http://www.webexhibits.org/causesofcolor/17B.html Birds Birds www.webexhibits.org/causesofcolor/17B.html http://www.webexhibits.org/causesofcolor/17B.html http://www.webexhibits.org/causesofcolor/17B.html http://www.webexhibits.org/causesofcolor/17B.html http://www.webexhibits.org/causesofcolor/17B.html discernible only if you have ultraviolet-sensing cones