Christopher B. Goguen

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

Nest desertion by blue-gray gnatcatchers in association with brown-headed cowbird parasitism

The blue-gray gnatcatcher, Polioptila caerulea, is one of the smallest regular hosts of the brown-headed cowbird, Molothrus ater. Gnatcatchers typically raise none of their own young when parasitized, yet anti-parasite behaviours have not been reported. A blue-gray gnatcatcher population in which pairs often desert nests when parasitized by cowbirds was studied to examine the success of a desertion response and to determine factors that stimulate desertion. Of 83 gnatcatcher nests, 76% were parasitized; of nests parasitized during the egg-laying stage, 45% were deserted. Desertion occurred independently of the timing of the parasitism event during the breeding season and the number of cowbird eggs laid per nest. All cowbird eggs laid after the completion of the gnatcatchers egg-laying stage were accepted. Parasitism rates were lower late in the breeding season, and nesting success of unparasitized nests was higher. Pairs that deserted often obtained cowbird-free clutches, allowing them to raise gnatcatcher young. Nest desertion appears to be a means of avoiding parasitism and increasing fitness. The nest desertion response was inconsistent, but it cannot be explained by the adaptiveness of acceptance or by the possibility that blue-gray gnatcatchers are in transition between being acceptors or rejectors of cowbird parasitism. Nest desertion by blue-gray gnatcatchers appears to be a generalized response to a disturbance at the nest induced by cowbirds. The cue that elicits the desertion response remains unclear. ? 1996 The Association for the Study of Animal Behaviour

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 females 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...

Cowbird (Molothrus spp.) Ecology: A Review of Factors Influencing Distribution and Abundance of Cowbirds across Spatial Scales

Department of Biology, Villanova University, Villanova, Pennsylvania 19085, USA; Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, California 93106, USA; Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA; Department of Wildlife Ecology, University of Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706, USA; Division of Biology, Kansas State University, 232 Ackert Hall, Manha an, Kansas 66506, USA; School of Forest Resources, Penn State University, 2C Ferguson Building, University Park, Pennsylvania 16802, USA; and Florida Museum of Natural History, University of Florida, P.O. Box 117800, Gainesville, Florida 32611, USA

Songbird Community Composition and Nesting Success in Grazed and Ungrazed Pinyon-Juniper Woodlands

Livestock grazing is a dominant land use of pinyon-juniper habitats in the western United States, yet the effects of grazing on breeding bird communities in this habitat have been poorly studied. We compared habitat structure, songbird abundance, and nesting productivity within pinyon-juniper woodlands on an actively grazed site and a site experiencing long-term relief from livestock grazing in northeastem New Mexico. From 1992 to 1995, we performed vegetation sampling, conducted songbird point counts, and located and monitored nests on 8 35-ha study plots. Four of these plots experienced moderate cattle grazing and 4 were ungrazed since 1973. We found no differences in habitat or vegetation features between grazed and ungrazed plots. Bird communities were similar, with only 1 of the 11 species we tested more abundant on the ungrazed treatment (western scrub-jay; Aphelocoma californicus). We detected no differences in nesting success or cause-specific rates of nest failure for 7 common bird species (P < 0.05), and we detected no differences in brown-headed cowbird (Molothrus ater) parasitism rates for the major hosts between grazed and ungrazed areas. Greater than 75% of the nests of the solitary vireo (Vireo solitarius), western tanager (Piranga ludoviciana), and blue-gray gnatcatcher (Polioptila caerulea) were parasitized on both treatments. These high parasitism rates may be the result of high densities of local cowbirds because of abundant feeding sites (i.e., livestock), the high mobility of cowbirds, and the close proximity of ungrazed plots to grazed areas (all <4 km). Our results suggest that 20 years of relief from grazing had little influence on the habitat structure or bird species composition of the pinyon-juniper woodlands on our study site. However, livestock grazing has indirectly affected the nesting success of some songbird species via the influence of grazing on cowbird abundance. Our findings highlight the need for studies that incorporate nest monitoring and landscape-scale approaches to better understand the relation between cowbirds, livestock, and songbirds, and the time required for recovery from grazing effects.

Effects of Parasitism by Brown-Headed Cowbirds (MOLOTHRUS ATER) on Reproductive Success of Three Frequent Hosts in New Mexico

Abstract We studied effects of parasitism by brown-headed cowbirds (Molothrus ater) on three frequent hosts from a pinyon-juniper (Pinus-Juniperus) community in New Mexico. During 1993–1999 and 2001–2002, we measured frequencies of parasitism and examined how differences in size of host, life-history traits, and behavior, as well as aspects of behavior of brown-headed cowbirds (i.e., removal of eggs of host and multiple parasitism), influenced reproductive success of blue-gray gnatcatchers (Polioptila caerulea), plumbeous vireos (Vireo plumbeus), and western tanagers (Piranga ludoviciana). Brown-headed cowbirds parasitized 79% of 288 nests of blue-gray gnatcatchers, 84% of 170 nests of plumbeous vireos, and 76% of 129 nests of western tanagers. Parasitism reduced productivity of hosts through removal of eggs, reduced hatching success, reduced survival of nestlings, and desertion of nest. Relative to unparasitized nests, size of clutch of singly parasitized nests was reduced, on average, by 0.3 eggs for the plumbeous vireo and by 0.8 eggs for the two other species. Both blue-gray gnatcatchers and plumbeous vireos rarely fledged any of their own young from parasitized nests, whereas western tanagers did so regularly. All three species deserted some nests in association with parasitism, but the primary response to parasitism was acceptance of eggs from brown-headed cowbirds. Multiple parasitism was common, however, mean number of young of hosts that fledged only differed between singly and multiply parasitized nests for the western tanager due to high costs of single parasitism for the blue-gray gnatcatcher and plumbeous vireo. Overall, costs of parasitism were negatively correlated to body size of host; parasitism caused failure of 58% of nests of blue-gray gnatcatchers, 49% of nests of plumbeous vireos, and 22% of nests of western tanagers.