Well, you probably know now. If not, it's a baby Turkey Vulture, taken on August 3, 1999. I found out about this bird from a man who called the helpline at work. He had seen this bird and its sibling on a hillside in Attleboro. When he discovered it on July 30, the body plumage was much whiter, giving a striking contrast between the white body and black wings. He hit the books, but was stymied - it was obviously a vulture, but with a white body? He sent me photos, but by that time he had gone back for a better look, and confirmed that the body plumage was down and not adult feathers, and concluded that it was indeed a Turkey Vulture.
When I arrived at the scene, there were originally two young birds, but one scuttered down into a crevice the moment I made eye contact. The second bird was totally unbothered by my presence; it barely even looked at me. I was able to approach relatively close, but hesitated to try to find the nest itself, since the first bird seemed bothered by my presence, and it might feel cornered if I approached the crevice. David (the person who called me) told me later that the bolder sibling appeared to be the older of the two (by size), and that both birds liked to spend the morning in the crevice, and come out for the afternoon to sun on the rocks.
Many thanks to Rick Desmarais for discovering this wonderful nest site, and to David Hammond for being willing to share its location with me.