The Scarlet Macaw (Ara macao) is an endangered species. In Costa Rica, the Scarlet Macaw population of the Central Pacific Conservation Area (ACOPAC, n =432 individuals) has undergone considerable study and has been used effectively as a flagship species for regional conservation. Costa Rica's only other viable Scarlet Macaw population, located in the Osa Peninsula Conservation Area (ACOSA, n=800-1200 individuals), remains virtually unstudied. We studied ACOSA Scarlet Macaw nest cavities from February 19th to March 22nd 2006. Through informal interviews with park guards and residents, we found a total of 57 potential nests in 52 trees. Eleven nests were reported as frequently poached. Scarlet Macaws used 14 identified tree species, ten of which are unrecorded in Costa Rica. The most common nesting trees were Caryocar costaricense (n=12, 24%), Schizolobium parahyba (n=9, 18.0%), Ceiba pentandra (n=7, 14.0%) and Ficus sp. (n=5, 10.0%). We compare nesting characteristics to those recorded in ACOPAC. A combination of bottom-up and top-down strategies are necessary to ensure the Scarlet Macaw's long-term success, including environmental education in local schools, community stewardship of active nests, and the advertisement of stricter penalties for poaching.