Conservation Evidence strives to be as useful to conservationists as possible. Please take our survey to help the team improve our resource.

Providing evidence to improve practice

Individual study: Effect of brown-headed cowbird Molothrus ater removal on parasitism rate of hooded warbler Wilsonia citrina nests in Crawford County, Pennsylvania, USA

Published source details

Stutchbury B.J.M. (1997) Effects of female cowbird removal on reproductive success of hooded warblers. Wilson Bulletin, 109, 74-81


This study is summarised as evidence for the intervention(s) shown on the right. The icon shows which synopsis it is relevant to.

Remove/control adult brood parasites Bird Conservation

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.