1533 Ecological Applications, 11(5), 2001, pp. 1533–1544 q 2001 by the Ecological Society of America BROWN-HEADED COWBIRD BEHAVIOR AND MOVEMENTS IN RELATION TO LIVESTOCK GRAZING CHRISTOPHER B. GOGUEN 1 AND NANCY E. MATHEWS Department of Wildlife Ecology, University of Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706 USA Abstract. The Brown-headed Cowbird (Molothrus ater) is a widespread brood parasite which often engages in a commensalistic feeding relationship with domestic livestock. We studied the behavior of female cowbirds breeding in pinyon–juniper woodlands in New Mexico, USA, on two adjacent sites, one an active cattle ranch, and the other a site that was not grazed by domestic livestock throughout the songbird breeding season. In 1994, we conducted morning and afternoon surveys of cowbird abundance in pinyon–juniper and prairie habitats; from 1995 to 1997 we used radio telemetry to monitor daily and seasonal movement and behavioral patterns of female cowbirds. Our objectives were to measure how closely cowbird feeding behavior was linked to livestock grazing, and how the presence or absence of active livestock grazing within a female’s breeding range influenced diurnal patterns of behavior. During morning surveys, we detected cowbirds primarily in pinyon– juniper habitat, but in similar numbers in the ungrazed and actively grazed woodlands. In the afternoon, we detected cowbirds feeding almost exclusively in actively grazed prairies but found that they deserted those sites when cattle were removed in early July. Radio telemetry confirmed that individual females were commuting daily between these habitats. Females (n 5 30) were generally located in pinyon–juniper habitats from 0500 to ;1200, presumably breeding. Females that bred within actively grazed pinyon–juniper habitat often fed on the ground with livestock on their morning ranges, while those breeding in ungrazed habitat did not. In total, 98% of cowbird feeding observations occurred with livestock. Although most females commuted ,3 km between breeding and feeding ranges, some individuals with breeding ranges located toward the center of the ungrazed property av- eraged 7.7 km. When cattle were rotated out of the main feeding pasture in early July, females immediately extended their commutes by ;1.2 km to access remaining actively grazed pastures. Overall home range sizes were large (160–4344 ha) and tended to increase with distance between the females’ breeding range and active livestock grazing. This in- crease was reflected mainly by differences in feeding range sizes rather than breeding range sizes. The observed link between cowbird behavior and the distribution of livestock suggests that in regions where livestock grazing is the dominant land use, manipulations of livestock grazing patterns may provide an effective tool to manage cowbird parasitism. Key words: brood parasitism; Brown-headed Cowbird; cattle; commensalism; foraging ecology; livestock grazing; Molothrus ater; radio telemetry; songbird conservation. The current widespread distribution of the brood- parasitic Brown-headed Cowbird (Molothrus ater) has made management of this species a principal issue in the conservation of many songbird populations and communities across North America (Robinson et al. 1995a). Cowbirds are thought to have been restricted by their foraging habitat requirements to shortgrass prairie regions west of the Mississippi River prior to European settlement (Hamilton and Orians 1965, May- field 1965). Cowbirds are omnivorous, foraging on the ground in areas of short vegetation (Friedmann 1929, Mayfield 1965). In the past, cowbirds likely relied on large grazing mammals, such as bison (Bison bison), Manuscript received 25 June 1999; accepted 3 July 2000; final version received 31 August 2000. 1 Present address: School of Forest Resources, Pennsyl- vania State University, 113 Ferguson Building, University Park, Pennsylvania 16802-4300 USA. E-mail: CBG10@psu.edu to create or facilitate foraging opportunities in what may have been a near-obligate, commensalistic rela- tionship (Friedmann 1929). Cowbirds appear to use these mammals as ‘‘beaters’’ to flush arthropods (Fried- mann 1929, Mayfield 1965), and may associate with grazers because their activities reduce vegetation height and increase arthropod densities (Morris and Thompson 1998). Domestic livestock have proved to be a suitable surrogate to native ungulate grazers for cowbirds and may now perform a role analogous to that of bison (Mayfield 1965). Other anthropogenic ac- tivities (e.g., agriculture, lawn maintenance, bird feed- ing) have also created persistent feeding habitats that cowbirds have exploited in their range expansion (Mayfield 1965, Rothstein 1994). Within the western United States livestock grazing is currently a dominant land use (Holechek et al. 1989), in some regions providing the primary foraging habi- tats for breeding cowbirds. As a result, it is possible