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Depredation Patterns of Predator Species

Striped Skunks

Striped skunks forage throughout uplands (Storm 1972). They are likely to depredate duck nests in all upland habitats but seldom depredate nests over water (Sargeant and Arnold 1984, Sullivan 1990).

Response to Hens and Nests--Striped skunks are not known to capture hens (Sargeant and Arnold 1984). We did not observe striped skunks encounter duck nests but occasionally observed them at nests. We encountered a radio-equipped striped skunk at a gadwall nest during the night the eggs hatched. The hen and brood were present the next morning, suggesting that some hens can defend their nests from skunks.

Striped skunks often did not completely depredate duck nests during their initial visit to the nest. Of 13 duck nests at which radio-equipped striped skunks were encountered without interrupting the depredation, skunks ate all eggs (n = 4-8 eggs/nest) of 8 nests and partially depredated the other 5 nests. At each of the latter nests, skunks ate 3-9 eggs and left 2-5 whole eggs. Each of these was completely depredated, presumably by striped skunks, when rechecked 1-2 days later. Maximum number of eggs known to be eaten by a radio-equipped striped skunk in 1 night was 13 (an entire gadwall clutch). The skunk ate 11.5 of the eggs early in the night. Then, after traveling ≥1.5 km from the nest, it returned at dawn and ate the remaining 1.5 eggs. Of 46 artificial duck nests in the compound depredated by striped skunks, 1-3 whole eggs of 6 placed in each nest remained in each of 4 (9%) nests the next day.

Treatment of Eggs--We found that striped skunks seldom removed eggs or eggshells from nest sites, unless the nest was near their den or retreat. We accounted for 157 (91%) of 173 eggs depredated by 3 of 4 skunks studied in the compound, based on eggshells found at the nests. The other 16 eggs seemed to be accounted for by shell fragments. In contrast, we accounted for only 51 (53%) of 96 eggs depredated by the fourth skunk studied in the compound from eggshells found at nests. That skunk had an underground den about 10 m from the nests. We found numerous eggshells along the trail between the nests and the den. We once found duck eggshells (n=2) near the den of a radio-equipped striped skunk. We have no other evidence of striped skunks removing eggs from nest sites or of striped skunks hiding eggs.

Appearance of Eggshells and Nest Site--Several investigators describe appearance of eggs depredated by striped skunks, but data are few and most descriptions vague (Appendix A). However, there is general agreement between their findings and ours that shells of eggs depredated by striped skunks usually have a large elliptical hole with coarsely broken pieces of shell caved inward (Appendix C, Fig. 4b, d, f-k). Most of 275 eggshells examined for damage by striped skunks had large holes (64%) or were severely damaged (25%; Appendix B, Table 3).

We examined 206 eggshells from artificial duck nests and 48 eggshells from natural duck nests destroyed by striped skunks for location of openings (Appendix B, Table 4). Collectively, 102 (40%) eggshells had the opening in the side, 96 (38%) in a side-end, and 56 (22%) in an end. However, 1 captive skunk opened eggs differently than the other skunks. Fifty percent of openings made by that skunk were in an end of the eggshell compared with 11 % of openings made by the other skunks. We seldom found an eggshell with puncture marks, a puncture hole, or conspicuous yolk residue.

We found no dug areas at duck nests destroyed by skunks. However, Baker (1978) found small holes had been dug at 4 (27%) of 15 artificial nests destroyed by striped skunks.

Skunks often displaced considerable nest material from nests and often matted vegetation where they ate eggs. Most (78%) of 46 artificial nests depredated by skunks in the compound had >30% of the material displaced; only 4 nests (9%) had no material displaced (Appendix B, Table 5). Displaced nest material usually was pulled out from 1 side of the nest. Darrow (1938) and Rearden (1951) reported that striped skunks may tear apart or demolish nests; however, none of the nests we examined were disturbed in that manner. We often noticed a trail of trampled vegetation extending out about 1 m from nests destroyed by the radio-equipped skunks, especially in dense vegetation. Displaced nest material and eggshells found outside those nests were on the trails.

Skunks customarily ate duck eggs ≤1 m from the nest. Of 151 eggshells found at nests depredated by 3 captive skunks without a den near the nests, most (72%) were ≤20 cm from the nest. Eggshells were often in the nest, and few (4%) were >1 m from the nest (Appendix B, Table 6).

Conclusions--The striped skunk is 1 of 2 principal predator species (the other is the raccoon) that customarily leaves eggshells and/or shell fragments of all depredated eggs at duck nests. Evidence that strongly indicates a nest was destroyed by striped skunks includes finding at the nest most or all of the following: (1) eggshells of ≥50% of eggs of the clutch and sufficient shell fragments to account for the other eggs, (2) each eggshell has a large hole or is severely damaged, (3) ≥50% of openings in eggshells are in the side or a side-end, (4) >10% of nest material displaced (on ground), and (5) all eggshells are ≤1 m from the nest (Appendix B, Table 2).

Evidence that indicates a nest was not destroyed by striped skunks includes finding at the nest any of the following: (1) the hen was killed, (2) cached egg, (3) <50% of eggs of clutch represented by eggshells, (4) ≥50% of eggshells have small hole openings, (5) dug area, (6) aerially displaced nest material, or (7) eggshell with a conspicuous yolk residue (Appendix B, Table 2).