A practical introduction to stable-isotope analysis for seabird biologists: Approaches, cautions and caveats

Stable isotopes of carbon and nitrogen can provide valuable insight into seabird diet, but when interpreting results, seabird biologists need to recognize the many assumptions and caveats inherent in such analyses. Here, we summarize the most common limitations of stable-isotope analysis as applied to ecology (species-specific discrimination factors, within-system comparisons, prey sampling, changes in isotopic ratios over time and biological or physiological influences) in the context of seabird biology. Discrimination factors are species specific for both the consumer and the prey species, and yet these remain largely unquantified for seabirds. Absolute comparisons across systems are confounded by differences in the isotopic composition at the base of each food web, which ultimately determine consumer isotopic values. This understanding also applies to applications of stable isotopes to historical seabird diet reconstruction for which historical prey isotopic values are not available. Finally, species biology (e.g. foraging behaviour) and physiologic condition (e.g. level of nutritional stress) must be considered if isotopic values are to be interpreted accurately. Stable-isotope ecology is a powerful tool in seabird biology, but its usefulness is determined by the ability of scientists to interpret its results properly.