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Foods and Foraging of Prairie Striped Skunks
During the Avian Nesting Season

Methods


Skunk capture and monitoring

We captured skunks each year in live traps, primarily during April and May (Sargeant et al. 1982). Captured skunks were anesthetized with ketamine HCl (Beck 1976), ear tagged, and equipped with radiocollars that had fluorescent tape affixed for night recognition. We determined sex, and we assessed reproductive condition of females. Nearly all resident adults were under radiosurveillance (Sargeant et al. 1982). Procedures complied with animal care and use guidelines at the Northern Prairie Wildlife Research Center.

Each year, from vehicles, we conducted dusk-to-dawn tracking of all collared skunks simultaneously at approximate 3-day intervals during 2-week sample periods (15-30 April, 1-15 May, 16-31 May, 1-15 June, 16-30 June, and 1-15 July). We located skunks at about 2-hour intervals during night-tracking. The next morning, we investigated suspected foraging sites (indicated by ≥2 radio locations in close proximity during night-tracking) and searched for evidence of foraging (e.g., digging) or food sources (e.g., carrion, destroyed eggs). To confirm specific locations of foraging sites, personnel on foot observed skunks during twilight periods.

We attempted to locate daytime retreats of all radiocollared skunks daily. Each retreat was assigned a unique number marked on a wooden lath placed at the site. When a retreat was first located, we collected and discarded all scats found at the site (striped skunks commonly defecate near their dens [Hamilton 1936, Verts 1967]). Thereafter, we revisited retreats within 1-3 days after each use by a radiocollared skunk and collected all fresh scats. We bagged individual scats separately, identified each by date, retreat number, and skunk number, and then froze scats until analyzed. Each scat was assumed to contain residue of foods eaten during the 24-hour period preceding deposition.

Scat analysis

We washed individual scats with water through stacked number 10 and number 40 soil sieves. We identified food remains to lowest taxon possible, using keys and by comparing with reference materials. Bird eggshell fragments were categorized based on thickness (Greenwood 1981). We considered fragments ≤0.16 mm (thin) to be from passerine birds and fragments >0.16 mm (thick) to be from larger birds, mostly ducks (Anatinae), but potentially including other taxa (e.g., Tetraonidae, Phasianidae, Rallidae, and Scolopacidae). We categorized remains of adult birds, based on size of feather and bone fragments, into small bird (e.g., blackbird [Icteridae] size and smaller) or large bird (e.g., duck size). Feathers in sheaths were considered to be from embryos or nestlings.

Diet determination

We determined percentage occurrence of food remains in scats (Scott 1941) by 2-week periods. We pooled results from the first 3 periods (spring) and last 3 periods (summer) and tested for effects of sex (female or male), season (spring or summer), year (1976, 1977, or 1978), and their interactions on occurrence of major food groupings and taxa. We used a split-plot repeated measures ANOVA (GLM Procedure, SAS Institute, Incorporated, 1989), because 39% of the individual skunks were represented by scats in both seasons (5 of 11 skunks in 1976, 6 of 26 in 1977, 12 of 22 in 1978). Each skunk within a year was the whole unit and season was the subunit. We used only skunks with ≥5 scats in a season. Although 9 skunks were present in more than 1 year, we considered individuals to be independent among years. We weighted percentage occurrence estimates for each skunk in a season by number of scats for that skunk that season, because precision of estimates is proportional to number of scats. We used least-squares means and their standard errors (SE) to estimate population mean percentage occurrence; least-squares means are unbiased estimates of population means (Milliken and Johnson, 1992). We used Fisher's protected least significant difference (Milliken and Johnson 1992) to make pairwise comparisons among means for significant main effects. When tests resulted in P≤0.05, we concluded that means were different.

Sampling food resources

At suspected foraging sites where we detected evidence of foraging on invertebrates, we estimated abundance of organisms with multiple soil excavations (0.25 m² × 5 cm deep), expressed as organisms/m², or multiple sweepnets (0.30 m² × sweep distance), expressed as organisms/m³. We identified food items found at foraging sites to the lowest taxon possible. We present results of sampling as arithmetic means and ranges.

We discovered some duck nests each year during study activities and monitored them to determine whether eggs were depredated by radiocollared skunks. We marked each nest with a flagged stick placed 4 m away (Klett et al. 1986). We checked nest fates every 2-3 weeks, or more often if we thought a nest may have been discovered by a skunk.


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