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Depredation Patterns of Predator Species

Raccoons

Response to Hens and Nests--Raccoons rarely capture hens (Sargeant and Arnold 1984). We did not observe raccoons encounter duck nests. We obtained information on depredation of 28 artificial nests by 4 captive raccoons in our studies. Raccoons often did not take all 6 eggs from a nest during the first night but returned subsequently to complete the depredation. Rearden (1951) reported an instance of partial depredation of a clutch of 8 chicken eggs in an artificial nest by a free-ranging raccoon. That raccoon ate 4 eggs when it first depredated the nest; the remaining eggs were eaten 9 days later.

Treatment of Eggs--Raccoons seldom removed eggs or eggshells from nest sites. Four captive raccoons depredated 159 (95%) of 168 eggs placed in the 28 artificial nests. We accounted for 144 (91%) eggs from eggshells found at the nests. We found eggshells from 5 of the 15 missing eggs near the nests. Shell fragments found at nests may have accounted for the 10 other missing eggs. Rearden (1951:389) noted that "When destroying a nest located in dry cover near water, a raccoon will frequently carry the eggs near the water to eat, at times leaving shells in shallow water." Nevertheless, he found eggshells <9 m (mostly <2 m) from all (n = 20) nests. We found no evidence of raccoons caching or otherwise hiding eggs.

Appearance of Eggshells and Nest Sites--Most (66%) of 144 eggshells examined for damage by raccoons had large holes; few (13%) had small holes (Appendix B, Table 3). Openings usually were round with numerous coarsely broken pieces of shell caved inward (Appendix C, Fig. 3a-e). We found no evidence of raccoons crushing duck eggs within their mouth. However, they often trampled eggshells (Appendix C, Fig. 3j-l), nest material, and vegetation at nests. We found no evidence of raccoons digging at nests. Greenwood (1979) also reported that raccoons opened duck eggs by biting a hole, and then ingesting contents.

Most (59%) of 226 eggshells of duck eggs examined for location of openings made by raccoons had the opening in an end (Appendix B, Table 4; Appendix C, Fig. 3a-f), presumably made while the raccoon held the egg upright. Only 27 (12%) of the eggshells had the opening in the side (Appendix B, Table 4). Other investigators (Darrow 1938, Rearden 1951, Baker 1978) also noted that raccoons tend to open most eggs in an end. Examined eggshells seldom had puncture marks or puncture holes in addition to the main opening, or conspicuous yolk residue.

Raccoons did not displace nest material at 18 (64%) of 28 artificial nests they depredated, although material was loosely packed. Only 2 (7%) nests had >30% of the material displaced (Appendix B, Table 5). Darrow (1938) and Baker (1978) also reported that raccoons tend to displace little nest material from nests when destroying clutches. However, Rearden (1951:389) reported that "...80 percent of the nests [n = 20] destroyed by raccoon were found to be pawed to some extent."

Most (64%) of 144 eggshells found at 28 artificial nests were ≤20 cm (including in nests) from the nest; few (4%) were >1 m from the nest (Appendix B, Table 6). Baker (1978) found eggshells (poultry eggs) ≤3 m from 15 (83%) of 18 artificial nests depredated by raccoons. We observed that eggshells outside nests often were clustered or in a line away from the nest.

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

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