Moonlight drives the energy balance and annual cycle of a nocturnal forager.

Behavioral, physiological, and life-history adaptations to the lunar cycle are global phenomena across trophic levels and ecosystems yet remain poorly understood because of the challenges of studying free-living organisms at night. We show that the lunar cycle regulates both daily foraging activity and foraging success in the red-necked nightjar-a nocturnal avian insectivore-and that moonless periods lead to energy deficits that trigger synchronized energy-conservation responses. These cyclical imbalances cascade into fluctuations in fuel reserves and influence the timing of key annual life-history events, including migration and reproduction, but not molt. Despite adaptations to offset lunar constraints, nightjars' annual cycle remains governed by the moon's monthly rhythm, underscoring its pervasive influence on nocturnal life.