Sorry this is so late, but it's been a long day in the field.  Thanks to Bob and Paul for posting info throughout the day.  
 
First, after I sent out my description last night, Marshall Iliff sent me a list of things that were wrong for Brown-chested Martin.  The one mark that was good for Brown-chested was the brown upperparts.  Well, that mark was obviously incorrect as the bird was very dark gray/black.  I have no interest in concocting a two-bird theory ;)
 
Second, we obtained a few poor photos from a long distance.  I *believe* that they will conclusively eliminate Purple Martin from the discussion.  The observed lack of pale collar and forehead should be enough, but I'd like the photographic evidence.  I am not prepared at this time to go any farther than eliminating Brown-chested.  
 
Today the bird was solely seen in the area around the Rum Pointe Golf Course Club House on South Point.  This is the golf course that is visible south of the Assateague Visitor Center.  There is a large field that separates the two.  Yesterday, there were 2000+ swallows swirling over that field and making trips to and from Assateague Island.  Today there may have been 200 swallows total all day.  There were some trickling south past the visitor center between 7:30 and 9am.  More seemed to be hanging out down by the golf course so we headed down there.  Within about 20 minutes we found the bird perched on the GPS antenna of the club house.  Since photographs were top priority I did not spend as much time observing the bird as I would have liked today.  Once the bird left the antenna (which it did before we were anywhere close) it was only observed very high in the air.  It would mill around with the Tree Swallows then drift behind the trees to the south.  It twice returned, but when it left the third time, we never saw it (or many swallows) again.  
 
I would recommend that martin searchers watch the field from either club house or the area behind the visitor center and see where the most swallows are congregated.  Both days the bird was seen between 9:30 and 10:30.
 
Good luck and get photos!!!
 
Matt Hafner
Bel Air, MD 
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