Variability in resource use within populations of free-ranging animals can influence demographic and evolutionary processes. Yet in many systems, the extent of intra-population trophic niche variation and the environmental factors that contribute to this variance remain poorly understood. For example, dietary studies commonly group individuals and jointly classify the study population along the generalist to specialist continuum, even though diets of sub-populations and individuals can deviate significantly from the population average, particularly in heterogeneous environments. To evaluate the degree of intraspecific diet variation in a population of wild birds and to test potential linkages between dietary specialization and environmental gradients in relative resource abundance, we used hierarchical Bayesian stable isotope mixing models to estimate (i) assimilated diets of juvenile lesser scaup (Aythya affinis; hereafter scaup) and (ii) diet variation within and across lakes that harbor distinct assemblages of aquatic macroinvertebrates. We fount that lake-level variation in diet accounted for most of the variation across the study population, with little variation attributed to individual-level specialization. Although we did not detect an effect of macroinvertebrate abundance on diets of scaup ducklings, our findings indicated that they have greater dietary flexibility than previously considered. By identifying landscape and resource heterogeneity as factors generating variation in resource use within populations of a generalist consumer, our results facilitate improved predictions of which species might be best adapted to respond to changing environmental conditions.