The Black-capped and Carolina chickadee’s range only barely intersects, so identification is typically as easy as finding out which side of the line you are on. Unfortunately, for those within a few tens of miles of the line, a confident ID may not be possible. 

The following paper contains a map showing the location of the hybrid zone: 

• https://doi.org/10.1002/ece3.5497 

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The following Cornell Lab | Macaulay Library recordings were used in this episode: 

• Black-capped chickadee song by Matthew D. Medler (ML508153) 
• Black-capped chickadee call by Geoffrey A. Keller (ML508154) 
• Carolina chickadee song and call by Wil Hershberger (ML508127) 

Sources and more: 

• https://www.allaboutbirds.org/guide/Black-capped_Chickadee 
• https://www.audubon.org/field-guide/bird/black-capped-chickadee 
• https://www.allaboutbirds.org/guide/Carolina_Chickadee 
• https://www.audubon.org/field-guide/bird/carolina-chickadee 
• Dobson C, Kassenbaum D, Oehmke D, et al. 2023. Field guide to hotspots and birds in Illinois. Champaign-Urbana: Scissortail LLC. 
• Kroodsma DE, Albano DJ, Houlihan PW, et al. 1995. Song Development by Black-Capped Chickadees (Parus Atricapillus) and Carolina Chickadees (P. Carolinensis). The Auk. 112(1):29–43. https://doi.org/10.2307/4088764. 
• National Audubon Society videoguide to the birds of North America. 2004. Fullscreen. Carrboro, NC: Godfrey-Stadin Productions. 
• Peterson RT, Peterson VM. 1980. A field guide to the birds : a completely new guide to all the birds of eastern and central North America. Fourth edition, completely revised and enlarged. Boston: Houghton Mifflin Company (The Peterson field guide series ; 1). 
• Robbins MB, Braun MJ, Tobey EA. 1986. Morphological and Vocal Variation Across a Contact Zone between the Chickadees Parus Atricapillus and P. Carolinensis. The Auk. 103(4):655–666. https://doi.org/10.1093/auk/103.4.655. 
• Sibley D. 2016. Sibley birds East : field guide to birds of eastern North America. Second edition. New York: Alfred A. Knopf (Field guide to birds of eastern North America).