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Ecology and Management of Islands, Peninsulas and Structures for Nesting Waterfowl

Duckling Survival and Habitat Use in an Island Nesting Population: Garrison Reservoir, North Dakota

Mark Willms and Rodney Sayler
Institute for Ecological Studies
Box 8278, Univ. of North Dakota
Grand Forks, North Dakota 58202

Large numbers of ducklings (possibly 15,000-20,000/yr) hatch annually from Mallard Island in Lake Sakakawea and numerous small islands at Lake Audubon National Wildlife Refuge, North Dakota. However, most hens must leave nesting islands in the Garrison Reservoir and travel to the mainland to reach suitable brood-rearing wetlands. In recent years, concerns developed regarding duckling survival, due to the small number of broods observed and the recent expansion of a California (Larus californicus) and ring-billed (Larus delawarensis) gull colony on Lake Audubon. We studied brood survival of 50 radio-equipped mallard (Anas platyrhynchos) and 26 gadwall (A. strepera) hens captured shortly before their nests hatched, during 1986-1987. Females were monitored on a regular basis after hatch to determine distances traveled, habitat selection, and brood survival.

Brood hens from Mallard Island traveled significantly further (mean = 4.7 km) to reach brood-rearing wetlands than hens from Lake Audubon (mean = 3.0 km). Mallard broods traveled farther (mean = 5.0 km) than gadwall broods (mean = 2.0 km). Brood hens from Mallard Island required an average of 2.8 days to reach destination wetlands compared to an average 1.9 days for Lake Audubon brood hens. Mallard brood hens often used smaller, shallower, and more vegetation-filled wetlands for brood-rearing than gadwalls when they were available.

Duckling survival estimates for mallard and gadwall broods combined averaged 12% for birds produced from Mallard Island and 35% for birds from Lake Audubon. About 81% (17/21 from Mallard Island) and 30% (5/15 from Lake Audubon) of the unsuccessful brood hens lost all ducklings while enroute to a wetland. Duckling survival from Mallard Island averaged about 30% (25/83) in 1986 during a year of high water levels in the Garrison Reservoir, but declined to 0% (0/129) in 1987 when the reservoir was low and wide shorelines were exposed.

Our studies suggest that several environmental factors impact duckling survival rates in this large-reservoir environment (not necessarily in order of importance): 1) relatively large post-hatch travel requirements, 2) predation by gulls and mink (Mustela vison), 3) severe weather events during lake crossings, and 4) relatively low quality or productivity of wetlands adjacent to the reservoir. Management actions, such as predator control, wetland construction, and wetland management, are probably warranted to insure at least average productivity rates from large numbers of waterfowl attracted to these reservoir nesting islands.

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