Behavioral Ecology of the Brown-Headed Cowbird (Molothrus ater) in a Bison-Grazed Landscape in New Mexico

Brown-headed cowbirds (Molothrus ater; hereafter "cowbirds") often forage with grazing ungulates. Although domestic livestock have largely replaced American bison (Bos bison) as the cowbird's foraging associate, recent restoration of bison to their former territories has increased opportunities for cowbirds to interact with this native grazer. During summer 2002, we studied the behavior of cowbirds and bison in a 27,000-ha shortgrass prairie pasture in northeastern New Mexico. We monitored afternoon distribution of bison and native ungulates within the pasture. We used radiotelemetry to monitor daily movements and behavior of female cowbirds breeding in adjacent pinyon and juniper woodlands; we located and monitored host nests to measure parasitism rates. Our objectives were to (1) measure the spatial and temporal distribution of bison, (2) examine effects of daily changes in bison distribution on breeding and foraging strategies of female cowbirds, and (3) compare the behavior of cowbirds in a bison-grazed landscape with that in a cattle-grazed one. The number of bison within 10 km of the radiotagged cowbirds' breeding grounds varied daily, ranging from 6 to 1,161 individuals. Radiotagged female cowbirds (n = 10) were typically involved in breeding activities in pinyon-juniper woodlands in the mornings and commuted an average of 7.24 km to prairie sites to feed during afternoons. Radiotagged cowbirds fed primarily with bison (77.0% of feeding relocations), but they also fed with elk (Cervus elaphus; 9.9%), without ungulates at a dry lakebed (10.6%), and in a bison-handling corral (1.9%). When few bison were within 10 km of the cowbird breeding ranges, cowbirds foraged with elk or without ungulates. Mean commuting distance was related to distance of the nearest bison from the breeding grounds. Females in the bison-grazed landscape commuted farther and had larger feeding areas and overall home ranges than those in the cattle-grazed landscape. Differences in commuting behavior resulted from bison mobility and differences in management strategies. Increased commuting distances caused by bison mobility may result in reduced egg-laying rates for cowbirds, potentially benefiting breeding songbirds. Resumen.Molothrus ater suele alimentarse junto a ungulados herbivoros. Aunque el ganado domestico ha reemplazado al bisonte americano (Bos bison) como especie asociada durante la alimentation de M. ater, la restauracion reciente de los bisontes a sus territorios habituales ha incrementado las oportunidades que tiene M. ater de interactuar con este herbivoro nativo. Durante el verano de 2002 estudiamos el comportamiento de los M. ater y bisontes en una pradera de pastos cortos de 27,000 ha en el nordeste de Nuevo Mexico. Monitoreamos la distribution de bisontes y ungulados nativos durante la tarde dentro de la pastura. Utilizamos radiotelemetria para monitorear los movimientos y el comportamiento diario de las hembras de M. ater reproduciendose en zonas adyacentes a bosques de pinones y enebros, y localizamos y monitoreamos nidos de los hospederos para medir las tasas de parasitismo. Nuestros objetivos fueron: (1) medir la distribution espacial y temporal de los bisontes, (2) examinar el efecto del cambio diario en la distribution del bisonte en la reproduction y estrategias de alimentation de las hembras de M. ater, y (3) comparar el comportamiento de los M. ater en un ambiente que es mantenido por el pastoreo de bisontes contra otro mantenido por el pastoreo de ganado 3E-mail: cbgl0@psu.edu