Seasonal and inter-annual variation in the feeding ecology of a tropical oceanic seabird, the red-tailed tropicbird Phaethon rubricauda

We studied seasonal and inter-annual variations in the diet, amount of food delivered to chicks, and body condition of chicks and adult red-tailed tropicbird at Europa Island (Mozambique Channel, Indian Ocean), during 3 consecutive seasons. The diet consisted of epipelagic fishes (13 families) and squid (3 families) totalling 32 taxa. Three families, the squid Ommastrephidae (mainly Sthenoteuthis oualaniensis) and the fishes Exocoetidae (flying-fish) and Coryphaenidae (dolphin-fish), accounted for 68 % of the number of prey and 83 % of the ingested biomass. There were considerable differences in size and mass of the prey items, dolphin fish being the largest and squid the smallest. There were important seasonal variations in diet, During chick-rearing, 41 % of the mass of food delivered to chicks consisted of dolphin fish, whereas these fish represented only 2 % of the mass of food during incubation. The squid were caught in similar proportion throughout the breeding season but were larger during chick rearing. We did not find any inter-annual variation in the diet of chicks, nor in the body condition of either fledglings or adults, suggesting that no major inter-annual changes in food availability occurred during the course of our study. However, there was some inter-annual variation in feeding frequency and body condition of chicks of less than 60 d. There were also significant day-to-day variations in the feeding frequency in one of the 3 seasons studied. This suggests that day-to-day variation in the marine environment or in foraging conditions may have short-term consequences in the food provisioning and body condition of growing chicks, but that these variations do not necessarily result in long-term (inter-annual) variations in diet and body condition of fledgling or adult red-tailed tropicbirds at Europa island. These results suggest that between 1995 and 1997, red-tailed tropicbirds of Europa relied on predictable and seasonal food resources when breeding.