Stable Carbon and Nitrogen Isotope Discrimination Factors for Quantifying Spectacled Eider Nutrient Allocation to Egg Production

Nutrient-allocation models based on stable-isotope analysis are used to determine the nutrient sources birds invest in eggs. This approach is particularly useful for birds that migrate between habitats with distinct stable-isotope compositions. A crucial variable is the difference in stable-isotope values of egg tissues relative to diet, so appropriate adjustments can be used in models comparing nutrients from tissues to putative food sources. We established discrimination factors (Δδ) between the diet and eggs of captive Spectacled Eiders (Somateria fischeri) fed a controlled diet. Relative to diet, values of Δδ13C were higher for albumen (2.6‰), yolk protein (2.9‰), eggshell (13.0‰), and shell membrane (3.9‰), and lower for whole yolk (-1.6‰) and yolk lipid (-3.5‰). Values of Δδ15N of egg components were higher relative to diet (albumen 3.7‰, yolk protein 4.4‰, shell membrane 4.7‰, and whole yolk 3.5‰). Except for egg proteins, these patterns are generally consistent with published values for other birds. We conclude that choice of discrimination factors could markedly affect estimates of source contributions to eggs and so recommend species-specific estimates. We also provide the first reported discrimination factors between the female's diet and embryonic down feathers (Δδ13C = 2.1‰ and Δδ15N = 5.2). Finally, we determined discrimination factors between lipid and protein in diet sources and eggs, thus enabling consideration of these nutrients separately. Our study enhances the framework for nutrient-allocation modeling in eiders and likely other sea ducks.