Jerry,
        Did you see the Black-throated Green Warblers?  I'd be suspicious of 
any out of range warbler this time of year that wasn't seen.  Warblers have 
been know to sing song of other warbler species and this late in the nesting 
season at least some species (e.g., parula) have a variety of vocalizations. 
I use this test in my work at McKee-Beshers WMA in Montgomery for Cerulean 
Warblers.  Unless I see it I don't count it since Northern Parulas give a 
very version of the cerulean song.
       On the other hand, I notice that in the first breeding bird atlas 
black-throated greens were listed in northern Frederick County.

Paul Woodward
Fairfax City, VA