Species Identity and Nest Location Predict Agonistic Interactions at a Breeding Colony of Double-Crested Cormorants (Phalacrocorax auritus) and Great Blue Herons (Ardea herodias)

Recent dramatic growth of the North American Interior population of Double-crested Cormorants (Phalacrocorax auritus) has led to concern about potential impacts of this species on co-nesting colonial waterbirds. Previous investigations of these concerns have focused on Double-crested Cormorant interactions with other species within homogeneous breeding environments, making broad patterns difficult to identify. The present study examined how nest location, nest density, and species identity mediate agonistic interactions among Double-crested Cormorants and Great Blue Herons (Ardea herodias) nesting at a colony site in central Minnesota, USA. Twenty-six Double-crested Cormorant nests and 27 Great Blue Heron nests were observed for 30 min weekly for 8 weeks during the breeding season to estimate frequency of agonistic behavior and identify species-level patterns of interaction. Most agonistic interactions observed (81%) were intraspecific interactions among Double-crested Cormorants; Great Blue Herons engaged in a higher combined total count of interspecific and intraspecific interactions when they nested near Double-crested Cormorants. Interspecific interactions were more common among ground-nesting birds than among tree-nesting birds. This study suggests that further investigation into impacts of Double-crested Cormorants on co-nesting birds is most warranted for ground-nesting colonies.