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).