Tissue- and diet-dependent stable carbon and nitrogen isotope discrimination: a calibration study in a captive shorebird species

In ecology, stable-isotope ratios are widely used to determine diets of organisms and reconstruct food webs. This is usually done by analyzing the stable-isotope ratios of nitrogen (delta N-15), which increase with increasing trophic level, and those of carbon (delta C-13), which correlate with the delta C-13 value of food source(s) and generally differ between terrestrial and marine food sources. Assimilation of food changes stable-isotope ratios, resulting in different values between the food source and its consumer. These differences are known as isotope trophic discrimination factors and, if known, can be used to determine from the stable-isotope ratios in the consumer's tissue what the consumer has been eating. What is often ignored is that discrimination factors can differ between consumer's food sources and also between tissue types. Therefore, we performed a controlled feeding study in red knots Calidris canutus to determine discrimination factors between different food sources and red knot tissues. We kept two groups of red knots in captivity on a stable diet, one group feeding on mudsnails and the other on Trouvit pellets, for several months, during which the birds molted their feathers. We analyzed delta C-13 and delta N-15 in both food sources and in five red knot tissues (blood cells, blood plasma and three feather types) and subsequently calculated the isotope discrimination factors. We confirmed that the discrimination factors differed between tissues, and also between diets. Our values deviated from general averages reported in reviews on a wide range of animals/birds, but were very similar to values from previous red knot and dunlin studies. We therefore think that our discrimination factors can be used in future stable isotope studies, not only on red knots, but also on other marine shorebird species and plea for careful consideration of using the right discrimination factors.Keywords: delta C-13, delta N-15, discrimination factor, red knot, shorebird, stable isotope