Hi Dave --

Not sure about your particular bird, but Barry and I had a immature Mourning Warbler about 3 years ago along the Potomac just north of Chain Bridge in late July. We also (at first) could not believe that was the correct ID but we ended up with superb looks and that's what it was! We figured ours was an early migrant or dispersing bird coming down along the river from western MD or VA.

Gail Mackiernan
Colesville, MD