Foraging Habitat Partitioning in Roseate and Common Terns

Abstract

I examined foraging by sympatrically breeding Roseate (Sterna dougallii) and Common (S. hirundo) terns to investigate whether habitat was partitioned, and whether use of foraging habitat was related to disparate population sizes between the species. Foraging Roseate Terns were associated with physical features such as shoals and drift lines more frequently than Common Terns were. Common Terns foraged under a wider range of habitat conditions than Roseate Terns did, and Common Terns were associated with predatory blue-fish (Pomatomus saltatrix) more often. Flocks in which proportions of Roseate Terns were higher than expected (based on the relative sizes of the breeding populations) were smaller and less dense than other foraging flocks, and they formed at sites which were shallower, closer to shore, and winder. Roseate Terns were more successful in flocks in which proportions of Roseate Terns were higher than expected than when in other flocks. Common Terns' foraging efficiency was lower in flocks in which proportions of Roseate Terns were higher than expected than in other flocks. Roseate Terns have a smaller area of preferred foraging habitat available than do Common Terns, and this may be reflected in their population sizes.