Evaluating carbon (δ13C) and nitrogen (δ15N) isotope discrimination in a captive model to better understand carry-over effects in oviparous species

A fulsome understanding of carry-over effects in migratory oviparous wildlife facilitates effective conservation and requires knowledge of how these species allocate nutrients during egg production. Accurate inference about oviparous nutrient allocation strategies – the contributions of nutrients from body reserves (endogenous) versus those from diet (exogenous) to eggs – is dependent on isotope tissue-diet fractionation values (TDFs). However, not only are these values highly variable and species specific, but they are also based on limited data. To determine carbon ( 13C) and nitrogen (15N) TDFs for birds’ eggs, we conducted a feeding trial with captive shorebirds and measured  13C and 15N in whole food, food constituents (protein and lipid-based), egg components (albumen, whole yolk, lipid-free yolk, and yolk lipid), and select somatic tissues (pectoral muscle, subcutaneous fat) in females. We calculated diet and component specific TDFs and then used these values in Bayesian stable isotope mixing models to evaluate changes in nutrient allocation over the course of the egg-laying period. Our results show that TDFs in shorebirds are distinct from those previously measured in other species and also indicate seasonally variable endogenous nutrient inputs to protein-based egg components. The first study to report these types of data for a small-bodied bird, our findings provide new insights about the relationships between avian nutrition, reproduction, and potential carry-over effects.