any adhered oils. Wing δ 15 N was similarly influenced by pigment (range 0.3‰), but this effect was not reduced through washing. We recommend future isotopic studies of monarchs and other butterflies for migration research to use the same region for subsampling consistently and to wash samples with solvent to reduce isotopic variance related to uncontrolled variance in discrimination (δ 2 H, δ 13 C, δ 15 N) and/or adsorbed water vapor (δ 2 H). These data also need to be included in description of methods.