Effects of Predator Control on Island-nesting Mallards in the
Upper Mississippi River National Wildlife and Fish Refuge
ROBERT B. DAHLGREN AND JOHN F. WETZEL
U.S. Fish and Wildlife Service, Office of Refuge Biology, La Crosse,
WI
54602; Wisconsin Department of Natural Resources, 3550 Mormon Coulee
Road, Room 108, La Crosse, WI 54601
In a 1981-87 study of four islands in Pool 8, Ron Nicklaus, Wisconsin Department of Natural Resources, found up to 51 mallard nests per island and 59 nests per acre with 69% nest success. In 1988, we expanded this study to include 16 islands in Pools 7, 8, and 13. Twelve islands (6.8 acres) were free of raccoon (Procyon lotor) and fox (Vulpes vulpes and Urocyon cinereoargenteus) (FRF) and had 42 nests per acre with 77% hatched. In contrast, four islands (14.9 acres) with fox and/or raccoon present had 2.3 nests per acre with 26% hatched. On islands with fox and/or raccoon present 49% of nests were depredated and 26% were abandoned, but on FRF islands 9% were depredated (by birds) and 13% were abandoned. Where fox and raccoon were present, predation was more than five times as great and abandonment three times as great as where they were absent. In 1989, we learned that nesting occured from Pools 4 to 18. For 10 FRF islands (7.4 acres), nest success was 53%, and for four islands (22.6 acres) with raccoon and/or fox present, success was zero.
In May 1990, 30 islands close together in shallow water were checked for nests in middle of Pool 8 and only three nests were found; raccoon tracks found regularly on these 30 islands. Many other islands and a large block of river forest land were searched in 1989 but few nests were found. For FRF islands in 1988-89, 2 islands less than 0.1 acre in size had 147 nests per acre, 5 islands 0.1-0.49 acres in size had 78 per acre, 3 islands 0.5-0.99 acres in size had 42 per acre, and 2 islands > 1.0 acre in size had 31 nests per acre. The smallest islands had highest nest densities, and would logically be least likely to harbor fox and raccoon. A Pool 13 FRF island 0.7 acres in size had 67 nests with 68% success in 1988. A fox was present on that island in early spring 1989; no duck nests were found on the island that year. The fox was gone in spring 1990 and by the end of May there were 36 nests on the island. Another Pool 13 FRF island (1.6 In island was 4 acres) had 43 nests with 89% success in 1988. The island was not monitored in 1989. A fox was present on the island in 1990 and no duck nests were found by the end of May.
Great-homed owls (Bubo virginianus) killed hens. We suspect gulls (Larus argentatus) were the chief avian nest predator, but crows (Corvus brachyrynchos) and great egrets (Casmerodius albus) may also have been involved. One FRF island in 1989 that had high nest predation by birds was being used for roosting by great egrets. The complex of islands separated by running sloughs in the upper end of most pools are heavily infested with fox and raccoon. Since this habitat is more typical in Pools 4, 5, and 5A in the northern end of the refuge, nesting density and success are low there and in the upper and middle ends of other pools where islands are not sufficiently isolated over distance by deep water. Island construction should emphasize tiny islands isolated by distance and deep water from land. We expect high returns in duck production from predator control on Mississippi River islands.
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