Factors affecting intraspecific diet variation of ducklings: Niche partitioning in a heterogeneous landscape?

Abstract Variability in resource use within populations of free‐ranging animals can influence demographic and evolutionary processes. Yet in many ecological systems, the extent of intra‐population variation in the resources that animals consume and the environmental factors that contribute to this variance remain poorly understood. For example, dietary studies commonly group conspecific individuals and jointly classify entire study populations along the generalist to specialist continuum, even though the diets of subpopulations and individuals can deviate significantly from the population average, particularly in heterogeneous habitats—where structural and biotic characteristics of the sampled habitat are diverse. To evaluate the degree of intraspecific diet variation in a population of wild birds and to test potential linkages between observed variation and environmental gradients in relative resource abundance, we used hierarchical Bayesian stable isotope mixing models to estimate (1) assimilated diets of juvenile lesser scaup ( Aythya affinis ; hereafter scaup) and (2) diet variation within and across lakes that harbor distinct assemblages of aquatic macroinvertebrates. We found that lake‐level variation in diet accounted for most of the variation across the study population, with little variation attributed to differences among individuals within a given lake. Although we did not detect an effect of macroinvertebrate abundance on the diets of pre‐fledging scaup, our findings indicated that ducklings have greater dietary flexibility than previously considered. By identifying variation in resource use within a population of a generalist consumer occupying diverse habitats, our results facilitate improved predictions of which species might be best adapted to respond to changing environmental conditions.