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Over 1100 nests, primarily of mallards (Anas platyrhynchos), were located in a subsample of 10 grass fields (99 ha) during the study. Adjusted nest success ranged from 6-70% annually and was highly correlated with the type and number of predators on the island. Mean annual nest densities ranged from 0.5-3.9 nests/ha and were depressed about 53% during the second year of increasing predator numbers. Successful nests tended to be located: 1) in significantly denser cover consisting of more alfalfa and fewer short forbs and grass, and 2) closer to the lake, but further from tree plantings and wetlands than unsuccessful nests. Tall, dense nesting cover provided approximately a 10-20% improvement in nest success under moderate to high predation rates. However, overall nest success rates declined precipitously in all types of nesting cover when red fox (Vulpes vulpes) were present on the island. A breeding pair of coyotes (Canus latrans) buffered waterfowl nests from fox predation. In 1986, uncorrected nest success rates declined from 58% on the western part of the island, which contained a coyote den, to 0% on the east end frequented by fox.

We conclude that although large blocks of dense nesting cover provide nesting waterfowl with a measure of extra security from mammalian predators, the effect is not dramatic. Overall nest success rates, even on a management area containing extensive areas of dense grass cover, were depressed substantially by relatively small numbers of red fox (e.g. 1-2 individuals). Predator control is probably necessary under existing predator population levels to achieve higher waterfowl production on developed wildlife management areas.