SREL Reprint #2608

Modeling the movements of cowbirds: application towards management at the landscape scale

Steven J. Harper1, James D. Westervelt2, and Ann-Marie Shapiro2

1Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802
2U.S. Army Construction Engineering Research Laboratories, Champaign, IL 61821

Abstract: Brood parasitism by brown-headed cowbirds (Molothrus ater) reduces the reproductive success of two endangered bird species at Fort Hood, Texas. A successful management program has focused on trapping cowbirds at feeding locations, grassland areas almost always associated with the presence of cattle. To enhance the efficacy of this effort, we developed an individual-based model that predicted visitation rates by cowbirds to all potential feeding locations at Fort Hood and the surrounding landscape. Utilizing GRASS-GIS and SWARM, the model incorporated the spatial arrangement of habitats, daily movements of cattle, and daily movements of cowbirds from their breeding sites to feeding locations. We simulated four types of movements by cowbirds as they searched for cattle: 1) random walk, 2) direct return to previous locations (i.e., memory), 3) return to previous locations with en route assessment (i.e., memory with limited perception of the current state of the system), and 4) omniscience. Straight-line distances between breeding and feeding locations for cowbirds that used the memory-with-perception rule were similiar to independent telemetry results, and total search distances for this movement type approached those of omniscience. Both movement distance measures were inversely related to perception distance. Maps of cowbird visits to feeding areas differed among movement rules but were not strongly affected by the density of breeding cowbirds. Maps also identified locations where trapping efforts could reduce parasitism within specific areas known to support endangered species. While it seems plausible that cowbirds are able to remember foraging locations and to perceive their surroundings as they travel, additional studies are needed to document their cognitive abilities. By simulating movements of individual cowbirds across a dynamic landscape, model results may help to strengthen ongoing cowbird control efforts.

Keywords: Individual-based model, spatial model, geographic information system, GIS, conservation management, control program, landscape ecology, endangered species, brown-headed cowbird, brood parasitism.

SREL Reprint #2608

Harper, S. J., J. D. Westervelt, and A. Shapiro. 2002. Modeling the movements of cowbirds: application towards management at the landscape scale. Natural Resource Modeling 15:111-131.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).