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1 February 2012 Frequent Vocalizing is Negatively Associated with Brood Parasitism in a Host of the Brown-Headed Cowbird
Sonya E. Steckler, Courtney J. Conway
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Brood parasitism by the Brown-headed Cowbird (Molothrus ater) can substantially affect host species' reproductive success. The “host-activity” hypothesis suggests that parasites eavesdrop on conspicuous behaviors to locate and parasitize hosts, and several studies of cowbird hosts support this hypothesis. In contrast, a recent study of the Least Bell's Vireo (Vireo bellii pusillus) reported a negative association between the host's vocalization rate near the nest and brood parasitism. This contradictory pattern is intriguing because Bell's Vireo is a common cowbird host and vocalizes near and on its nests. We tested a key assumption of the host-activity hypothesis in a different subspecies (V. b. arizonae) to determine whether the contradictory pattern reported in V. b. pusillus is an anomaly or could be generalized to other subspecies. Unparasitized vireos vocalized more frequently than parasitized birds, confirming that the pattern in Bell's Vireos is the opposite of that reported for other cowbird hosts. Nesting stage played a role: unparasitized birds vocalized more than parasitized birds only during the nest-building and incubation stages. Given that vocalization rate and other behaviors change through the breeding season, future tests of the host-activity hypothesis should control for nesting stage. Moreover, future efforts to identify the underlying cause for the association between vocalization rate and probability of parasitism should consider the possibility of reciprocal causal relationships between them. We propose five additional hypotheses to explain why in Bell's Vireo the pattern between these two traits is opposite of what has been reported in other birds.

© 2012 by The Cooper Ornithological Society. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press's Rights and Permissions website,
Sonya E. Steckler and Courtney J. Conway "Frequent Vocalizing is Negatively Associated with Brood Parasitism in a Host of the Brown-Headed Cowbird," The Condor 114(1), 219-226, (1 February 2012).
Received: 10 January 2011; Accepted: 1 August 2011; Published: 1 February 2012

Bell's Vireo
brood parasitism
brown-headed cowbird
host behavior
host-activity hypothesis
Molothrus ater
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