Bill Ellis wrote:

>On Saturday afternoon, I heard the song of a Black-capped
>Chickadee twice, and the call of a Black-capped Chickadee once,
>in my neighbor's yard.  There were no mockingbirds in the area,
>nor any active birders.  This is only an interesting observation,
>since I believe that chickadees in the zone of overlap between
>Carolina and Black-capped are known to exchange vocalizations, so
>heard-only birds may not be reliably identified.  I did not see
>the bird(s).
>
>Bill Ellis
>Eldersburg, MD
>
>  
>
I have heard that in the overlap zone either species may learn to 
sing either or both songs, but calls are genetic and therefore 
more reliable.  Of course if you get a hybrid individual all bets 
are off, but that applies to visual marks as well.  Unfortuantely 
I don't have a direct reference for the information about calls, 
but the call has seemed reliable to me on birds that I have 
identified both visually and vocally.

-- 
Maurice Barnhill
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Department of Physics and Astronomy
University of Delaware
Newark, DE 19716