Just to add to some comments made by others.

Black-and-White Warblers are on their breeding territories in May, June and early July.
The latest egg date in Maryland is June 19.
The latest egg date in Virginia is June 20.
As their breeding cycle is completed and the territories break down, the birds begin a period of “post breeding dispersal”, prior to fall migration.  I suspect this “post breeding dispersal” is poorly understood in many species; and made much more difficult if the dispersal occurs within the normal breeding range of the species, as it may generally be with the Black-and-White Warbler.

At some point you may have several activities all taking place concurrently:  late breeders trying to finish up, early (or failed) breeders beginning post breeding dispersal movements, and some early migration.  Without banding individuals it is hard to accurately place a bird in this complex and daily changing life-cycle environment. 

Black-and-White Warblers are clearly early migrants based on banding records.  In the banding that was done at Kent Point, MD in the ’60.  Even a minor cold front in early August would produce 20-25 Black-and-White Warblers.  However, the assumption is that these migrants originated 200-300 miles north of Maryland (one nights flight).  The earliest migrant date in the “Yellow Book” (1996) is July 24.  But, the real “earliest migrant date” could easily be earlier than July 24, because of the difficulty of identifying true migrants.

David Bridge