Dan Small, unable to post to the list, asked that I post this for him:
 

At 0930 this morning I had a Barn Swallow and what I thought was a Cave Swallow feeding over the Chestertown Sewage Lagoons. Viewing was from a great distance, but the Barn was easily identified (long wings, deep fork and dark shiny blue back). The other bird seemed totally different (shorter wings, i couldn't make out a fork, the back was not dark shiny blue, the chin was light and the forehead was dark). This bird also appeared smaller and had a different feeding style, it frequently landed on the small ice patches while the Barn never landed. Unfortunately I was unable to make out the rump color. 
 
The other possibility is that it was a immature Barn swallow with a really short tail. I can't remember the molt patterns for barns at the moment to know whether they molt before they reach winter grounds. If they do then I would think this bird is definitely a Cave.
 
Dan Small
Chestertown, MD