Temporal fluctuation in abundance of Brown-headed Cowbirds and their hosts in riparian habitat in the Okanagan Valley, British Columbia, Canada

ABSTRACT.   We tested the hypothesis that the abundance of Brown-headed Cowbirds ( Molothrus ater ) and their hosts, as well as parasitism rates, changed between 1992–1993 and 2001–2003 in riparian habitats in the Okanagan Valley, British Columbia, Canada, where riparian habitat has been reduced in area by more than 85% over the past 60 years. Cowbird abundance declined from a mean of 2.1 and 1.9 individuals per census plot in 1992 and 1993, respectively, to 0.66 individuals per plot in 2001–2002. The mean number of potential host individuals per census plot was also lower in 2001–2002 (5.5) than in 1992 (7.0) and 1993 (7.8). Although the percentage of Yellow Warbler ( Dendroica petechia ) nests parasitized declined (77% in 1992–1993 to 50% in 2002–2003), Yellow Warblers and Song Sparrows ( Melospiza melodia ) in the Okanagan Valley continue to be parasitized at high rates and have low nesting success. Host species and the distance of nests from the edge of nest patches were the strongest predictors of both nest success and parasitism, indicating the importance of large continuous patches of shrubs that allow nests to be located further from edges.