Evaluating carbon (δ13C) and nitrogen (δ15N) isotopic discrimination in egg components of a captive shorebird to better understand carry-over effects in oviparous species.

An understanding of carry-over effects in oviparous wildlife - and thus effective conservation - requires knowledge of how nutrients are allocated during egg production. Stable isotope analysis is commonly used to estimate the contribution of nutrients from body reserves (endogenous) versus those from diet (exogenous) to eggs. However, reliable inference about oviparous nutrient allocation strategies is dependent on isotopic discrimination factors, which are highly variable, species specific, and often based on data for a limited number of species. To determine carbon (δ13C) and nitrogen (δ15N) discrimination factors for birds' eggs, we conducted a feeding trial with captive killdeer (Charadrius vociferus) and measured δ13C and δ15N values in whole food, food constituents (protein and lipid-based), egg components (albumen, whole yolk, lipid-free yolk, and yolk lipid), and somatic tissues (pectoral muscle, subcutaneous fat) in females. We calculated diet and component specific discrimination factors and used Bayesian stable isotope mixing models to evaluate changes in nutrient allocation over the course of the egg-laying period. Our results show that discrimination factors in killdeer are distinct from those reported for other species and suggest seasonally variable endogenous nutrient inputs to protein-based egg components in this system. The first study to report these types of data for a shorebird, our findings provide new insights about the relationships between avian nutrition, reproduction, and potential carry-over effects.