Northern Prairie Wildlife Research Center
Ermines and long-tailed weasels forage primarily in uplands (Sargeant et al. 1993). They likely destroy duck nests in all types of upland habitats but are not known to destroy duck nests located over water.
Response to Hens and Nests--Hens have been reported killed at nests by long-tailed weasels (Keith 1961), but methods used to identify cause of mortality were not provided. Hens were not killed at nests destroyed by smaller ermines (Fleskes 1988, J. P. Fleskes, Iowa Coop. Fish and Wildlife Res. Unit, Iowa State Univ., Ames, pers. commun.). Long-tailed weasels, being similar to minks in morphology and habits, probably kill ducks by biting them in the cranium, and may drag the carcass away from the nest site.
We found no descriptions of weasels encountering duck nests. However, destruction of a nest by weasels usually involves gradual removal of eggs during a period of several days (Fleskes 1988). Fleskes (1988) found that weasels commonly partially depredate nests, and that hens attending such nests usually continued laying or incubating until most eggs were depredated. In that study, 36 (95%) of 38 nests believed destroyed by weasels (probably all by ermines) had repeated losses of eggs between visits (3-10 day intervals) to nests by investigators. Each nest had ≥1 whole egg remaining when it was abandoned by the hen, and the clutch was considered destroyed.
Treatment of Eggs--Weasels remove many duck eggs from nest sites. Barkley (1972) observed an ermine carry 6 whole eggs in its mouth (1 at a time) from the nest of a ruffed grouse. Fleskes (1988) accounted for only 20% of eggs from duck nests that failed because of depredation by weasels, based on eggshells found at or near the nest. Twenty-nine percent of the eggs of those nests were accounted for by whole eggs; 51% were missing without a trace of shell. Most (79%) whole eggs were in the nest. Most eggshells and a few whole eggs were in narrow trails under dense vegetation, sometimes in small groups.
Appearance of Eggshells and Nest Sites--Weasels have difficulty opening duck eggs (Teer 1964, Fleskes 1988). Generally, they begin opening an egg by biting into an end or side-end. They enlarge the hole by biting- and prying-out pieces of shell. This action often results in an elongated narrow slot with finely chipped edges across or down the egg (Appendix C, Fig. 7e, g, i, j; also see Teer  and Fleskes ). Fleskes (1988:15) described the openings as "...ringed with numerous small shell fragments and "bite-outs"...."
A factor in identifying destruction of some duck nests by weasels is presence of paired canine puncture marks on ≥1 eggshells. Fleskes (1988) found such marks on ≥1 eggshell at 4 (17%) of 24 duck nests at which eggshells of eggs depredated by weasels were found. Teer (1964) found paired canine puncture marks on duck eggs depredated by long-tailed weasels. Distance between paired canine puncture marks usually will distinguish eggshells of eggs depredated by ermines from those depredated by long-tailed weasels, and between eggs depredated by weasels and other carnivores (Appendix B, Table 1).
Most of 15 eggshells examined for damage by weasels had small (60%) or large (33%) holes (Appendix B, Table 3). We examined 13 eggshells of duck eggs depredated by weasels for location of openings and found little difference in proportions with openings in the end, side, or side-end (Appendix B, Table 4). Fleskes (1988) found conspicuous egg residue (presumably included yolk) in ≥1 eggshell at 10 (42%) of 24 nests at which eggshells of eggs depredated by weasels were found.
We found no evidence of weasels digging at nests. Fleskes (1988) found no displaced nest material at 38 nests apparently destroyed by weasels.
Conclusions--Scant information exists concerning destruction of duck nests by weasels. Where weasels are major predators of duck nests, many clutches should have eggs missing before any egg hatches or the clutch is destroyed. Evidence that strongly indicates a nest was destroyed by weasels includes finding at the nest most or all of the following: (1) ≥1 whole egg, (2) eggshells of <50% of clutch, (3) eggshells with numerous puncture marks, (4) eggshell with fine serrations along edge of an opening, and (5) no displaced nest material (Appendix B, Table 2). Finding a dead hen with bite marks in the cranium may indicate a nest was destroyed by long-tailed weasels.
Evidence indicating a nest was not destroyed by weasels includes the following: (1) cached egg, (2) eggshell in nest, (3) eggshells of ≥50% of clutch, (4) displaced nest material, or (5) dug area (Appendix B, Table 2).