THE IMPACT OF BROWN-HEADED COWBIRD PARASITISM ON POPULATIONS OF THE NUTTALL WHITE-CROWNED SPARROW

Brown-headed Cowbirds, Molothrus ater, are brood parasites on many spe cies of North American Passerines. Cowbird parasitism is frequently ci ted as a contributing factor in the decline of United States passerine populations, based on three lines of evidence. First range expansion and population increases by Cowbirds in this century are dramatic and well documented Second, increasing rates of parasitism have been disco vered in a variety of host species populations. Finally, direct negati ve effects of parasitism on certain vulnerable species. notably Kirtla nd's Warbler Dendroica kirtlandii, have been demonstrated It remains u nproven however, that Cowbirds have significant negative effects on mo re abundant and widely-distributed hosts. We report here on the impact of Cowbird parasitism on populations of Nuttall's White-crowned Sparr ow, Zonotrichia leucophrys nuttalli, in the San Francisco Bay Area Thi s widespread songbird is now subject to a 40-50% rate of parasitism in San Francisco, an increase from 5% only 15 years ago. Using known mor tality and fecundity values, we calculate that this population of Wbit e-crowns cannot maintain its numbers when the parasitism rate exceeds approximately 20%. Present levels of parasitism thus appear to threate n the long-term survival of this adaptable songbird in the San Francis co Bay Area.