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Increased Nest Success with Predator Management at Union Slough
National Wildlife Refuge

JAMES A. GALLAGHER AND ERWIN E. KLAAS

Iowa Cooperative Fish and Wildlife Research Unit, Iowa State University,
11 Science Hall II, Ames, IA 50011


We conducted a waterfowl nesting study at Union Slough National Wildlife Refuge (NWR) in northern Iowa in 1988-89 to evaluate the effect of predator removal on nest survival of ground-nesting waterfowl. An earlier study (1984- 85) at the refuge (Fleskes 1986) estimated nest success of all ground-nesting ducks to be 11.9% (Mayfield estimator). Mammalian predation was the major cause of mortality to eggs and nesting hens (82% of all nest failures).

In 1988, one fulltime trapper set box traps and leghold traps from 6 March through 13 July, for a total of 7,067 trap-nights. Nine red foxes (Vulpes vulpes), 111 raccoons (Procyon lotor), 46 striped skunks (Mephitis mephitis), 56 opossums (Didelphis virginiana), and 15 domestic cats (Felis domestica) were caught at a rate of 0.033 captures per trap-night and removed from the Refuge. In 1989, a similar trapping program was conducted from 13 March through 16 July, for a total effort of 7,522 trap-nights. Eleven red foxes, 102 raccoons, 45 skunks, 43 opossums, 13 cats, 3 badgers (Taxidea taxus), and 1 mink (Mustela vison) were captured at a rate of 0.029 per trap-night.

Breeding-pair density increased from 56 pairs per km² in 1988 to 96 pairs per km² in 1989. We believe this increase was caused by relatively attractive wetland conditions in the spring at Union Slough NWR compared with conditions elsewhere in the state and region. Drought conditions existed in Iowa in 1988 and continued in 1989. By the end of the brood- rearing season in 1989, nearly all of the pools at Union Slough NWR were dry.

We conducted three searches of upland nesting cover each year during April through July using a chain-drag and 4-wheel drive all-terrain-vehicles. We monitored nests until hatch or termination. Nest success for all species combined in 1988 was 17.7% (Mayfield estimator) for 261 nests of mallard (Anas platyrhynchos), blue-winged teat (A. discors), and northern shoveler (A. clypeata). In 1989, nest success for all species combined was 26.5% for 313 nests of mallard, blue-winged teal, northern shoveler, American wigeon (A. americana), and gadwall (A. strepera). Nest success for all species combined in 1989 was significantly greater (z=2.41 p<0.02) than in 1988. Nest success with predator management in 1988-89 was 22.2% and was significantly higher (z=3.54 p<0.00l) than the 1984-85 average of 11.9% without predator management. Mammalian predation was still the major cause of nest failure, accounting for 73% of failures among 390 unsuccessful nests. The red fox was responsible for 24% of all failures.

Based on these short-term results, we believe predator management has been effective in increasing nest success at Union Slough National Wildlife Refuge and should be continued. Evaluation of nest success should also be continued to determine whether this response will be sustained.


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