Comparative foraging ecology of five sympatric terns at a sub-tropical island in the eastern Indian Ocean

Abstract

Over one million pairs of seabirds breed annually on the Houtman Abrolhos island group, off the mid-western coast of Australia, the largest seabird breeding station in the eastern Indian Ocean. Eight of the 13 species that breed annually on Pelsaert Island are terns. Dietary samples (regurgitates) were collected from the five most numerous tern species from 1993 to 1999. The prey items identified were related to observations of foraging seabirds around the island. The largest species studied, the crested tern Sterna bergii, foraged mainly for reef fish over shallow reef flats near the breeding islands, as well as for schooling clupeid fish over coastal shelf waters. The smallest species, the roseate tern S. dougallii, also foraged within sight of its colonies, but over deeper waters than crested terns. Three larval fish species characteristic of continental shelf waters (a gonorhynchid, a goatfish and a bellowfish) figured prominently in the diets of roseate terns and three other tern species studied. Of these, only the sooty tern S. fuscata also ate squid and lanternfishes, foraging farthest (480–600 km) from its breeding island, off the shelf edge. The lesser noddy Anous tenuirostris and brown noddy A. stolidus took very similar taxa of prey and also overlapped considerably in their foraging ranges, about 180 km from their colonies, between the islands and the continental shelf edge. However, the lesser noddy took smaller fish than did the larger brown noddy. Although these five abundant tropical tern species, that bred sympatrically, showed some segregation in their diets and, more clearly, in their foraging ranges, considerable overlap of both aspects remained.