Social groups form when the costs of breeding independently exceed fitness costs imposed by group living. The costs of independent breeding can often be energetic, especially for animals performing expensive behaviors, such as nest construction. To test the hypothesis that nesting costs can drive sociality by disincentivizing independent nest founding, we measured the energetics of nest construction in a facultatively social carpenter bee (Xylocopa sonorina ), which bores tunnel nests in wood. We used flow-through respirometry to measure metabolic rates of bees excavating wood and used computerized tomography (CT) images of nesting logs to measure total excavation volumes. From these data, we estimated the energetic cost of nest construction under alternative social/reproductive strategies. Our results indicate that new-nest excavation entails a significant energetic cost, equivalent to approximately 150 hours of flight. Alternatively, a female may minimize or even eliminate this cost by inheriting an existing nest, a strategy associated with group living. These results suggest that females join (or remain in) social groups to avoid steep energetic costs, supporting the costly nest-founding hypothesis for the origins of group living.