Diet of tropical Roseate Tern chicks on Aride Island and the role of local oceanographic conditions and age of chicks on food provisioning

Small seabirds are very sensitive to changes in the availability of prey and thus may be good indicators of changes in species composition and availability of prey. However, long-term data on dietary variation is scarce for tropical tern species. This paper assembles 9 years of data on deliveries of fish prey to Roseate Tern chicks of known-age on Aride Island, the Seychelles, in the western Indian Ocean, to examine: (1) the role of intrinsic (age of chicks) and extrinsic (local oceanographic conditions) factors on chick food provisioning, and (2) the temporal variation in intake rate of chicks. Our results show low annual variability in dietary composition: 75-97% of all ingested items were goatfish (Mullidae), which agrees with previous studies showing that these form the main prey fish for the Seychelles seabird community. Intake rate of chicks varied considerably among years, from 0.86 g fish ingested chick-1 h(-1) in 1997 to 3.12 g chick(-1) h(-1) in 2005, with poor years showing a much higher coefficient of variation than good years. Intake rate of chicks was significantly correlated with local chlorophyll-a concentration and sea-surface temperature of the preceding 2 weeks. This suggests that food provisioning of tropical Roseate Tern chicks is sensitive to fluctuations in local oceanographic conditions that affect the availability of prey fish at a short temporal scale, and that Roseate Terns are suitable sentinel organisms to indicate local changes in environmental conditions.