Effects of sodium heparin on δ¹³C, δ¹⁵N and δ³⁴S values in avian whole blood

Anticoagulants are often added to avian blood samples immediately after collection to allow plasma to be extracted at a later time, but the effects of anticoagulants on stable isotope studies are not well understood. We use a Bayesian approach to assess the effects of sodium heparin, one of the most commonly used anticoagulants, on δ¹³C, δ¹⁵N and δ³⁴S values in avian whole blood using Herring Gulls Larus argentatus and Great Black-backed Gulls Larus marinus as study species. We found that the addition of sodium heparin only had a significant effect on δ³⁴S values (−0.4‰) and that this effect did not meaningfully impact the estimated diet proportions of broad-scale stable isotope mixing models, and only slightly affected the output of finer-scale stable isotope mixing models. Our results highlight that researchers should be aware of the possible impacts of sodium heparin on measured stable isotope values in avian blood, particularly for δ³⁴S, and that this effect should be carefully considered relative to the scale and particulars of the study (e.g. generalist versus specialist species and broad-scale versus fine-scale analyses).