Our friend Josh Eisen alerted us to the great birdsong.fm, a site that streams only birdsong (you can also download its app or mp3):

Just birdsong. For me the walls and confinement of my apartment fall away and I am in the woods, a jungle, sunlight flooding through the foliage; the air changes and peace descends. It makes my office that summer porch I don’t have. It brings longer hours and summer light and makes them palpable even in the midst of winter. It can be great to meditate with too.

Thanks a million Josh!