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 delta N-15 and delta C-13 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 delta N-15 and delta C-13 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 delta N-15 of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC delta N-15, with chicks exhibiting lower delta N-15 when they were growing the fastest. As growth slowed, delta N-15 increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted delta N-15 in chick RBCs by 0.92aEuro degrees. We propose that increased nitrogen-use efficiency is responsible for N-15 depletion in both growing and food-restricted chicks. delta N-15 values in RBCs of free-ranging auklets fell within a range of only 1.03aEuro degrees, while feather delta N-15 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.