Um, that quoted bit is a much later corruption of "Babes in the Wood" (originally, there was one robin, he spread strawberry leaves, and he whistled plaintively; he was the branches among, they didn't throng), so your quote only illustrates broad, relatively modern usage. That said, I'm sure "carol" in the sense of birdsong or joyful singing also goes back quite a ways, probably even before "carol" became applied to the more lugubrious Christmas hymns.