LAUSR

Abstract. Hat Island, Lake Michigan, Michigan, USA, is an important historical breeding location for Caspian Terns (Hydroprogne caspia) and other co-nesting species, including Double-crested Cormorants (Phalacrocorax auritus). Although a bird sanctuary of the Michigan Islands National Wildlife Refuge, Double-crested Cormorant management (egg oiling and killing adults) was allowed on Hat Island in 2010; in 2011 and 2012, management actions were restricted to shooting Double-crested Cormorants from at least 500 m offshore. Because the Double-crested Cormorant colony was spread across much of Hat Island, in 2010, a small area of Double-crested Cormorant nests nearest the Caspian Tern colony was left undisturbed to minimize negative impacts on Caspian Terns. Yet, data indicate that management actions had indirect negative impacts on Caspian Terns. Game camera images show that Caspian Terns experienced more frequent disturbances in 2010 as compared to 2011 and 2012. Furthermore, nest inventories and chick banding, in combination with direct chick re-sighting and camera images, show Caspian Tern reproductive success was lower and the colony was abandoned earlier in 2010, as compared to other years. This illustrates that Double-crested Cormorant control actions can cause unintended consequences to co-nesters, even when those conducting management report no impact, and exemplifies that there should be greater consideration of the impacts on co-nesters prior to management.