Northern Prairie Wildlife Research Center
Insects occurred frequently in scats throughout the nesting season, as reported by Dixon (1925), Hamilton (1936), and Verts (1967). Bird eggs and birds were second in occurrence in skunk diets, contrary to findings reported by Verts (1967). Our finding of great use of birds and bird eggs is consistent with knowledge that prairie skunks are important predators of ground-nesting birds (Kalmbach 1938, Sargeant and Arnold 1984, Greenwood 1986). Vickery et al. (1992) concluded that skunk depredation of bird nests occurred incidental to foraging for insects. Roseberry and Klimstra (1970) suggested depredation of meadowlark (Sturnella magna) nests increased coincidental to increased foraging by skunks as populations of prairie voles (M. ochrogaster) increased.
Vertebrates occurred less frequently in scats of female skunks early in the nesting season than later, suggesting that females may depredate fewer nests early in the nesting season when their movements may be affected by whelping. Skunks whelp in early May in North Dakota (Greenwood and Sargeant 1994). Females may not leave their natal den for several days after whelping and subsequent travels may be restricted for 1-2 weeks thereafter (R. Greenwood and A. Sargeant, unpublished data). Travels of male skunks are not affected by whelping; they do not assist in rearing of young.
Among insects, adult ground beetles and scarab beetles, scarab beetle larvae, and noctuid moth larvae occurred most frequently in scats. Examination of foraging sites confirmed sources of scarabs and noctuids, but we seldom found sources of ground beetles. Ground beetles occur in low densities, however, are highly mobile, usually nocturnal, and hide in deep burrows during daytime in dry weather (Kirk 1974). This may be why we seldom sampled them during daytime, even though they occurred frequently in skunk diets. Kirk (1974) reported that ground beetles are highly vulnerable to skunks at night. He found H. erraticus densities of 0.7/m² in South Dakota cropland, with a maximum approaching 3/m². At the few sites where we recorded ground beetles, densities were within this range. Kelker (1937) found ground beetles to be among the most commonly occurring insects in diets of skunks in Michigan.
Abundance of scarab beetles fluctuates annually due to multi-year life cycles; the most common cycle is a period of 3 years (Little 1957). Mean densities of phytophagous scarab larvae we observed at foraging sites were within the range reported in prairie by Fluke et al. (1932); the maximum density we detected (70/m²) was similar to theirs (82/m²). Larvae of Phyllophaga anxia, a species common in our study area, are found near soil surface in spring where soil is moist (Jarvis 1966). We found phytophagous scarab larvae to be most abundant in 1977 at sites in pastures, such as patches of buckbrush, where snow trapped in winter provided moist soil in spring. McDaniel et al. (1971) reported A. fimetarius to be the most prevalent scarab beetle in cow dung in the grasslands of eastern South Dakota; the greatest density they reported from literature was 100/pad.
Skunks foraging in areas with abundant soil invertebrates used slow, deliberate movements while locating prey (R. Greenwood and A. Sargeant, unpublished data). Vickery et al. (1992) reported that >90% of the time when skunks foraged, their noses were within 1-3 cm of the ground. Slobodchikoff (1978) concluded that skunks locate sedentary prey primarily by olfaction. Langley (1979) reported that auditory cues are important to skunks foraging on moving prey, such as ground beetles.
Grassland areas often were dimpled with thousands of shallow digs after prolonged periods of intensive foraging by skunks for invertebrates. We occasionally observed several (3-5) skunks foraging simultaneously in the same general locality, similar to an observation by Roseberry and Klimstra (1970). Jarvis (1966) reported foraging by skunks for scarab larvae sometimes was so intense in Nebraska hay fields that the sod was severely damaged. Although grasslands with thin sod (in poor condition) are more likely to be invaded by scarab beetles than grasslands with dense sod (in good condition, Fuelleman and Graber 1936), even dense sod can support scarab larvae at a density of 20/m² (Fluke et al. 1932), sufficient to attract skunks in our study.
As availability of invertebrate resources changed, focal areas of foraging also changed. For instance, during summer we found noctuid larvae at 56% of the foraging sites in 1977, but at only 5% of the sites in 1978 (Table 3). Coincidently, grasshoppers occurred at only 19% of the foraging sites in summer of 1977, but at 68% of the sites in summer of 1978. Such changes in skunk foraging patterns, fueled by changes in invertebrate availability and abundance, probably account for some of the extreme variability observed in daily survival rates of duck nests in grasslands (Klett et al. 1988, Greenwood et al. 1995). When invertebrate densities in grasslands are sufficiently great to stimulate persistent nightly foraging, it is almost inevitable that most nests in the focal area will be destroyed. When large expanses of grassland are available in a locality, however, intensive foraging by skunks may not recur in the same area until the next pulse of invertebrates becomes available. This would benefit nesting birds that initiate clutches in the interim and may enhance nest survival rates.