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Depredation Patterns of Predator Species

Minks

Minks forage primarily in or near permanent and semipermanent wetlands and probably destroy few duck nests in uplands (Sargeant and Arnold 1984, Arnold and Fritzell 1990). Nests over water or on small islands are especially vulnerable to minks (Aufforth et al. 1990).

Response to Hens and Nests--Minks prey on nesting hens (Sargeant and Arnold 1984, Aufforth et al.1990). Minks customarily kill ducks by biting them in the cranium or upper neck (pers. observations). Minks killed the hen at several of 31 nests destroyed by predators in the exclosure, and often removed the carcass from the nest site (e.g., to a retreat). Aufforth et al. (1990) also found dead hens and dead ducklings with severely bitten heads and upper necks at or near duck nests depredated by minks. Darrow (1938) and Sargeant et al. (1973) commented that minks often drag prey into shelters, such as dens or retreats.

We have no information on the response of minks to hens or duck nests. However, minks seem to be especially attracted to nests with eggs in late stages of incubation (Eberhardt and Sargeant 1977), when noise is present from pipping and hatching ducklings.

Treatment of Eggs--We have little information about treatment of eggs by minks. Eggshells often were scattered along trails, sometimes in small groups, leading away from 31 duck nests in the exclosure. Whole eggs were present at 6 nests. Investigators found no eggshells at 7 (23%) nests and no shell fragments at 16 (52%) nests. (The latter included 12 nest sites with eggshells with damage that indicated shell fragments may have been present but overlooked by investigators). Thus, minks may partially depredate nests and remove eggs from nest sites.

Appearance of Eggshells and Nest Sites--Captive minks often opened eggs by biting or prying with their canines an elongated irregular slot across the side of the egg (Appendix C, Fig. 6d-f, i). Darrow (1938:837) commented that, "Shell remains left by a weasel [and minks] usually show a longitudinal sectioning of the eggs and the edges of the remaining portion are finely chewed...." Aufforth et al. (1990) noted that small tooth marks were visible on edges of openings in duck eggs depredated by minks. Rearden (1951:390) concluded that, "Tooth marks found on unbroken portions of egg shells are one of the most helpful identifying signs left by mink." Distance between paired canine puncture marks can be used to help distinguish between eggs depredated by minks and other carnivores (Appendix B, Table 1).

Most of the 28 eggshells examined for damage by minks had small (36%) or large holes (50%) (Appendix B, Table 3). Most of 27 eggshells examined for location of openings made by minks had the opening in the side (63%) or a side-end (30%) (Appendix B, Table 4). Conspicuous residue of egg yolk often was present in eggshells.

We found no evidence of minks digging at nests. Additionally, no nest material was displaced from most (65%) of 31 nests in the exclosure presumably destroyed by minks; only 1 (3%) nest had >30% of the material displaced (Appendix B, Table 5). Rearden (1951) and Aufforth et al. (1990) noted that little nest material was displaced from duck nests destroyed by minks.

Nests destroyed by minks in the exclosure often had ≥1 well-defined narrow trail leading away from the nest, sometimes to a retreat. Rearden (1951:390) noted that "...mink runways frequently are found quite close to destroyed nests."

Most (70%) eggshells found at 31 nests destroyed by minks in the exclosure were outside the nest but ≤1 m from the nest.

Conclusions--Scant information exists concerning destruction of duck nests by minks. Presence of a hen carcass with bite marks in the cranium and/or neck suggests mink predation. Other indications that a nest was destroyed by minks include finding at the nest most or all of the following: (1) ≥1 whole egg, (2) ≥50% of eggs missing without a trace, (3) eggshells with numerous puncture marks, (4) no displaced nest material, and (5) well-defined narrow trails leading away from the nest (Appendix B, Table 2).

Evidence that indicates a nest was not destroyed by minks includes finding at the nest any of the following: (1) cached egg, (2) all of a clutch represented by eggshells and shell fragments, (3) all eggshells are in nest, (4) aerially displaced nest material, (5) dug area, and (6) ≥30% of nest material displaced (Appendix B, Table 2).