The Yellow-billed Cotinga (Carpodectes antoniae) is one of Central America’s rarest and most endangered species. A regional endemic to Panama and Costa Rica, between 250 and 999 individuals appear to now survive mainly within Pacific coastal mangroves and adjacent lowland forest within the Térraba-Sierpe National Wetlands and the Osa Peninsula of Costa Rica, an Alliance for Zero Extinction Site. We used radio telemetry to determine seasonal movements and habitat use and requirements of three Yellow-billed Cotingas at the Rincón River and mangrove estuary on the Osa Peninsula. During the breeding months from approximately December to June, radio-tagged cotingas used mangroves primarily for courtship display, insect feeding, protection from inclement weather, and night roosts. They used lowland forest for feeding on fruit and insects and for infrequent courtship display. Birds fed on fruits from 23 lowland forest tree species throughout the year. Although the female ranged farther than the males during the non-breeding season, no bird showed evidence of migratory behavior. They did not wander from the Rincón mangroves during the non-breeding months from July to November, and they roosted in the same mangrove plot each night. The juxtaposition of mangroves and lowland forest rich in fruit trees may be critical to the survival of the species.