1. Capital breeders accumulate nutrients prior to egg development, then use these stores to support offspring. In contrast, income breeders rely on local nutrients consumed contemporaneously with offspring development. Understanding such nutrient allocations is critical to assessing wildlife reliance on different habitats.

2. Despite the contrast between these strategies, it remains challenging to trace nutrients from endogenous stores or exogenous food intake into offspring. Here, we tested a new solution to this problem.

3. Using tissue samples collected opportunistically from wild emperor penguins (Aptenodytes forsteri), which exemplify capital breeding, we hypothesized that the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of individual amino acids (AA) in endogenous stores (e.g., muscle) and in egg yolk and albumen reflect the nutrient sourcing that distinguishes capital versus income breeding. Unlike other methods, this approach does not require untested assumptions or diet sampling.

4. We found that over half of essential AA had δ¹³C values that did not differ between muscle and yolk or albumen, suggesting that most of these AA were directly routed from muscle into eggs. In contrast, almost all non-essential AA differed in δ¹³C values between muscle and yolk or between muscle and albumen, suggesting de novo synthesis. Over half of AA that have labile nitrogen atoms (i.e., “trophic” AA) had higher δ¹⁵N values in yolk and albumen than in muscle, suggesting that they were transaminated during their routing into egg tissue. This effect was smaller for AA with less labile nitrogen atoms (i.e., “source” AA).

5. Our results indicate that the δ¹⁵N offset between trophic-source AA (Δ¹⁵N_{trophic-source}) may provide an index of the extent of capital breeding. The value of emperor penguin Δ¹⁵N_Pro-Phe was higher in yolk and albumen than in muscle, reflecting the mobilization of endogenous stores; in comparison, the value of Δ¹⁵N_Pro-Phe was similar across muscle and egg tissue in previously-published data for income-breeding herring gulls (Larus argentatus smithsonianus). Our results provide a quantitative basis for using AA δ¹³C and δ¹⁵N, and isotopic offsets among AA (e.g., Δ¹⁵N_Pro-Phe), to explore the allocation of endogenous versus exogenous nutrients across the capital versus income spectrum of avian reproduction.

These are stable carbon isotope and stable nitrogen isotope values measured in amino acids from hydrolyzed proteins in samples of emperor penguin egg yolk, egg albumen, and skeletal muscle.