Compound‐specific (15)N analysis of amino acids: A tool to estimate the trophic position of tropical seabirds in the South China Sea

Compound‐specific (15)N analysis of amino acids (AAs) is a powerful tool to determine the trophic position (TP) of organisms. However, it has only been used in a few studies of avian ecology because the AA patterns in the consumer‐diet nitrogen trophic discrimination factor (TDF(G) (lu‐Phe) = ∆(15) N(G) (lu)−∆(15) N(P) (he)) were unknown in birds until recently, and tropical seabirds have never been investigated with this methodology. Here, we explore the application of this method to tropical seabirds. In this study, we recovered the fossilized bones of tropical seabirds from ornithogenic sediments on two coral islands in the Xisha Islands, South China Sea, as well as the bones and muscle of their predominant food source, flying fish (Exocoetus volitans). Compound‐specific (15)N and (13)C analyses of AAs in both seabird and fish bone collagen were conducted. The TP of flying fish was calculated based on a widely used single TDF(G) (lu‐Phe) approach. We then calculated the TP of tropical seabirds in three different ways: (a) according to the composition of their diet; (b) based on the single TDF(G) (lu‐Phe) approach; and (c) using a multi‐TDF(G) (lu‐Phe) approach. The results of the multi‐TDF(G) (lu‐Phe) approach were much closer to the results based on the composition of the seabird diet than the results of the single TDF(G) (lu‐Phe) approach, confirming its applicability for tropical seabirds. For seabird bone samples of different ages, TP determined from the multi‐TDF(G) (lu‐Phe) approach was most similar to that of bulk δ(15)N of bird collagen, with seabirds occupying higher TPs during the Little Ice Age, as previously shown. In addition, the (13)C Suess effect was reflected in the AAs δ(13)C in our samples. This study applied a compound‐specific (15)N analysis of AAs to determine the TP of tropical seabirds that has potential to extend to all tropical seabirds many of which are widely distributed and play a key role in the evolution of coral island ecosystems.