The period of growth prior to fledging is a critical life stage for most birds, and shifts in diet during ontogeny may affect growth and development. We used nitrogen (δ¹⁵N) stable isotopes in feather tissue to quantify dietary composition and evaluate the relationship between diet and growth in prefledging Greater Sage-Grouse (Centrocercus urophasianus). Sequential sampling of δ¹⁵N from feather tissue that was synthesized throughout growth allowed us to evaluate changes in plant versus invertebrate contributions to chick diet during the first 28 days posthatch. Feathers became depleted in δ¹⁵N throughout growth, and Bayesian mixing models suggested that the proportional contribution of invertebrate nitrogen declined with chick age. We estimate that invertebrate contributions to the protein in chick diets decreased from 39% at 1 week to 23% at 4 weeks of age, which is consistent with previous research on sage-grouse that used traditional diet-sampling methods. Chicks with feather δ¹⁵N values that spanned a larger range, but had an intermediate mean, had the longest tarsi and greatest mass at 28 days. These chicks also displayed a more rapid transition to herbivory during the prefledgling period. These patterns are consistent with greater importance of invertebrates during early growth but also suggest that a rapid transition to a more herbivorous diet ultimately results in the highest growth rates. Sequential sampling of feather δ¹⁵N provided useful information on temporal patterns in chick diet that were directly relatable to growth during the prefledging stage, and we encourage replication of our approach in other systems.