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Depredation Patterns of Predator Species

Red Foxes

Red foxes forage in uplands but tend to avoid wet areas (Sargeant 1972). We seldom saw red foxes in the enclosure enter the pond, even to retrieve readily accessible prey. Thus, red foxes likely depredate duck nests in all upland habitat, but seldom depredate nests over water (Sargeant and Arnold 1984, Sargeant et al. 1984).

Response to Hens and Nests--Red foxes often capture nesting hens (Sargent et al.1984) Sargeant and Eberhardt (1975) reported that ducks did not struggle when captured by red foxes, and foxes customarily killed ducks by biting them in the body or by severing their neck. Red foxes killed most ducks restrained on nests in our study. They often laid the duck at or near the nest temporarily, but seldom ate it there. Foxes customarily left no more than a few breast and/or tail feathers at nests. We inspected a mallard nest in a grassy fence row after watching a red fox depart carrying the hen. The only evidence of depredation was a few breast feathers and a smal area of slightly matted vegetation, where the hen had been laid. When eating ducks, red foxes usually ingest nearly all feathers and bones, except wings or wing tips (Sargeant 1978).

All but 5 of the 144 artificial nests offered to foxes in the enclosure in 1972-1973 had all eggs depredated during the first night. Foxes observed discovering the nests seldom left the nest without depredating the clutch.

Treatment of Eggs--Ten of 12 foxes studied in the enclosure in 1972-73 were observed removing 454 eggs form 77 artificial nests. A fox would grab an egg (always 1), often after first pawing it from the nest, and immediately depart to cache it. This process was repeated until all eggs were cached, which resulted in foxes spending little time at the nest. Nearly all (99.8%) of the 454 eggs observed taken were cached; only 1 (0.2%) was eaten at the nest.

The straight-line distance from nest to cache of 420 eggs ranged from 4 to 163 m ( = 43.9 ±27.6m [SD]). Eggs were cached separately, usually in different distances and directions from the nest. Cached eggs were inconspicuous but barely covered (usually by ≤1 cm) with soil and/or other debris.

Records for a pair of foxes placed in the enclosure in 1985 and for 2 free-ranging foxes revealed the same pattern of depredation described above. All 87 eggs in 14 nests (13 artifical nests containing 78 eggs and 1 natural nest with 9 eggs) were missing when exposure periods ended. No cached eggs, eggshells, or shell fragments were found at the nests.

No eggs of any artificial nest known to have been discovered by foxes in the enclosure were at the nest when the 1-night exposure period ended. When 3 nests were offered simultaneously, foxes often discovered the second or third nest while caching eggs from the earlier discovered nest. This resulted in the foxes caching eggs, often interchangebaley, from different nests until all eggs were taken. This behavior may also occasionally result in red foxes partialy depredating duck nests, especially where nests are numerous.

Other investigators have noted the tendancy of red foxes to remove usually all eggs from the nests and to cache or eat eggs away from nests of birds (Darrow 1938, Rearden 1951, Kruuk 1964, Tinbergen 1965) and turtles (Macdonald et al.1994).

Appearance of Eggshells and Nest Sites--We found hundreds of eggshells of duck eggs depredated by foxes in the enclosure but only 1 was at a nest. Eggshells in Appendix C, Figure 2 were of eggs eaten in the enclosure or pens. Most (73%) of 42 eggshells examined for damage by the foxes had large holes, but small holes were common (20%). Few eggshells (7%) were severely damaged (Appendix B, Table 3; Appendix C, Fig. 2). We seldom found puncture marks on eggshells. We occasionally found an eggshell with ≥1 puncuture hole in addition to the main opening (Appendix C, Fig. 2g, i). We never observed a fox crush a duck egg within its mouth.

Most (88%) of 42 eggshells examined for location of opening made by foxes had the opening in the side; only 1 (2%) had an opening in an end (Appendix B, Table 4). We seldom found a conspicuous yolk residue in an eggshell.

We found no evidence of red foxes digging at nests. Although no nest material was used in construction of 77 artificial nests observed depredated by foxes in the enclosure during 1972-73, the pattern of egg removal indicated little nest material would have been displaced. Nest material was present in the 14 other nests depredated by foxes. Seven (50%) nests had no nest material displaced; 4 (29%) nests had >30% displaced (Appendix B, Table 5). Darrow (1938) reported that nests of ruffed grouse (Bonasa umbellus) destroyed by red foxes were also undisturbed.

Little vegetation was trampled at nests destroyed by foxes in the enclosure, because foxes spent little time at the nests and seldom ate ducks or eggs there.

Figure 2. Typical appearance of a duck nest destroyed by a red fox.

Conclusions--Red foxes have the most distinct pattern of destruction of duck nests, because they cache all eggs of the clutch away from the nest site. The following combination of evidence at nests is characteristic of destruction of a nest by red foxes: (1) all eggs missing, (2) no disturbance of ground surface or vegetation, and (3) ≤10% of nest material displaced (Appendix B, Table 2; Fig. 2). Although red foxes often kill the hen, they customarily leave little or no evidence of hen mortality at the nest.

Evidence at the nest that indicates a nest was not destroyed by red foxes includes the following: (1) all or part of a hen carcass, (2) cached egg, (3) eggshell or shell fragment, (4) dug area, or (5) aerially displaced nest material (Appendix B, Table 2).