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Depredation Patterns of Predator Species

Franklin's Ground Squirrels

Franklin's ground squirrels forage in dense grass, forb, or brush habitats and hence are unlikely to destroy duck nests in sites with short or sparse vegetation or those in nests over water (Sowls 1948, 1955, Choromanski-Norris et al.1989).

Response to Hens and Nests--Franklin's ground squirrels rarely attack nesting hens but occasionally prey on ducklings (Sowls 1948, 1955). Sargeant et al.(1987) found that free-ranging Franklin's ground squirrels customarily began depredating duck eggs placed near their trails or burrows ≤2 min after encountering the eggs. Ground squirrels took an average of 2.2 days (range 1-5 days) to depredate 6 eggs in an artificial duck nest. Thus, Franklin's ground squirrels probably often partially depredate natural duck nests.

We compiled information for 15 artificial duck nests with eggs thought to have been depredated exclusively by free-ranging Franklin's ground squirrels. Each nest was available to the ground squirrels for 1-3 days after being discovered by them. After discovering a nest, ground squirrels occasionally left all eggs undamaged. At other times they consumed eggs at the nest site or removed them from the site.

Treatment of Eggs--Duck eggs are not easily opened by Franklin's ground squirrels. Sowls (1948) described the process as follows. First, the egg is usually moved to a site where it will not easily roll. Then, the egg is embraced lengthwise under the body and hard thrusting and even vigorous rolling is employed to obtain the leverage needed to penetrate the shell. The initial opening, often near an end (Appendix C, Fig. 8a), is enlarged by biting off small pieces of shell. Close examination of eggshells often reveals finely serrated edges along openings.

In our studies, free-ranging Franklin's ground squirrels were observed opening duck eggs on 3 occasions, twice by straddling the egg lengthwise to hold it in place and then biting (no hard thrusting or rolling) into an end, and once as described by Sowls (1948). Franklin's ground squirrels were observed 3 times holding shell fragments off the ground with their front feet and eating portions of the shell.

Seventy-three (81%) of 90 eggs from 15 artificial nests depredated exclusively by Franklin's ground squirrels were depredated. Number of eggshells and amount of shell fragments found at individual nests ranged from none to portions of all eggs. Only 35 (48%) of 73 depredated eggs were represented by eggshells at the nest. A few additional eggs (exact number undetermined) were represented by shell fragments.

Appearance of Eggshells and Nest Sites--Most (57%) of 35 eggshells of duck eggs examined for damage by Franklin's ground squirrels had large holes, but eggshells with small holes and severely damaged eggshells were also common (Appendix B, Table 3). Openings in eggshells usually were broad and irregularly shaped, often extended lengthwise along the shell, and usually had few pieces of shell caved inward (Appendix C, Fig. 8).

Most (65%) of 31 eggshells of duck eggs examined for location of openings made by Franklin's ground squirrels had the opening in a side-end, but openings often were in the side (23%) or in an end (13%; Appendix B, Table 4). Usually, there was no conspicuous yolk residue in eggshells. However, ground squirrels often did not complete eating contents of depredated eggs during a single visit to the nest or in a single day. Hence, partially eaten eggs with fresh evidence of depredation occasionally may be found at duck nests depredated by Franklin's ground squirrels.

There was no digging at any of 15 artificial nests depredated by Franklin's ground squirrels. Nest material was displaced from each nest; >30% of nest material was displaced from each of 10 (67%) nests (Appendix B, Table 5).

Franklin's ground squirrels occasionally covered whole eggs left in artificial nests with nest material. These nests looked the same as natural nests with eggs covered by the hen before taking a recess. Narrow indistinct trails, under dense vegetation, occasionally were present at nests destroyed by Franklin's ground squirrels.

Fifteen (43%) of 35 eggshells found at 15 artificial nests depredated by Franklin's ground squirrels were ≤20 cm from the nest (including in the nest); 11 (31%) were >1 m from the nest (Appendix B, Table 6). Eggshells usually were scattered around the nest, sometimes in indistinct trails. We found an eggshell at the entrance to a Franklin's ground squirrel burrow, indicating that they take some duck eggs into burrows.

Conclusion--Where Franklin's ground squirrels are major predators of duck eggs, many nests should have eggs missing before any egg hatches or the nest is destroyed. Evidence indicating a nest was destroyed by Franklin's ground squirrels includes finding at the nest most or all of the following: (1) ≥1 whole egg in nest, (2) eggshells of <50% of clutch, (3) eggshells with irregularly shaped openings and fine serrations along openings, (4) recently opened eggshell with much content present, and (5) ≥50% of eggshells >1 m from the nest (Appendix B, Table 2).

Evidence indicating a nest was not destroyed by Franklin's ground squirrels includes finding at the nest any of the following: (1) the hen was killed, (2) cached egg (3) eggshells of >50% of clutch, (4) aerially displaced nest material, or (5) dug area (Appendix B, Table 2).