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Depredation Patterns of Predator Species

Gulls

Gulls are most likely to depredate duck nests near, but not in, gull nesting colonies (Williams and Marshall 1938, Odin 1957, Anderson 1965, Vermeer 1970, Dwernychuk and Boag 1972). We found no evidence in literature of gulls depredating duck nests over water.

Response to Hens and Nests--We found no evidence in literature of gulls killing or harassing hens at nests. Ducks commonly nest among gulls on nesting islands (Anderson 1965, Vermeer 1970, Dwernychuk and Boag 1972). We found weak responses by gulls to duck eggs. Only 17 (27%) of 63 artificial nests offered to gulls were visited by ring-billed gulls and/or California gulls. Loose eggs offered to gulls at 2 landfill sites usually were ignored by the gulls.

Responses by gulls to the artificial nests ranged from infrequent brief visits by individuals to lengthy ones (up to 38 min) by ≤6 birds. Two (13%) of 16 nests visited exclusively by gulls were not depredated; 5 (31%) nests had all eggs depredated. Of 84 eggs placed in 14 nests depredated exclusively by gulls, 30 (36%) eggs were whole, 28 (33%) were missing, and 26 (31%) were represented by eggshells when exposure periods ended. One or more eggs were missing, mostly without a trace of shell, from 12 nest sites. All eggs were missing from 1 site.

Treatment of Eggs--Odin (1957:188) reported that California gulls used the following methods to open duck eggs: "...(1) pecking a hole in the egg at the nest, (2) dropping the egg over water and retrieving the contents, (3) carrying the egg intact to the gull nesting islands or to a favorite loafing site, and (4) swallowing the egg whole." We observed several instances of a gull carrying an intact egg with its bill (Fig. 4). Once, we observed a California gull swallow a whole blue-winged teal egg. Gulls usually punctured, and sometimes partially consumed, an egg before carrying it off. Shell fragments were found at 2 (33%) of 6 nests at which there were missing eggs and no eggshells (but shell fragments are easily overlooked). We found no evidence indicating gulls hide eggs.

Figure 4. California gull holding a whole duck egg in its bill.

Appearance of Eggshells and Nest Sites--Both species of gulls customarily opened duck eggs by pecking, and California gulls occasionally bit eggs to open them. Eggs often were turned while being opened or eaten. This action resulted in the same or a different gull making another opening on the opposite side of the egg, and spillage of egg contents. Multiple openings (Appendix C, Fig. 11a, i, j) were present in 3 (6%) of 48 eggshells.

Most of 80 eggshells examined for damage by gulls had small (35%) or large (44%) holes (Appendix B, Table 3). Each opening usually was an elongated slit with pointed ends extending across 1 side of the egg (Appendix C, Fig. 1la, b, e, f). Twenty-seven (50%) of 54 eggshells from loose eggs depredated by gulls at landfills had openings of this shape.

Most (89%) of 75 eggshells examined for location of openings made by gulls had all openings in the side; no eggshell had an opening in an end (Appendix B, Table 4). Conspicuous yolk residue usually was present in eggshells.

We found no evidence of gulls digging at nests. Nest material was displaced from only 4 (29%) of 14 nests depredated exclusively by gulls. In each instance, ≤30% of the material was displaced (Appendix B, Table 5).

Most (81%) of 26 eggshells at nests depredated exclusively by gulls were ≤20 cm from the nest (Appendix B, Table 6).

Conclusions--Gulls probably destroy few duck nests. Evidence indicating a nest was destroyed by gulls includes finding at the nest most or all of the following: (1) ≥1 whole egg in nest, (2) part of clutch missing without trace, (3) ≥50% of openings in eggshells are small or large holes, (4) all openings in eggshells are on the side or a side-end, (5) eggshell with 2 openings of similar size, (6) eggshell with opening that is narrow elongated slit with pointed ends, (7) all eggshells have conspicuous yolk residue, and (8) no displaced nest material (Appendix B, Table 2).

Evidence indicating a nest was not destroyed by gulls includes finding at the nest any of the following: (1) the hen was killed, (2) cached egg, (3) eggshell with opening in an end, (4) all eggshells are clean inside, (5) dug area, and (6) >30% of nest material was displaced (Appendix B, Table 2).