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Duck Brood Movements Delayed by Predator Exclosures


U.S. Fish and Wildlife Service, Northern Prairie Wildlife Research Center,
Route 1, Box 96C, Jamestown, ND 58401

High rates of predation on eggs and nesting hens have prompted widespread use of large predator exclosures in the Prairie Pothole Region in efforts to increase recruitment of dabbling ducks. Although the effectiveness of exclosures as a means of increasing nest success is well documented, little information is available concerning the fate of broods once they leave their nests within exclosures. As part of the Small Unit Management Project in eastern North Dakota and west-central Minnesota, we monitored mallard and gadwall broods hatched inside predator exclosures during 1988-90. We found that ducklings easily moved through the 5x8-cm mesh of our exclosure fences; however, hens, which we expected would fly over the fences, were reluctant to leave the ground while accompanying broods. We added some 10x10-cm openings for the hens to use as exits, but they apparently had trouble finding them. Observations and time-lapse photography at these openings indicated that hens tended to pace back and forth over short distances and thus reduced their chances of encountering the widely spaced openings. Several radio-tagged mallard broods experienced long delays in leaving exclosures with few or no hen exits.

Concerns raised by this information prompted us to more fully assess effects of fence design and time required by broods to exit the exclosures. We conducted an experiment with radio-tagged gadwall ducklings in two exclosures that had different numbers and sizes of exits available to hens. Results indicated that hens with broods left four times faster from the exclosure with larger, more numerous exits. During the 1990 field season, we tested another type of hen exit intermediate in size to those we had tried previously. The test also compared exits to which hens were directed by means of a lead and exits without leads. Preliminary results of this work will be addressed in the presentation.

Long delays at exclosure fences increase the potential for duckling mortality from several combined or independent factors: exhaustion, exposure, abandonment, and predation. Predation events we observed near exclosures emphasized the need to maintain high-quality escape cover both within an exclosure and in the corridors that broods are likely to use while enroute from the exclosure to water. To minimize the time broods are delayed by exclosure fences, we recommend that they be designed to allow hens with broods to exit the structures on the ground.

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