RECONSTRUCTING AVIAN DIETS USING STABLE-CARBON AND NITROGEN ISOTOPE ANALYSIS OF EGG COMPONENTS - PATTERNS OF ISOTOPIC FRACTIONATION AND TURNOVER

Because eggs are composed of nutrients that are ultimately derived from the diet of adult females, the relative abundance of naturally-occurring stable isotopes of carbon and nitrogen in eggs should be related to those in their diet and this may form the basis of a method for tracing diets. Before such dietary reconstructions can be established, however, it is necessary to know how isotopic signatures change (or fractionate) from the diet during the synthesis of various egg components. In this study, stable-carbon and nitrogen isotopic fractionation factors between diet and whole yolk, lipid-removed yolk, albumen, and shell membranes of eggs of captively-raised Mallards (Anas platyrhynchos), Japanese Quail (Coturnix japonica), Prairie Falcons (Falco mexicanus), Peregrine Falcons (Falco peregrinus), and Gyrfalcons (Falco rusticolis) were established. In addition, carbon isotope fractionation patterns between diet and eggshell carbonate and yolk lipid were determined for quail, Mallards and falcons. On average, yolk, albumen and membranes were enriched in N-15 relative to diet for all birds by 3.4 parts per thousand, a value typical of processes associated with protein synthesis. For quail and Mallards, albumen, membranes, and shell carbonate were enriched in C-13 relative to diet by 1.5, 3.6, and 14.9 parts per thousand, respectively, and whole yolk was depleted in C-13 by 2.6 parts per thousand due to the presence of yolk lipids. Falcons showed lower carbon diet-tissue fractionation for all tissues (albumen: +0.9, whole yolk: -1.9, membranes: +2.7, carbonate: +11.2 parts per thousand) and this may be due to a greater reliance on dietary lipids vs. carbohydrates for the synthesis of egg components or on fractionation differences associated with fermentation of carbohydrates. Patterns of isotopic fractionation between diet and Mallard eggs were not influenced by clutch order. A diet-switch experiment using quail indicated that albumen, shell membrane and shell carbonate values reflect diet integrated over 3-5 days and yolk over eight days prior to laying. Because birds were fed ad libitum, these data provide baseline data on fractionation patterns for birds optimally mobilizing nutrients from diet to egg components. These values may differ for those wild birds that rely more heavily on endogenous reserves.