Brown-Headed Cowbird Parasitism and Habitat Disturbance in the Sierra Nevada

Abstract

I studied nest parasitism by the brown-headed cowbird (Molothrus ater) in the northern Sierra Nevada to determine parasitism rates of host species, effects of differing availabilities of humanand livestock-based foods (habitat disturbance) on parasitism rates, and impacts on host populations. Twenty-six of 206 nests and family groups with dependent young (including 8 species) were parasitized. Parasitism was strongly associated with habitat disturbance; 27.1% of groups of susceptible host species in highly disturbed areas contained cowbird young vs. 8.7% in less disturbed areas. Parasitized family groups contained 76% fewer host young than unparasitized groups. Productivity of susceptible species was reduced by 20.6 and 6.6% in highly and moderately disturbed areas, respectively, and by 6.1% in the total study area. Hosts that prefer riparian habitats are most likely to decline due to cowbird parasitism. J. WILDL. MANAGE. 50(4):571-575 Recent invasion of the Sierra Nevada by the brown-headed cowbird has raised concerns over its effects on native birds. Because Sierran birds have not previously been in contact with cowbirds, and thus may not have evolved effective defenses, cowbird brood parasitism could cause reductions in vulnerable species (Gaines 1977, Rothstein et al. 1980). Such impacts have been documented for the Kirtland's warbler (Dendroica kirtlandii) (Mayfield 1977) and have been suggested for other species in California (Gaines 1974, Goldwasser et al. 1980) and elsewhere (Brittingham and Temple 1983). An indirect method for evaluating potential population impacts is to determine rates of parasitism of host nests and family groups with dependent young. Differences in parasitism rates have been found in various parts of the Sierra Nevada. Gaines (in Rothstein et al. [1980]) reported high rates of parasitism associated with declines in certain hosts in Yosemite Valley. Rothstein et al. (1980) also found high rates of parasitism at Mammoth Lakes in the eastern Sierra Nevada. In contrast, Verner and Ritter (1983) found low parasitism rates in the southern Sierra Nevada. Availability of human-based foods has been suggested as important in determining numbers and dispersion of cowbirds and the intensity of host parasitism in the Sierra Nevada (Rothstein et al. 1980, 1984; Verner and Ritter 1983). Cowbirds frequently feed in disturbed areas where high energy foods are concentrated, including residential areas with bird feeders, campgrounds, livestock concentration areas, and garbage dumps. Rothstein et al. (1984) reported that cowbirds travelled up to 6.7 km from feeding sites to parasitize host populations. Verner and Ritter (1983) showed a decrease in abundance of the warbling vireo (Vireo gilvus), a frequently parasitized host, near cowbird feeding sites. To date, however, no study has specifically related amounts of human habitat disturbance and associated foods with rates of cowbird parasitism at a variety of sites within a single study area. Study objectives were to: (1) identify host species and rates of parasitism, (2) determine if parasitism rates differed between areas with different availabilities of human-based foods, (3) evaluate the potential impacts of parasitism on host populations, and (4) compare rates and patterns of parasitism in the northern Sierra Nevada to those found previously in the central and southern Sierra. I thank J. Verner, S. I. Rothstein, W. F. LauThis content downloaded from 157.55.39.114 on Mon, 25 Apr 2016 06:49:54 UTC All use subject to http://about.jstor.org/terms 572 COWBIRDS AND HABITAT DISTURBANCE * Airola J. Wildl. Manage. 50(4):1986 Table 1. Criteria used to define foraging habitat suitability classes for brown-headed cowbirds and to assign records of nests and family groups of potential hosts, northern Sierra Nevada, 1983-85. Site distance (km) from areas of Human occupancy Livestock occupancy IrregIrregSuitability Regulara ularb Regular ular High 4.0 >2.0 >4.0 >0.8 a Occupied >75% of days during host species' nesting season. b Occupied <75% of days during host species' nesting season. denslayer, Jr., and E. C. Beedy for comments on the manuscript and B. D. Marcotte for field