Responses of Nesting Yellow-headed Blackbirds and Yellow Warblers to Wrens

We studied nest defense behavior of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and Yellow Warblers (Dendroica petechia) in response to two species of common nest destroyers. We presented freeze-dried models of Marsh Wrens (Cistothorus palustris) at Yellow-headed Blackbird nests and House Wrens (Troglodytes aedon) at Yellow Warbler nests during the incubation stage. We presented a Clay-colored Sparrow (Spizella pallida) as a control for both species. Male Yellow-headed Blackbirds responded more intensively to the Marsh Wren model than the control, and female blackbirds responded intensively to both models but were more aggressive toward the Marsh Wren. Most Yellow Warblers did not respond to the House Wren model with their typical predator responses (e.g., chip alarm calls). Some female warblers were aggressive toward the wren model, whereas others sat in their nest. Sitting in the nest as a defense to deter nest destruction by House Wrens needs further investigation. Differences in response levels between blackbirds and warblers may be related to differences in levels of nest destruction experienced by the two species or differences in nest defense behaviors used by the two species. Received 15 September 2010. Accepted 28 March 2011.