Long-distance Commuting by Brown-headed Cowbirds in New Mexico

The Brown-headed Cowbird (Molothrus ater) is a widespread brood parasite that has attracted considerable recent attention as a possible threat to forestdwelling songbirds (e.g. Finch 1991, Robinson et al. 1995). Cowbirds are unusual among passerines in that their parasitic nature allows some populations to have spatially and temporally separate breeding and feeding activities (Rothstein et al. 1984). Cowbirds are ground foragers that feed in open habitats such as grazed grasslands, agricultural fields, livestock corrals, and mowed lawns (Friedmann 1929, Mayfield 1965, Ortega 1998). For breeding, however, cowbirds occupy a wider variety of habitats where their passerine hosts occur in higher densities (Robinson et al. 1995). This reliance on distinct habitat types for essential activities gives rise to daily commuting patterns in landscapes where breeding and feeding habitats are spatially separated (Rothstein et al. 1984, Thompson 1994, Gates and Evans 1998). Cowbirds are restricted to parasitizing nests that are within commuting distances of their feeding habitats. In recent years, recognition of the relationship between feeding and breeding habitats of cowbirds has led to management strategies involving the manipulation of feeding habitats to alter cowbird breeding distributions for the benefit of potential hosts. For example, in the Coconino National Forest, Arizona, managers rotate livestock (which enhance feeding opportunities for cowbirds) away from nesting sites of endangered Southwestern Willow Flycatchers (Empidonax traillii extimus; Goguen and Mathews 1999). For such strategies to effectively protect hosts, the scale at which they are implemented must be sufficient to discourage cowbirds from commuting. The maximum commuting distance between breeding and feeding sites of cowbirds is one of several factors needed to determine the correct scale of management efforts for cowbirds. In previous radio-telemetry studies, the longest observed commute between breeding and feeding areas of female cowbirds has been around 7 km (Rothstein et al. 1984, Thompson 1994, Gates and Evans 1998). This distance has been used as an estimate of the maximum commuting distance for female cowbirds (e.g. Gustafson and Crow 1994, Coker and Capen 1995) and is the distance used for livestock removal in some cowbird management efforts (e.g. Goguen and Mathews 1999). Given the conservation implications, it is necessary to