Demographic Consequences of a Catastrophic Event in the Isolated Population of Common Terns at Bermuda

Abstract. A small, isolated population of Common Terns (10–30 pairs) has bred at Bermuda since at least the 1920s. In September 2003, a hurricane eliminated all the adult males; only females returned in 2004 and these paired together and laid clutches of three–seven infertile eggs. The breeding population was re-established in 2005–06 by four adult males that had been too young to breed in 2003–04. These males paired with females that were probably young and bred with high success (mean 2.6 fledged chicks/pair) through 2009. Two males apparently each mated with two females to form productive trios. The old females continued to pair together and lay infertile eggs, even though males were raising chicks on the same islets. Consequently, the effective population size at the ‘bottleneck’ was only four males and four females. Although the population is now increasing rapidly, it remains critically endangered.