Effects of preservation methods on stable isotope signatures in bird tissues

Increasing use is being made of stable isotopes as indicators of habitat use and trophic ecology of animals. Preservation of tissues can alter stable isotope signatures. We investigated the effects of addition of ethanol and NaCl solution (hereafter 'salt'), and of freezing and drying, on carbon and nitrogen isotopic values in blood of the spectacled petrel Procellaria conspicillata, and compared these with those from simultaneously growing feathers. The mean delta(13)C values of blood preserved in ethanol was significantly higher, and of blood preserved in salt was significantly lower than that of dried or frozen samples. delta(13)C values in ethanol showed high variation according to brand and batch and could account for the differences found in delta(13)C ratios in ethanol-preserved blood samples. Mean delta(13)C and delta(15)N values in growing feathers were higher than in blood, suggesting tissue-specific fractionation. We conclude that different methods of preserving tissues such as blood may bias stable isotope values, and urge researchers to consider this issue. Air drying is proposed as a practical and unbiased method for blood preservation in field situations where freezing is not a practical option, and a mathematical approach is suggested to permit comparison between studies using different preservation methods or tissues.