Use of Earthen Islands by nesting ducks in North Dakota

We studied duck nests on 1 natural island and 21 constructed islands on the J. Clark Salyer National Wildlife Refuge (NWR), North Dakota in 1982-83. We found 286 nests of 7 species; mallard (Anas platyrhynchos), gadwall (A. strepera), and blue-winged teal (A. discors) represented >90% of all nests. The mean annual nest density was 8.7/ha with a mean Mayfield-adjusted nest success of 24%. Islands on which the most ducks were produced were 335-1,085 m offshore, surrounded by 150-200 m of open water, and contained tall, dense, brush or forb cover. Three islands with these characteristics held 37% of the nests and produced 55% of total ducklings. Although gadwalls and mallards formed dense nesting concentrations, bluewinged teal showed little nest-clumping behavior or nest cover preferences. Other factors influencing production rates included a seasonal decline in nest success and between-year variability. To be cost effective, islands located in wetlands with large mink (Mustela vison) and raccoon (Procyon lotor) populations should have adequate spatial isolation and/or predator control. J. WILDL. MANAGE. 53(2):411-417 Recruitment rates of ducks in the intensively farmed portion of the prairie pothole region are low (Cowardin et al. 1983) because of declining habitat and concentration of nest and female duck predators (Sargeant and Arnold 1984). Recent efforts to improve waterfowl production have centered on restricting predator access (e.g., electric fencing) (Lokemoen et al. 1982), cover management (e.g., dense nesting cover [DNC], burning, mowing), and construction or improvement of nesting islands. Some island habitats attract large populations of nesting ducks (Drewien and Fredrickson 1970, Browne et al. 1983) and have high nest success due to the reduced impacts of predators (Duebbert 1966, Vermeer 1968). Artificial islands can potentially produce more ducks/unit area than other habitats, but they are expensive to construct and maintain, and in many areas may not improve nest success enough to justify costs (Lokemoen 1985). Several authors have made recommendations concerning island construction (Hammond and Mann 1956, Duebbert 1982), but only recently have they been tested quantitatively (Giroux 1981, Carlsen 1984). Our objectives were to quantitatively assess selected environmental variables that best explain duck nesting use and production for a group of earthen islands, and to recommend ways to improve waterfowl production on islands. We thank R. A. Stomstad and T. B. Brokke for field assistance. We are grateful to D. Walls for logistic support and personnel to assist with field work. Additional support was provided by the University of North Dakota Biology Department and the Institute for Ecological Studies. Special thanks are extended to R. D. Sayler who provided helpful advice and reviewed the manuscript. The final draft benefited from comments made by R. C. Drewein and H. F. Duebbert. This publication is Contribution 8 of the Institute for Ecological Studies, University of North Dakota.