"at a study site in Pennsylvania *all birds sing Black-capped songs*, and
about 60% of those same birds also sing Carolina songs, *even though
genetic tests indicate all of these birds are hybrids and Carolinas*
(Curry). Thus, contrary to many published reports, song is of little value
for identification within the contact zone, since a young chickadee there
has the opportunity to learn both songs, or to incorporate elements of both
songs into a “hybrid” song."
See the rest of the article and refs to see that the chickadees learn each
other's song in the border regions so in Maryland, I don't think one can
rely on songs.