We determined whether lipid extraction of seabird muscle, liver and feathers is required for δ¹³C and δ¹⁵N analysis, based on a study of Barau’s Petrels (Pterodroma baraui) and White-tailed Tropicbirds (Phaethon lepturus). Samples were analyzed for stable isotopes before and after lipid removal. Lipid-free muscle and liver were significantly enriched in ¹³C compared to those containing lipids (0.52‰ and 0.61‰ mean differences in δ¹³C 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 δ¹⁵N values in tropicbirds. Researchers should thus run samples separately for δ¹³C and δ¹⁵N analysis, extracting lipids only in aliquots in which δ¹³C is measured, or apply arithmetic δ¹³C normalization methods on non lipid-extracted samples. Predictive models developed from our data, inferring δ¹³C values of lipid-extracted material from δ¹³C 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.