Generalist populations are often composed of individuals each specialized on only a subset of the resources exploited by the entire population. However, the traits underlying such niche variation remain underexplored. Classically, ecologists have focused on understanding why populations vary in their degree of intraspecific niche variation, with less attention paid to how individual-level traits lead to intraspecific differences in niches. We investigated how differences in behavior, morphology, and microhabitat affect niche variation between- and within-individuals in two species of spider Anelosimus studiosus and Theridion murarium. Our results convey that behavior (i.e., individual aggressiveness) was a key driver of intraspecific trophic variation in both species. More aggressive individuals capture more prey, but particularly more Coleoptera, Hymenoptera, and Diptera. These findings suggest that behavioral traits play a critical role in determining individuals’ diet, and that behavior can be a powerful force in driving intraspecific niche variation.