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1 June 2011 Interactions Between Double-crested Cormorants and Other Ground-Nesting Species
Christopher M. Somers, Jennifer L. Doucette, D. V. Chip Weseloh, Victoria A. Kjoss, R. Mark Brigham
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Increasing populations of Double-crested Cormorants (Phalacrocorax auritus; hereafter cormorants) are of concern regarding the displacement of other species from breeding colonies, but little is known about such interactions. Agonistic interactions between ground-nesting cormorants and American White Pelicans (Pelecanus erythrorhynchos; hereafter pelicans) were investigated and compared to those involving cormorants and Herring Gulls (Larus argentatus). Also, a survey of cormorant eggs in nests of other ground-nesting species was conducted. Of 158 agonistic interactions between cormorants and pelicans, 83% were threat displays, 9% attempts to steal nest material and 8% involved some form of physical contact. Pelicans defended their nests less frequently and engaged in fewer physical contacts (8%) in colonies mixed with cormorants compared to areas with only pelicans (33%). Herring Gulls engaged in more physical contact (26%) with cormorants in a mixed colony than when nesting only with other gulls (4%). Cormorant eggs were found in 1–26% of Herring and Ring-billed Gull (L. delawarensis) nests searched in the Great Lakes, and 0.3% of Caspian Tern (Hydroprogne caspia) nests at one location. The results indicate cormorants routinely engage in agonistic interactions with other ground-nesting birds; however, the type and importance of interactions varies. Site- and species-specific evaluation of interactions involving cormorants and other birds is recommended before taking management steps to reduce nest site competition.

Christopher M. Somers, Jennifer L. Doucette, D. V. Chip Weseloh, Victoria A. Kjoss, and R. Mark Brigham "Interactions Between Double-crested Cormorants and Other Ground-Nesting Species," Waterbirds 34(2), 168-176, (1 June 2011).
Received: 8 April 2010; Accepted: 1 September 2010; Published: 1 June 2011

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