Question regarding black sludge in birdbath

During the migration of Robins in early November, I had a strange occurance that I am hoping someone can identify. I have two heated birdbaths and as the last of the Robins were coming through I would get up to four or five at a time bathing in the baths. (not unusual for me during migration). They tend to knock alot of water from the baths, but I just refill. However, over a couple of days in addition to the Robins several Starlings were coming as well. This is when the "black sludge" began appearing in the bottom of my baths. What happened was that groups of both the Starlings and Robins would be bathing and when I would check later (sometimes as little as 1/2 hour) most the water would be gone and there the bath would be filled on the bottom with a chunky black sludge. It smelled like tar and some pieces were about 1/2 inch long. I didn't notice that the birds were particularly dirty, but I didn't get that close. It wasn't bird poopies either. Does anyone have any idea what it was?

Unfortunately, I didn't think to take a picture of it.

maybe it really was tar? i don't have a clue how they could have tar on them and have it wash off in plain water but i'll be watching this thread with interest.

Or maybe just some really dirdy birdys. Good thing you had a bath for them...