SREL
Reprint #2608
MODELING
THE MOVEMENTS OF COWBIRDS: APPLICATION TOWARDS MANAGEMENT
AT THE LANDSCAPE SCALE
STEVEN
J. HARPER
Savannah River Ecology Laboratory
University of Georgia, Drawer E
Aikeli, SC 29802
E-mail: harper@srel.edu
JAMES
D. WESTERVELT
U.S. Army Construction Engineering Research Laboratories
Champaign, IL 61821
E-mail: j-westervelt@cecer.army.mil
ANN-MARIE
SHAPIRO
U.S. Arniy 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 feedirig
locations, grassland areas alniost 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 individuals
cowbirds across a dynamic landscape, model results may help
to strengthen ongoing cowbird control efforts.
KEY WORDS:
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
