The Atlantic Forest in eastern South America harbors 849 bird species, of which 216 are endemic and 122 threatened with extinction. It is also one of the 25 biodiversity hotspots recognized throughout the world. Within this biome, the Serra do Mar stands out as an important area of endemism in South America. Nonetheless, ornithological knowledge of the Serra do Mar remains underestimated and incomplete. One lacunae of information for the Serra is a region called Curucutu, which is adjacent to the largest urban area in South America. The avifauna of Curucutu has been sampled occasionally since 1900, but all of the available data have yet to be published. Therefore, we compiled ornithological data published on Serra do Mar over the last 118 years and undertook a 16-year-long field inventory using 3 methods of data collection simultaneously (visual observations, point counts, and mist nets) for a total of 395 field days. Sampling was performed in forest and natural grasslands along an elevational gradient from 5 to 850 m.a.s.l. A total of 422 species of birds, 128 of which are endemic to the Atlantic Forest and 29 that are threatened with extinction, were documented, thereby illustrating the importance of this region. Of this total, 382 species occur in Núcleo Curucutu do Parque Estadual da Serra do Mar, demonstrating the importance of this reserve to the regional avifauna. We made several significant additions to the avifauna of the region, with only one species, the Black-necked Aracari (Pteroglossus aracari), being considered regionally extinct because of a lack of records since 1900—thus representing a kind of poorly studied extinction that has yet to be investigated and may be happening with much greater frequency in large forested areas of Serra do Mar. Even though the region is partially protected, being a state reserve, its proximity to the largest urban area in South America automatically puts the area under threat, especially considering the disorganized urban growth that has been encroaching upon natural areas of the region. The expansion of current reserves, and the establishment of new protected areas in this region of Serra do Mar is essential for guaranteeing the integrity of this very rich and threatened bird community of the Atlantic Forest.