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Wildlife Monographs

Red Fox Predation on Breeding Ducks
in Midcontinent North America

Results and Discussion


Species Compositions

Effects of Population Composition. Of the 1,857 adult ducks found at dens in the midcontinent area, 1,798 (96.8%) were dabbling ducks, 48 (2.6%) were diving ducks, and 11 (0.6%) were unidentified (Table 1). The dabbling ducks were comprised of the 7 most common duck species in the midcontinent area plus 1 wood duck (Aix sponsa). The percentage species composition of the dabbling ducks varied among sample areas but was strongly related to species abundance in breeding populations. This relationship is apparent in the North Dakota and South Dakota data that we combined into 2 samples, 1 representing pothole habitat east of the Missouri River and 1 representing stockpond habitat west of the Missouri River (Table 2). The percentage of each dabbling duck species in the east river data approximated the percentage in the breeding population, although significant (P < 0.05, standardized residual) discrepancies were noted. Mallards and northern pintails exceeded their occurrence in the population, whereas gadwalls occurred less frequently than in the population.

In the west river areas of North Dakota and South Dakota, mallards were the most common duck at dens and most abundant species in the breeding population. The representation of both mallards and northern pintails at dens, however, greatly exceeded their relative abundance in the population (P < 0.05). These 2 species ac counted for 88% of the ducks found at west river dens but made up 51% of the population. All other duck species, except green-winged teal, occurred less frequently at west river dens than in the population.

The relationship between vulnerability and relative abundance in breeding populations is also evident in the 1969-73 data from the 3-county intensive study area in east river North Dakota (Table 3). The relative abundance of each duck species at dens in the intensive study area each year was similar to its relative abundance in the population; only small differences separated the 5-year average values. However, as with the combined data for east river North Dakota and east river South Dakota, percentages of both mallards and northern pintails among ducks at dens exceeded percentages in the population (each year), whereas percentage of gadwalls at dens was less than in the population (4 of 5 years). The relative abundance of blue-winged teals at dens was less than in the population, likely because of a disproportionate number of teals (presumably nearly all blue-winged teals) among excluded unidentified ducks. Data for American wigeons and green-winged teals indicated they too were taken in proportion to their abundance.

The 48 diving ducks found at dens consisted of 46% lesser scaups (Aythya affinis), 27% ruddy ducks (Oxyura jamaicensis), 21 % redheads (A. americana) or canvasbacks (A. valisineria), and 6% scaups or ring-necked ducks (A. collaris). Thirty-nine of these were from east river North Dakota where they comprised 2.4% of the group-identified (dabbling ducks or diving ducks) ducks found at dens but where diving ducks made up 14.5% of the duck population (USFWS Breeding Pair Survey, Stratum 46). Lesser scaups accounted for at least 44% of that sample but made up 10% of the breeding diving duck population.

Eight diving ducks were among 31 group-identified ducks found at dens in Manitoba (Table 1). The relatively high percentage of diving ducks at the Manitoba dens (26%) corresponded to a higher proportion of diving ducks in the Manitoba population (Kiel et al. 1972:53). Only 1 diving duck was found among 207 group-identified ducks from dens in the other sample areas although many migrant and some breeding diving ducks were present in all areas.

From these data it is apparent that dabbling ducks were preyed on more extensively and diving ducks less extensively than would be expected from their relative abundance in the population. Among diving ducks, the upland nesting lesser scaup was taken most often.

Effects of Nesting Chronology.Although abundance of each duck species in populations is a major factor affecting the number of each species taken by foxes, other factors such as nesting chronology also affect vulnerability. Nesting chronology is especially important because most adult dabbling ducks taken by foxes are hens believed caught at nests and because timing and duration of nesting by each duck species is different.

Data on nesting chronologies of the 5 principal dabbling duck species in our 3-county intensive study area (mallard, northern pintail, northern shoveler, blue-winged teal, and gadwall) during 197677 revealed that mallards and northern pintails nest early in the season, northern shovelers and blue-winged teals nest in midseason, and gadwalls nest late in the season (Fig. 6). Approximately 1 month separated the onset of nesting by the early and late nesting species, but nesting by all 5 species extended into July. Because of extensive renesting resulting from high rates of nest destruction early in the nesting seasons, hens of early nesting species were exposed on nests to foxes longer than hens of late nesting species. These data and the data in Tables 2 and 3 show the earlier and longer a species nested, the greater was its vulnerability to foxes.


gif -- Nesting Chronologies
Fig. 6. Nesting chronologies (percentage distribution of active nests by week) of 5 principal dabbling duck species nesting on a 37-km highway right-of-way in Stutsman County, North Dakota, for combined years, 1976-77. Eight hundred nine nests were involved; numbers of nest observations for each species ranged from 233 to 1,189.

The effect of differences in nesting chronology on the vulnerability of dabbling ducks is also revealed by changes in relative abundance of each of the 5 principal species at early and late season dens in the 3-county intensive study area (Table 4). For this comparison we separated our data into early (22 Apr-11 Jun) and late (12 Jun-20 Jul) samples (no dens were visited before 22 Apr or after 20 Jul). Although the separation date is about 2 weeks after the middle of the duck nesting season (Fig. 6), it conformed to the timing of our den searches. In addition many ducks taken by foxes during our designated early period were not recovered from dens until the late period. The proportion of mallards among ducks at dens decreased, northern shovelers remained relatively constant, and gadwalls increased as the nesting season progressed reflecting the early, mid, or late season nesting habits, respectively, of each species. Results for the northern pintail (an early nesting species) and blue-winged teal (a midseason nesting species) do not conform to the expected pattern. The tendency for northern pintails to nest on cultivated lands where they suffer high rates of nest destruction from tillage early in the nesting season (Krapu 1977) may have increased their vulnerability to foxes later in the season during renesting attempts. Identification bias was likely a factor with the blue-winged teals because a higher percentage of the unidentified teals were in the late season sample when fox pups were larger and consumed ducks more completely.
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