D. M. O’Brien | USGS Science

USGS2010

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 ?? 13 C, ?? 15 N, and ?? 34 S of avian egg yolk and explored the utility of an arithmetic lipid correction model to adjust whole yolk ?? 13 C for lipid content. We analyzed the dried yolk of 15 captive Spectacled Eider (Somateriafischeri) and 20 wild King Eider (S. spectabilis) eggs, both as whole yolk and 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 ?? 1.1% in ?? 13 C, 1.1 ?? 0.5% in ?? 15 N, and 2.3 ?? 1.1% in ?? 34 S. Arithmetic lipid correction provided accurate values for lipid-extracted S13C 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 ?? 13 C of migratory birds, because these methods assume that all egg macronutrients are derived from the same dietary sources. ?? 2010 The American Ornithologists' Union.

The Auk↗

USGS2008

Disentangling effects of growth and nutritional status on seabird stable isotope ratios

A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather δ 15 N and δ 13 C in rhinoceros auklet chicks ( Cerorhinca monocerata ), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing δ 15 N and δ 13 C in control chicks to those of captive, non-growing subadult auklets ( n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska ( n = 15 for each), as well as adult feather samples ( n = 13). In the captive experiment, moderate food restriction caused significant depletion in δ 15 N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC δ 15 N, with chicks exhibiting lower δ 15 N when they were growing the fastest. As growth slowed, δ 15 N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted δ 15 N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for 15 N depletion in both growing and food-restricted chicks. δ 15 N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather δ 15 N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.

Alaska↗