LAUSR

ABSTRACT Intraspecific variation is a fundamental component of biodiversity, shaping species interactions and coexistence dynamics. While numerous mechanisms have been proposed to shape the degree of phenotypic variation within species, many remain largely untested or poorly explored at broad spatial and taxonomic scales. Using data from nearly 200,000 bird captures from 99 species across North America, we investigated hypothesized drivers of within‐population phenotypic variation, using body mass and wing length as traits of interest. The magnitude of observed phenotypic variation was modulated by a combination of geographic, environmental, and life history factors. This was true whether considering differences in within‐population phenotypic variation within or among species. The impact of these non‐mutually exclusive mechanisms has resulted in substantial variation in the observed magnitude of within‐population phenotypic variation. These results provide empirical evidence for a set of long‐standing hypotheses regarding the processes that regulate observed patterns of this understudied, but important, component of biodiversity.