The Osa Peninsula is in the South Pacific region of Costa Rica supporting some of the largest stands of mature tropical forest left on the Pacific coast of Mesoamerica. The Osa is well know for being one of the most biologically intense places on earth holding 2.5% of the world’s biodiversity. It is an extremely important wintering area for Neotropical migratory birds and well over half of Costa Rica’s avifauna including a handful of threatened and endemic species. This checklist includes birds from the Osa Peninsula and the entire Osa Conservation Area (ACOSA) where you will find nearly 500 species spread across 15 parks, reserves and refuges including Costa Rica’s crown jewel Corcovado National Park. These irreplaceable reserves protect a wide range of natural habitats that offer visitors the opportunity to enjoy birds in breathtaking settings. The Front Cover: This image is the Scarlet Macaw, an iconic bird that is abundant on the Osa Peninsula. This beautiful species has lost a considerable amount of habitat across the Central American portion of its range. Fortunately, the Osa is one of the Scarlet Macaws’ remaining strongholds in Meso America and can be found just about anywhere! The taxonomy and nomenclature follow the American Ornithological Society (AOS) checklist of North and Middle American birds, 7th Edition. The Spanish names come from A Guide to the Birds of Costa Rica by Stiles and Skutch. Key to Residency M = Migratory Neotropical migratory birds PM = Passage Migrant Birds that pass through the area during migration but don’t stay E = Endemic Birds that are endemic to Costa Rica RE = Regional Endemic Birds that are endemic to Costa Rica and Panama M/Res = Migrant/Resident Birds that have both resident and migrant populations BM = Breeding Migrant Birds that migrate to Costa Rica to breed * International Union for the Conservation of Nature (IUCN) red list species are listed and highlighted in red.