I drove to a friend's house in MD the other day, and as soon as I got out of the car, I heard what seemed to be a Worm-eating Warbler song. This seemed rather strange to me, since I was in a residential neighborhood. I decided to play a Worm-eating Warbler song to see what was going on. As soon as I did, a Chipping Sparrow flew over my head and landed in a tree behind me. Well, that makes a bit more sense. But this Chipping Sparrow's song had a faster trill that I've come to identify with the Worm-eating Warbler. So what's interesting to me is that while this bird fooled me into thinking it was a warbler, I also fooled it, since it thought my warbler song was the song of a Chipping Sparrow. If you're interested in how these birds sound, Sibley has an excellent article about the similarity of their songs and how to distinguish them. Here are some photos of the Chipping Sparrow I found.