EFFECTS OF FEMALE COWBIRD REMOVAL ON REPRODUCTIVE SUCCESS OF HOODED WARBLERS

Female Brown-headed Cowbirds (Molothrus ater) were systematically remo ved from two adjacent Hooded Warbler (Wilsonia citrina) breeding popul ations for 2-5 years. All female cowbirds detected using playbacks wer e removed. Although an average of only 17 female cowbirds were removed annually from each population, this reduced the frequency of parasiti sm to < 10% of nests in most years. The average percentage of nests pa rasitized (53%) in populations with no cowbird control (N = 3) was sig nificantly higher than for populations where female cowbirds were bein g removed (9%, N = 7). Experimental reduction of parasitism, however, did not result in a significant increase in number of young hedged per nest. Predation of entire clutches and broods effectively swamped the gains achieved by controlling female cowbirds. The effectiveness of c owbird removals as a management tool will likely vary with cowbird abu ndance and host species, but this study suggests that beneficial effec ts for the host species can be minimal.