EFFECTS OF LIPID EXTRACTION ON STABLE ISOTOPE RATIOS IN AVIAN EGG YOLK: IS ARITHMETIC CORRECTION A RELIABLE ALTERNATIVE?

Many studies of nutrient allocation to egg production in birds use stable isotope ratios of egg yolk to identify the origin of nutrients. Dry egg yolk contains >50% lipids, which are known to be depleted in (13)C. Currently, researchers remove lipids from egg yolk using a chemical lipid-extraction procedure before analyzing the isotopic composition of protein in egg yolk. We examined the effects of chemical lipid extraction on delta(13)C, delta(15)N, and delta(34)S of avian egg yolk and explored the utility of an arithmetic lipid correction model to adjust whole yolk VC for lipid content. We analyzed the dried yolk of 15 captive Spectacled Eider (Somateria fischeri) and 20 wild King Eider (S. spectabilis) eggs, both as whole yolk an after lipid extraction with a 2:1 chloroform:methanol solution. We found that chemical lipid extraction leads to an increase of (mean +/- SD) 3.3 +/- 11 parts per thousand in delta(13)C, 1.1 +/- 0.5 parts per thousand in delta(15)N, and 2.3 +/- 1.1 parts per thousand in delta(34)S. Arithmetic lipid correction provided accurate values for lipid-extracted VC in captive Spectacled Eiders fed on a homogeneous high-quality diet. However, arithmetic lipid correction was unreliable for wild King Eiders, likely because of their differential incorporation of macronutrients from isotopically distinct environments during migration. For that reason, we caution against applying arithmetic lipid correction to the whole yolk delta(13)C of migratory birds, because these methods assume that all egg rnacronutrients are derived from the same dietary sources. Received 12 June 2008, accepted 27 May 2009.