Trophic discrimination of amino acid-specific nitrogen stable isotopes in raptor nestlings: implications for estimating trophic position

Abstract

Bulk stable isotope analysis of carbon and nitrogen is commonly used to assess trophic relationships. However, compound-specific stable isotope analysis of individual amino acids may be a more accurate approach for resolving food web structure. Nevertheless, recent studies suggest amino acid-specific nitrogen trophic discrimination factors (TDFs) can vary depending on the type of nitrogenous waste produced and the quality of the diet, potentially limiting inference. We compared the ability to discriminate between primary and secondary consumers using bulk and compound-specific methods in an Arctic tundra ecosystem. Specifically, we determined bulk δ13C and δ15N values, as well as amino acid-specific δ15N values, for red blood cells from nestling golden eagles (Aquila chrysaetos), rough-legged hawks (Buteo lagopus), and gyrfalcons (Falco rusticolus), as well as for muscle samples from common prey species. We subsequently used high-precision diet estimates from nest cameras to estimate TDFs for gyrfalcon nestlings and compare these against TDFs for other taxa from the literature. Although bulk δ15N values of secondary consumers were enriched relative to primary consumers, overlap occurred across groups, and greater separation was apparent using δ15N of amino acids. Comparing red blood cell δ15N values to prey muscle δ15N values, bulk TDFs and compound-specific TDFs for trophic amino acids were lower than values from the literature that have typically been used to estimate trophic position. Amino acid-specific TDFs for raptor nestlings may be particularly low due to their rapid growth, consumption of high-quality protein, and excretion of nitrogenous wastes as uric acid, which have previously been identified as factors influencing isotopic discrimination. Consistent with recent studies, our findings indicate that the use of ‘universal’ TDFGlu-Phe values will result in an underestimate of nestling trophic level, and more work is needed to establish appropriate TDFs that reflect the physiology and life-history stage of the consumer.