Phenology and breeding ecology of Common Terns (Sterna hirundo) in Bermuda: An ecologically distinctive island population, now critically endangered

Abstract

Breeding Common Terns (Sterna hirundo) have been studied extensively in North America and Europe, but little is known of the small, isolated populations south of the main continental ranges. We conducted a 69 year study of Common Terns in the Bermuda archipelago (in the western North Atlantic Ocean at ∼32°N) to document their phenology, distribution, numbers, and breeding biology. Common Terns arrive at Bermuda in March and start to nest about 10 April, 2–3 weeks earlier than any other population of the species studied to date. Unlike the colonially nesting mainland populations, Bermudian Common Terns nest singly or in very small groups on many small islets and artificial nest sites (n = 50 sites) scattered throughout the archipelago. The population included 15–36 pairs in the 1970s and 1980s, but declined by about 50% following each of 3 major hurricanes in 1987, 2003, and 2010, and is now critically endangered. Since the 2003 hurricane, the population has included many (up to 7) female–female pairs laying infertile eggs. In contrast, the mean clutch size of male–female pairs was 3.01, with mean productivity >2.2 fledglings/pair over the 69 year period, the highest values reported for the species anywhere in its range; in at least 10 cases, pairs raised 4 chicks to fledging. The Bermudian population of Common Terns is intensively managed but remains critically endangered, and its recent shift to nesting on small rafts and buoys about 3 m in diameter poses new challenges for management.