Resource partitioning within a tropical seabird community: new information from stable isotopes
Résumé
Characteristics of the tropical oceanic environment (low productivity, little seasonality) and poor diversity of tropical seabird foraging methods and prey relative to temperate and polar species suggest that tropical seabirds overall encompass a narrow range of isotopic niches, with large overlaps among species. To test this hypothesis, we examined the stable carbon (δ13C) and nitrogen (δ15N) isotopic composition of blood and feathers of 5 seabird species from Europa Island, Mozambique Channel. While differences were small, blood δ13C and δ15N values characterized 5 distinct and non-overlapping trophic niches during the breeding period. Seabirds used 2 distinct foraging areas (δ13C), one used by sooty tern Sterna fuscata and white-tailed tropicbird Phaethon lepturus and one used by red-footed booby Sula sula and great Fregata minor and lesser F. ariel frigatebirds. Seabird species overall encompassed less than 1 trophic level (δ15N), which is in agreement with a diet mainly based on flying fish and squid. Feather δ13C and δ15N values showed that the trophic structure of the community was different during the breeding and non-breeding (moulting) periods, suggesting a shift in the feeding ecology when adult birds were no longer central-place foragers. The stable isotope method underlined sex-related (red-footed booby) and age-related (great frigatebird immatures and adults) feeding strategies. It also suggested that breeding adults could feed themselves on different prey than those given to their chicks (sooty tern). Within the tropical pelagic ecosystem, seabirds overall shared the same trophic level as large predatory fishes (albacore, yellowfin and skipjack tunas), but they had lower δ15N values than the deeper-dwelling bigeye tuna and swordfish. We conclude that analyzing stable isotope values in blood and feathers appears to be a promising alternative method for investigating food and feeding ecology of tropical seabirds year round, and for determining sex- and age-related differences in their foraging strategies. A limitation of the method is the lack of information on marine isoscapes; future studies aimed at isotopically characterizing the tropical marine environment could help to associate consumer signatures to geographic origins.