Nature made the bluebird she wished to propitiate both the sky and the earth, so she gave him the color of the one on his back and the hue of the other on his breast, and ordained that his appearance in spring should denote that the strife and war between these two elements was at an end.

The Writings of John Burroughs, Chapter VII (p. 205), Houghton Mifflin & Co. 1904