Turnover rates of stable isotopes in avian blood and toenails: Implications for dietary and migration studies

Abstract

Most stable isotope (SI) applications in avian ecology are based on the analysis of feathers and blood, whereas toenails are much less used. These structures grow slowly and continuously, thus integrating information over comparatively longer periods which may be useful for migration connectivity studies, while avoiding some of the difficulties posed by incomplete information on feather molt.In spite of this, interpretation of data from toenails is limited by lack of accurate figures on their turnover rates and discrimination values. To improve our understanding of SI ratios in toenails and surpass these drawbacks we measured the change in carbon (δ13C) and nitrogen (δ15N) SI ratios in plasma, red blood cells and toenails of captive dunlins Calidris alpina after a controlled diet switch. Discrimination values were estimated by comparing isotopic values at the end of the experiment with those of artificial food. We also estimated toenail growth rates in this species.Toenails showed much lower SI turnover rates than blood components, with half-lives of 27days and 35days for δ13C and δ15N, respectively. Isotopic ratios in toenails reached equilibrium with the new diet after 100–120days, roughly coinciding with the duration of toenail replacement.The discrimination values of 2.74±0.68‰ for δ13C and 4.06±0.27‰ for δ15N found for toenails are higher than those of both blood components. These values are also higher than most published values for blood in other avian species, but similar to those previously reported for feathers which, like toenails, are metabolically inert after synthesis and are mainly composed of keratin.This study highlights the potential role of toenails as sources of SI data for ecological studies, particularly to determine geographic origin of birds migrating between isotopically distinct environments. Indeed, their long turnover rates might allow for the detection of SI signals from the wintering or breeding ranges, by sampling individuals at their staging sites. Isotopic turnover in toenails follow their replacement rates, and these seem to be rather similar across several avian taxa. Thus, the turnover rates described here may potentially be used to interpret SI data for other birds.