Reproductive and Foraging Ecology of Roseate Terns (Sterna dougallii) in Two Tropical Colonies in the Indian Ocean Differing in Local Environmental Conditions

Over 2006-2010, we compared key aspects of the breeding ecology of Roseate Terns (Sterna dougallii) at two tropical colonies in the Seychelles archipelago (western Indian Ocean) that differ in oceanographic and geological (granite vs. coral) conditions: Aride Island and St. Joseph Atoll. Seasonal increases in chlorophyll concentration peaked in July at both sites, but the rate of increase during the breeding season was usually higher around Aride. This pattern matched the birds' breeding phenology, with laying typically starting earlier on Aride than on St. Joseph in four out of five study years. In 2008, a strong La Niña year, terns refrained from laying at St. Joseph, while on Aride a complete reproductive failure was recorded. Compared to Aride, the density of breeding birds was much higher on St. Joseph, coinciding with the presence of avian predators. Diet sampling and stable isotope analyses (d13C, d15N) of chick and adult feathers indicated that Roseate Terns at both colonies relied mainly on goatfishes (Mullidae) and shared broadly similar foraging ecology. Our study suggests that inter-annual variation in marine primary productivity is a key driver of breeding phenology and success at both colony sites. However, differences in nesting habitat and possibly colony age also contribute to variation in breeding parameters between these two colonies.