10). One feature of the strategy was a quaternary sex ratio that we conclude was ordinarily only slightly biased toward males. The mallard's adaptations made the species fit for an environment that existed before European settlement. It is perhaps too much to expect these adaptations to have the same positive survival values in areas so drastically altered by man. As A. Dzubin (personal communication) pointed out, the first sign of this new maladaptation is a distorted quaternary sex ratio. We conclude that, in keeping with the survival advantages of having "surplus" drakes, many such birds are dispersed in spring populations where they are available to fulfill the roles described by Aldrich (1973). Over and above some as-yet-undefined level, however, surplus drakes do not represent valuable additions to populations. In the absence of more definitive data, we suggest that it would be unwise to strive for a sex ratio of parity or less but that males in excess of 110:100 likely have little or no positive effect on productivity.

Previous Section -- Part Four: Implications of a Distorted Sex Ratio
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