Predation and breeding success in roseate terns (Sterna dougallii)

Predation limits the breeding success of many colonial seabirds and is a major factor in the decline of a number of marine bird species. The goal of our study was to determine how predation affects the breeding success of threatened roseate terns (Sterna dougallii) nesting on Country Island, Nova Scotia, Canada. This site is one of only three Canadian breeding colonies and the first to be systematically studied. Forty-five pairs of roseate terns, 330 pairs of arctic terns (Sterna paradisaea), and 130 pairs of common terns (Sterna hirundo) nested at this colony in 1996. Predation was the most important source of egg and chick mortality for roseate terns on Country Island. Twenty-four percent (18/75 eggs) of roseate tern eggs were depredated, presumably by corvids, while 77% (24/31 chicks) of roseate tern chicks were depredated by gulls. Overall, roseate terns at this site produced a maximum of 0.11 fledglings per nest. Roseate terns abandoned Country Island in 1997, presumably because of predation in 1996. We discuss the implications of our results for the long-term survival and management of the Canadian roseate tern population.