Brown-headed Cowbird Behavior and Movements in Relation to Livestock Grazing

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 averaged 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 increase 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.