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

Changes in Canid Predation Rate

We now consider if the expansion of red fox populations in the Prairie Pothole Region resulted in increased sex-specific predation on breeding mallards. During pristine times, populations of gray wolves were high while populations of coyotes and red foxes were relatively low (a population of wolves ranked high contains considerably fewer individual than similarly ranked populations of the smaller canids). Since then, gray wolves have been eliminated and a transition from multi-species, wolf-dominated canid populations toward single-species, red fox populations has occurred.

This change no doubt affected the predation rate on breeding mallards. Outside of the data we presented in Part One for red foxes, however little is known of the nature of canid predation on breeding ducks. We singled out the red fox as the predator most likely to have the greatest impact on adult mallards, but we suggested that coyotes too could prey heavily on mallards if populations of that canid were high. We suggest that, among the prairie predators, canids as a group have food habits, hunting strategies, physical characteristics, and population densities that enable them to inflict the greatest losses among breeding mallards, especially hens. We believe that they were among the principal predators of breeding mallards during pristine times but that their impact then was less than now. As we speculate about the history of canids and their impact on waterfowl, two primary questions must be resolved: (1) Have canid population changes since early times resulted in increased numbers of canids per unit area, and (2) is the red fox a more effective predator on hen mallards than are the other species?

Canid Density Changes

The density of canids (total individuals per unit area) is a strong determinant of the potential for predation. Density, in turn, is dependent on the spatial requirements, intra- and inter-specific tolerances, and social organizations of the individual species.

The spatial requirements of wolves, coyotes, and red foxes were discussed by Mech (1970), Gipson and Sealander (1972), and Sargeant (1972), respectively. Individual wolves generally occupy areas in excess of 130 km2, coyotes often occupy areas of less than 26 to 65 km2, and red foxes occupy areas of 2.6 to 8 km2. All three species exist in social groups (families) and, at least for the wolves and red foxes, family members simultaneously occupy the same territory. There is a tendency for the territories of wolves and foxes to be largely contiguous and nonoverlapping. The spatial relations of coyotes are probably similar to these species.

Because of these spatial characteristics and because of broad habitat tolerances of canids, nearly all land in the Prairie Pothole Region was likely occupied by one of the three principal species when populations of that species were high. At those densities individual family groups would have been spaced rather uniformly across the landscape. As densities declined, territory sizes would have increased and the intensity of use of habitat diminished until maximum sizes were reached. Thereafter, space would have been freed from the presence of that canid, first as small tracts of least preferred habitats and then, when populations declined further, as large expanses of terrain.

A. B. Sargeant, D. B. Siniff, and D. W. Warner (in preparation) suggested that maximum territory size for red foxes is 10.4 to 13 km2. Uniform spacing of fox families is apparent from the locations of rearing dens as shown for a moderate population on a 269-km2 block within our three-county study area (Fig. 12). Each den represents the focal point of activity for a family. Further evidence of spacing among foxes was presented by Scott (1943), Sargeant (1972), and Sargeant et al. (1975).

The mechanisms whereby large canids suppress populations of smaller canids are largely unknown but in instance after instance, coyotes have fared poorly in wolf-dominated areas and red foxes have fared poorly in coyote-dominated areas. What is known about the relationships among these three species suggests that the degree of tolerance is much higher between species of greatest size difference. Wolves are more tolerant of foxes than they are of coyotes (Murie 1944; Young and Jackson 1951; Mech 1970). Confusing the issue is that, at times and in certain situations, smaller canids may actually benefit from the larger canids by feeding on the remains of prey left by the larger species (Young and Jackson 1951; Mech 1970). Direct killing of small species by larger species occurs, but there is insufficient evidence to suggest that killing is the principal mechanism for suppression.

Canid composition changes are often gradual, such as during the 9-year period when coyotes disappeared from Isle Royale (Mech 1970). There are many examples of intermediate multiple-species populations which probably represent situations where populations of the larger canids are insufficient to occupy or extensively utilize the available space and/or where localized habitat differences favor different species in nearby areas. Behavioral traits that cause smaller canids to avoid larger canids may partly determine population compositions. Density of the larger species coupled with environmental factors such as availability of food would determine the amount of contact between individuals. As avoidance becomes increasingly difficult, smaller species would be at a competitive disadvantage and decline in number.

Social groups of the larger canids tend to include more adults than those of the smaller species; wolves travel in packs, red foxes live primarily as pairs, and coyotes are probably intermediate. Thus, even though wolves occupy more space than the smaller canids, their territories are in turn used by more individuals. However, even with this density-equalizing factor there are considerable differences in the actual density of comparably ranked populations. Relatively high breeding populations of each species are about one wolf per 26 km2 (Mech 1970), one coyote per 5-8 km2 (Gipson and Sealander 1972; Gier 1968; Young and Jackson 1951) and one red fox per 2 km2 (Sergeant 1972). While not precise, these estimates are adequate for a general comparison.

Canid Predation Effectiveness

Food habits studies have shown that the types of prey taken by red foxes are also used by coyotes and wolves (Scott 1943; Fichter et al. 1955; Mech 1970), but that not all foods available to wolves and coyotes are available to foxes. Because of their size, the wolf and coyote can prey on species too large for red foxes. The hunting of large prey by wolves and coyotes has resulted in group hunting strategies (Cahalane 1947; Mech 1970), which are rarely observed among red foxes (A. B. Sargeant, D. B. Siniff, and D. W. Warner, in preparation). The red fox hunts alone and because of its size turns its attention to small prey. For these reasons the red fox has very likely evolved as a more effective predator on upland-nesting ducks than is either the wolf or coyote.

The amount of travel by the different canids also has a direct bearing on their chances of encountering individual mallards. Adult foxes travel on the average about 14.5 km every 24 h (A. B. Sargeant, D. B. Siniff, and D. W. Warner, in preparation). Limited data presented by Gipson and Sealander (1972) suggest that travel by coyotes may average little more than that of foxes. Wolves may travel 32 km from dens (64 km round trip) during one night to obtain food (Mech 1970), but where food is closer to dens the average distance traveled may not greatly exceed that of foxes. Thus, foxes, because of their relatively small spatial requirements and solitary movements, rather thoroughly cover all parts of their territories regularly (Fig. 13); their chances of encountering individual waterfowl on nests are high. Wolves and coyotes, on the other hand, occupy much larger areas and spend much time traveling in pairs or larger groups. They seldom visit much of the area they occupy and their chances of detecting an individual nesting hen are decidedly less than those of a fox.

These factors in concert suggest that the vulnerability of breeding mallards to canids has increased since pristine times.

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