Summary The Área de Conservación Osa (ACOSA) contains the largest population of Scarlet Macaws Ara macao in Costa Rica. Despite their influence on ecosystem dynamics and status as a flagship species, empirical data on the foraging patterns of this population is lacking. This information is crucial in implementing effective conservation strategies, particularly reintroduction attempts. Observations of feeding behaviour were made systematically over a 12-month period to provide the first direct examination of Scarlet Macaw diet within the ACOSA region. Scarlet Macaws feed on various items including seeds, flowers, bark, and leaf-gall larvae. Key findings included a demonstration of a smaller dietary niche breadth than that recorded for other Central American populations, use of button mangrove Conocarpus erectus, a species not previously recognised as a food source for Scarlet Macaws, and a heavy reliance on an exotic non-native species, Terminalia catappa. We argue that whilst human-modified coastal locations may present viable habitat for Scarlet Macaws, anthropogenic influences including the removal of native food sources and proliferation of exotic and cultivated species have left the Scarlet Macaws of the ACOSA particularly dependent on a small number of species.