Distribution, Abundance and Trends of Gulls and Terns Breeding on Sable Island, Nova Scotia, Canada

Abstract

Sable Island is the most isolated seabird colony site in eastern Canada and the United States, offering a unique opportunity to study the population dynamics of terns and gulls in an area removed from human activities. Sable Island likely supported tens of thousands of terns prior to 1900, but the population declined during the first half of the 20th century, coinciding with colonization by breeding gulls. An island-wide census of terns and gulls was conducted in 2012 and 2013, and those results were compared with surveys conducted over the previous 45 years to assess changes in population abundance and distribution. The current island-wide population of Common (Sterna hirundo) and Arctic (S. paradisaea) terns was approximately 6,500 and 4,200 combined breeding pairs in 2012 and 2013, respectively, down from about 9,000 pairs estimated in 2008/2009, but higher than all estimates between 1970 and 2006. Population growth of these tern species has been concentrated at two large colonies, each with over 2,000 breeding pairs, whereas the number of small colonies on the island has been in decline since 1998. Fewer than six pairs of Roseate Terns (S. dougallii) have nested on Sable Island since 1993, down from counts of more than 100 individuals in the 1970s. Estimates of breeding pairs ranged from 744 to 951 Herring Gulls (Larus argentatus) and 398 to 472 Great Black-backed Gulls (L. marinus) during 2012 and 2013. A survey of gulls documented 33 to 50% fewer Herring Gulls and 23% fewer Great Black-backed Gulls than in 1970. Sable Island's tern and gull populations have fluctuated asynchronously over the past 100 years, and may be returning to a previous ecosystem state when gulls were absent and terns were abundant, but mechanisms underlying these trends are unknown.