Brown-headed cowbird behavior and movements in relation to livestock grazing

The Brown-headed Cowbird (Molothrus ater) is a widespread brood parasite wh ich often engages in a commensalistic feeding relationship with domestic li vestock. We studied the behavior of female cowbirds breeding in pinyon-juni per woodlands in New Mexico, USA, on two adjacent sites, one an active catt le ranch, and the other a site that was not grazed by domestic livestock th roughout the songbird breeding season. In 1994, we conducted morning and af ternoon surveys of cowbird abundance in pinyon-juniper and prairie habitats : from 1995 to 1997 we used radio telemetry to monitor daily and seasonal m ovement and behavioral patterns of female cowbirds. Our objectives were to measure how closely cowbird feeding behavior was linked to livestock grazin g, and how the presence or absence of active livestock grazing within a fem ale's breeding range influenced diurnal patterns of behavior. During mornin g surveys, we detected cowbirds primarily in pinyon-juniper habitat, but in similar numbers in the ungrazed and actively grazed woodlands. In the afte rnoon, we detected cowbirds feeding almost exclusively in actively grazed p rairies but found that they deserted those sites when cattle were removed i n early July. Radio telemetry confirmed that individual females were commut ing daily between these habitats. Females (n = 30) were generally located i n pinyon-juniper habitats from 0500 to similar 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 bree ding and feeding ranges, some individuals with breeding ranges located towa rd the center of the ungrazed property averaged 7.7 km. When cattle were ro tated out of the main feeding pasture in early July, females immediately ex tended their commutes by similar to1.2 km to access remaining actively graz ed pastures. Overall home range sizes were large (160-4344 ha) and tended t o increase with distance between the females' breeding range and active liv estock grazing. This increase was reflected mainly by differences in feedin g range sizes rather than breeding range sizes. The observed link between c owbird behavior and the distribution of livestock suggests that in regions where livestock grazing is the dominant land use, manipulations of livestoc k grazing patterns may provide an effective tool to manage cowbird parasiti sm.