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Depredation Patterns of Predator Species

American Crow

American crows range widely over uplands and wetlands, but duck nests in uplands appear more vulnerable to destruction by crows than those in wetlands (Sullivan and Dinsmore 1990).

Response to Hens and Nests--We found evidence of American crows harassing but not killing nesting hens. An American crow was observed jumping up and down on a nesting duck (large dabbling duck). When the duck flushed, the crow immediately flew off with an egg.

We obtained information for 54 artificial duck nests known to have been visited by American crows and at which we detected no interference from other potential predators. Each nest was monitored 1-3 days (n = 62 exposure-days) after detection by a crow was confirmed. Fifty-one (94%) nests were partially or completely depredated. Three nests not depredated were visited at least twice by crows.

Depredations of artificial nests usually were by individuals. Maximum number of crows photographed simultaneously at a nest was 1 individual during 48 (77%), 2 during 12 (19%), and 3-4 during 2 (3%) of 62 exposure-days. The latter records of crows at a nest site occurred during July, and may have included recently fledged individuals.

Crows usually depredated duck eggs placed in 54 artificial nests. Film records showed crows visited nests ≥352 times during 62 exposure-days. Two hundred fifty-seven (79%) of 324 eggs placed in the nests were depredated. Thirty-two (59%) nests had all eggs depredated on the day the nest was discovered. All eggs were missing without a trace of shell from 19 (59%) nest sites. Sowls (1948), Rearden (1951), Einarsen (1956), and Montevecchi (1976) also reported that American crows often remove eggs from nest sites.

Treatment of Eggs--Depredations of 51 artificial nests by American crows usually involved removal of eggs from nest sites. Of 257 depredated eggs, only 61 (24%) were accounted for by eggshells found at the nests. Forty-one (80%) nests had ≥1 egg missing; 19 (37%) had all eggs missing. Thirty-six (70%) nests had all eggs depredated, but eggshells of all 6 eggs were present at only 2 (6%) of those nests. Rearden (1951:392) concluded, "...it seems practically impossible for a crow to puncture an egg shell and pick it up without leaving some shell fragments, however small, in the nest." We found shell fragments at only 4 (21 %) of 19 nests from which all eggs were missing, but shell fragments are easily overlooked. Only 1 (1%) of 67 whole eggs at depredated nests was outside the nest when the exposure periods ended.

Montevecchi (1976:316) found that, "The most common predation method of the crows [American crows] was to fly off with eggs [large and medium-size domestic fowl eggs and small Japanese quail (Coturnix coturnix) eggs] and to cache (bury) or eat them at a distance from the site of predation. The larger eggs were more frequently pecked open at the egg site and were less effectively picked up and carried off by the crows." Crows carried off eggs by puncturing the egg and inserting a mandible to grasp the shell, or by grasping eggs with their bill without puncturing the shell (Montevecchi 1976:308). We photographed 14 instances American crows holding a duck egg with their bill; in 13 instances, the egg was pierced, and in 1 instance, the egg was grasped without being pierced.

Crows usually spent little time at nests when removing an egg, often too little to be photographed during 15-30 s photo intervals. For 19 nests with all eggs missing on day of nest discovery, crows visited the nests 3 times on average (range = 1-6). All 6 eggs of 1 clutch were missing from the nest site 20 min after a crow was first photographed at the nest. In contrast, individual crows visited another nest 47 times during a 3-day period before all 6 eggs were depredated.

Appearance of Eggshells and Nest Sites--Most of 61 eggshells examined for damage by American crows had small (51%) or large (43%) holes; only 4 (7%) were severely damaged (Appendix B, Table 3). Occasionally, there were ≥2 openings in an eggshell, sometimes in opposite sides of the egg (Appendix C, Fig. 10j, l). Most openings were round or irregularly shaped, with coarse edges but few inwardly caved pieces.

Most (60%) of 42 eggshells examined for location of openings had the opening in the side; only 4 (10%) had the opening in an end (Appendix B, Table 4). Conspicuous yolk residue often remained in eggshells after feeding by crows was completed. We occasionally found depredated eggs with small pecked holes and little contents removed. Sowls (1948) and Einarsen (1956) reported that American crows and common ravens, respectively, leave little egg contents in eggshells or nests after feeding on eggs.

We never observed crows digging at nests. The proportion of nest material displaced from each of 51 artificial nests depredated exclusively by crows ranged from 0 to 100%, but exceeded 30% at 32 (63%) nests (Appendix B, Table 5). Characteristically, crows tossed and/or dropped nest material, which often became entangled on erect vegetation.

Most (84%) of 61 eggshells found at the 51 artificial nests depredated exclusively by American crows were ≤1 m from the nest, usually ≤20 cm from the nest (Appendix B, Table 6).

Conclusions--Evidence of destruction of duck nests by American crows varies greatly depending on whether depredated eggs were removed from the nest site (most common in our studies) or eaten at the nest. American crows usually removed depredated eggs from nest sites, but a few shell chips often are left at the site and may be overlooked. If eggs were eaten at the nest, eggshells and numerous shell fragments customarily are present. Other evidence that indicates a nest was destroyed by American crows includes finding at the nest most or all of the following: (1) ≥1 whole egg in nest; (2) eggshell with ≥2 openings, sometimes on opposite sides; (3) eggshell with conspicuous yolk residue; and (4) aerially displaced nest material (Appendix B, Table 2).

Evidence that indicates a nest was not destroyed by American crows includes finding at the nest any of the following: (1) the hen was killed, (2) cached egg, (3) whole egg outside the nest, and (4) dug area (Appendix B, Table 2).