Chickadee songs were recorded in 1997–2000 at 185 sites in 15 Illinois counties to map the distributions of song types of Black-capped (Poecile atricapillus) and Carolina chickadees (P. carolinensis). These species are parapatrically distributed from New Jersey to Kansas. Hybrid zones form in areas of sympatry and chickadees in these areas frequently exhibit unusual (i.e., aberrant) singing behaviors. The presence of aberrant vocalizations and/or the co-occurrence of the two species' song types at a site or by a single individual were considered evidence of a hybrid zone. Four hybrid zones were detected in Illinois along the range interface of these species. A comparison of the distributions of Black-capped, Carolina, and aberrant song types from this and a previous study conducted in Illinois in 1954–59 revealed little change has occurred in the distributions of these two species' song types. This overall stability of song type distributions indicates the northward movement of hybrid zones seen at other locations has probably not occurred in Illinois.