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Depredation Patterns of Predator Species

Coyotes

Responses to Hens and Nests--Coyotes occasionally capture hens at nests but often discard them. They killed nesting Canada geese (Branta canadensis) on islands in Washington (Hanson and Eberhardt 1971) and ducks restrained on nests in our studies. Coyotes in our studies killed ducks by biting them in the body or by severing the neck, and often left the whole, headless, or partially eaten carcass at or near the nest. We occasionally found a whole dead hen at or near a natural nest that we suspected was destroyed by coyotes. When eating ducks, captive coyotes usually pulled off and discarded feathers and stripped flesh from the carcass, leaving most of the skeleton intact. Remains often resembled those left after raptors fed on bird carcasses (Einarsen 1956).

We observed coyotes in evening visit 52 of 64 artificial nests in the enclosure. By the next morning, when investigators visited nests, coyotes had partially or completely depredated 59 nests. However, we often observed coyotes visit a nest without taking the restrained duck or eggs.

Treatment of Eqgs--We observed coyotes remove 141 of 312 eggs during 303 visits to 52 artificial nests; 90 (64%) eggs were eaten, 34 (24%) cached, and 17 (12%) dropped and at least temporarily abandoned. Eggs were always taken singly, often by first pawing the egg and some nest material from the nest.

The 34 cached eggs were from 16 nests, 1-4 from each clutch. Coyotes cached each egg separately 6-80 in ( = 26.8 ±16.6 m [SD, n = 34]) from the nest. Cached eggs were barely covered (usually ≤l cm) with soil and/or debris but were inconspicuous nevertheless. Sooter (1946) also noted that coyotes cached duck eggs individually in shallow cavities and barely covered eggs with debris and soil. On small islands, he found a few duck eggs cached by coyotes <5 m from the nest of origin.

Our examination of nest sites of 59 nests depredated by coyotes revealed highly variable treatment of eggs. One to 5 whole eggs (n = 49 eggs) or eggs with puncture holes only and no contents removed (n = 3 eggs) remained at 18 (31%) nests. We accounted for 105 (35%) of the remaining 302 eggs placed in the nests from eggshells found at the nests. The other 197 (65%) eggs were missing from the nest sites, but 37 were accounted for by whole eggs (n = 3 eggs) and eggshells (n = 34 eggshells) found 3-7 m from 11 nests. Shell fragments were found at 31 (53%) nests, but in each instance were deemed to be less than amount of 1 egg. The number of eggs missing from individual nest sites ranged from 1 to 6. In 8 instances, all eggs were missing and no shell fragments were found at the nest. However, eggshells were found near 2 of those nests.

Appearance of Eggshells and Nest Sites--Most (93%) of 129 eggshells examined for damage by coyotes had small, usually round, holes (Appendix B, Table 3; Appendix C, Fig. 1a-c). Coyotes apparently also often make large holes in eggs (Sooter 1946) and severely damage eggshells (Appendix C, Fig. 1), but few (7%) examined eggshells were damaged in that manner (Appendix B, Table 3). Coyotes occasionally crush an egg within their mouth (Appendix C, Fig. 11). Sooter (1946) and Baker (1978) also noted that coyotes often make small holes in eggs and sometimes crush eggs within their mouth.

Most (97%) of 75 eggshells examined for location of openings had been opened by coyotes in the side; none had been opened on an end (Appendix B, Table 4). Sooter (1946) showed a destroyed gadwall nest with 9 eggshells remaining in the nest, each with a small or large round opening in the side. He attributed the destruction to coyotes.

Eggshells of eggs depredated by coyotes occasionally had ≥l puncture holes in addition to the main opening (Appendix C, Fig. 1b, h, i), often opposite the main opening. Eggshells seldom contained a conspicuous yolk residue.

We found no evidence of coyotes digging at nests. However, coyotes in the enclosure occasionally laid at or near nests, which resulted in trampled vegetation.

The proportion of nest material displaced from 59 artificial nests with eggs depredated by coyotes ranged from 0 to 100%; 40 nests (68%) had >30% of the material displaced (Appendix B, Table 5).

There was no consistent pattern in location of eggshells (n = 129) at nests (Appendix B, Table 6). Thirty eggshells (23%) were ≤20 cm from the nest, often in the nest, and 32 (25%) were >1 m from the nest. Eggshells found outside nests often were clustered.

Conclusions--Destruction of duck nests by coyotes is difficult to identify because evidence at nests is variable and often similar to that produced by other predator species. For example, Sooter (1946:38) stated, "...had the coyotes not been watched while cracking the eggs, some of the work would have been thought to be that of birds ......"

Finding a whole carcass of a hen without bite marks in the cranium at or near a nest is a strong, but infrequent, indicator of depredation by coyotes. The following indicates a nest may have been destroyed by coyotes: (1) part of clutch unaccounted for by eggshells and shell fragments, (2) most eggshells have small holes, (3) all openings in eggshells are in the side of the shell, (4) eggshell with ≥1 puncture hole in addition to the main opening, and (5) most eggshells are >1 m from the nest (Appendix B, Table 2).

Evidence indicating a nest was not destroyed by coyotes includes the following: (1) cached egg, (2) dug area, (3) amount of shell fragments exceeds that of 1 egg, (4) eggshell with an opening in an end, or (5) eggshell with a conspicuous yolk residue (Appendix B, Table 2).