Effects of Lipid Extraction on δ13C and δ15N Values in Seabird Muscle, Liver and Feathers

We determined whether lipid extraction of seabird muscle, liver and feathers is required for δC and δN analysis, based on a study of Barau’s Petrels (Pterodroma baraui) and Whitetailed Tropicbirds (Phaethon lepturus). Samples were analyzed for stable isotopes before and after lipid removal. Lipid-free muscle and liver were significantly enriched in C compared to those containing lipids (0.52 ‰ and 0.61 ‰ mean differences in δC respectively) implying that lipids should be extracted from these tissues to avoid effects of uncontrolled differential lipid content. However lipid extraction also slightly increased muscle δN values in tropicbirds. Researchers should thus run samples separately for δC and δN analysis, extracting lipids only in aliquots in which δC is measured, or apply arithmetic δC normalization methods on non lipid-extracted samples. Predictive models developed from our data, inferring δC values of lipid-extracted material from δC values and C:N ratios of non lipid-extracted material are presented and compared to five normalization methods taken from literature. Our models showed the best prediction efficiencies followed by that developed by McConnaughey and McRoy (1979). In feathers, no difference in stable isotopic measurements were noted between aliquots treated with sodium hydroxide or 2:1 chloroform:methanol. Both protocols seem suitable for preparing feather samples for isotopic analysis.