When I came in to Wild Bird Rescue this morning, Bob told me he had heard a bird singing in the mulberry tree and couldn't ID the bird.
This afternoon, I went out to feed the Mississippi Kites in the outdoor aviary and saw a little warbler in the mulberry, so I stopped to watch in hopes that I could see it well enough for long enough to figure out what it was. Lo and behold, it was a northern waterthrush, bobbing his tail.
Although this is in their migratory pathway, they are not often seen, so I consider myself very lucky. Whether this is Bob's songster, I can't say, but they do have a pretty song, so it's possible.