Naturally occurring stable isotopes in foodwebs can be used to determine the relative contributions of endogenous and exogenous nutrients to avian eggs in cases where birds move between isotopically distinct biomes or habitats to breed. We measured δ¹³C and δ¹⁵N values in somatic muscle tissues and eggs of Barrow's goldeneye (Bucephala islandica) together with those isotope values in amphipods from wetlands used by birds breeding on the Chilcotin Plateau in central British Columbia, Canada. Females that had recently arrived on the breeding grounds had muscle tissue isotope values similar to those found in coastal wintering birds and were considerably more enriched in ¹³C than were samples from local foodwebs. However, δ¹⁵N values of amphipods were highly variable among wetlands, resulting in a nondistinct exogenous δ¹⁵N endpoint for our dual-isotope mixing model. Therefore, we only used the model based on δ¹³C values to estimate nutrient sources to eggs. In 2000, first-laid eggs were more enriched in both isotopes than fourth- or eighth-laid eggs. Considerable endogenous protein input to egg yolk and albumen was detected for the first laid egg (yolk: range = 0–92.7%, median = 23.7%; albumen: range = 0–78.6%, median = 28.7%) with less endogenous contribution of somatic lipids (first egg: range = 0–100%, median = 4.9%). Using archived tissue samples of muscle and developing ovarian follicles from birds collected in 1993–1994, we found no δ¹³C isotopic evidence for endogenous protein contribution to egg yolk. Our results demonstrate the utility of the stable isotope approach in cases where isotopic endpoints are well established. Barrow's goldeneye showed a mixed strategy of endogenous vs. exogenous nutrient allocation to reproduction that varied by individual females, laying order, and year. We encourage managers to use this approach to quantify nutrient allocations from various biomes to reproduction in waterfowl to better understand the importance of wintering sites to reproduction.