Winter Wren. These guys are very secretive 90% of the time. In the early spring though they start to sing their melodious song.
When they do that you can be sure they are exposed on some branch up around eye level. If you hear this song one can move towards the sound. Stopping just as or before the song ends, about 5 to 10 seconds per song. Only moving closer when the song starts again, about every 5 seconds.
The trick is to figure out what branch the little bugger is singing on.
Because the song, at least for me, does not give away the birds location with any degree of accuracy I've sometimes found myself looking for it in a specific location only to find that it is 5 feet from me and not anywhere near where I was looking when I startle it into hiding.
From Mostly Twigs