Effects of nest parasitism by the Brown-headed Cowbird on nesting success of the California Gnatcatcher

California Gnatcatcher (Polioptila californica californica) nests, from 1992-1995 at five sites in western Riverside County, California, were examined to determine the effects of Brown-headed Cowbird (Molothrus ater obscurus) nest parasitism and cowbird trapping on nest fates. Before cowbird trapping, nest parasitism was 31.5% over the entire breeding season and 45.7% during the period when cowbirds were active. Nest parasitism appeared to decrease gnatcatcher nest success by increasing nest abandonment. However, nests were lost to depredation versus parasitism at a 3:1 ratio. Gnatcatcher eggs were significantly correlated with cowbird eggs beginning the week of 5 May in 1992 and 1993, indicating the onset of parasitism pressure within a breeding season. Considering nests initiated after 5 May, nest parasitism was significantly reduced from 45.7% before cowbird trapping to 9.8% after cowbird trapping. and nest success increased significantly from 11.2% before cowbird trapping to 21.7% after cowbird trapping. Considering nest fates regardless of initiation date, there were no significant differences between no-trap and trap periods. Comparisons of no-trap versus trap nest fates before 5 May indicated that nest abandonment increased significantly from 3.3% for the no-trap period to 28.7% during the trapping period. At the same time, nest success decreased significantly from 35.0% during the no-trap period to 15.7% during the trap period. Gains in nest success from decreased nest parasitism were negated by increased nest abandonment before cowbirds were active.