Potential impacts of nesting double-crested cormorants on great blue herons and black-crowned night-herons in the U.S. Great Lakes region

With recovery of double-crested cormorants (Phalacrocorax auritus) in the Great Lakes region, their numbers have increased significantly leading to concern about potential impacts on other species. Cormorants are thought to affect co-occurring colonial waterbirds by usurping limited habitat and destroying vegetation used as nest sites by these species. This paper summarizes initial results from a study to assess potential impacts of double-crested cormorants on great blue herons (Ardea herodius) and black-crowned night-herons (Nycticorax nycticorax) in the Great Lakes. The study examined population trends, interspecific interactions, and cormorant impacts on vegetation. Despite a steady increase in breeding cormorants in the U.S. Great Lakes over the past two decades, population trends of great blue herons and black-crowned night-herons do not indicate cormorants have negatively influenced breeding distribution or productivity of either species at a regional scale. Cormorants have caused total or partial loss of forest cover at a number of islands in the U.S. Great Lakes and these initial data suggest soil chemistry at cormorant colony sites will affect normal plant growth and survival. However, site use data and field observations indicate double-crested cormorant presence has not caused black-crowned night-heron or great blue heron declines or abandonment except under special circumstances. Although preliminary, these results suggest cormorant control policy should not be justified by assumption of potential impacts on other waterbird species without careful documentation.