ABSTRACTThe semideciduous forest is a prominent yet highly degraded phytophysiognomy within the Atlantic Forest. Historically, bird species inhabiting these forests occurred throughout central and western São Paulo state, south‐eastern Brazil, until the mid‐20th century. Many of these species have not been observed again or are nearing extinction within the state's inland forest fragments. This study reviews and compares historical and recent ornithological records, using museum specimens, literature, citizen science and recent field surveys to understand these species' current lack of records. In the early 20th century, extensive deforestation occurred statewide, resulting in the current forest fragments, which are currently in various stages of regeneration, conservation and isolation. Many of these fragments lack the specific habitats some species require, or they have not been recolonised due to insufficient connectivity with other forest fragments where these species still exist (particularly in the eastern Atlantic Forest). The non‐detection of forest species in semideciduous forest fragments strongly suggests an unprecedented and largely unnoticed extinction scale. This pattern of defaunation, as predicted 30 years ago, may be prevalent across numerous Semideciduous Atlantic Forests in Brazil.