Effects of female cowbird removal on reproductive success of hooded warblers

Published source details Stutchbury B.J.M. (1997) Effects of female cowbird removal on reproductive success of hooded warblers. The Wilson Bulletin, 109, 74-81.
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This study is summarised as evidence for the following.

	Action	Category
	Remove/control adult brood parasites Action Link

Remove/control adult brood parasites

A replicated, controlled before-and-after study between 1984 and 1995 at three 50-100 ha (2 experimental and 1 control) hardwood forest sites in Pennsylvania, USA (Stutchbury 1997) found that the proportion of hooded warbler Wilsonia citrina nests parasitised by brown-headed cowbirds Molothrus ater was significantly lower during years when cowbirds were removed (11% of 241 nests parasitised in 1991-5 vs. 64% of 28 nests parasitised in 1984-90). When comparing sites, parasitism rates were lower on sites with cowbird removal (0-11% of 280 nests parasitised at removal sites vs. 38-58% of 32 nests at controls). However, there were no changes in warbler nesting success between sites with low (?5%) and high (>30%) levels of parasitism (average of 1.7 fledglings/nest in sites with low parasitism vs. 1.6 fledglings/nest in sites with high parasitism). The authors suggest that nesting success may be more determined by high rates (22-52%) of predation, than by parasitism. An average of 11-20 female and 7.5-17 male cowbirds were removed each year.