Posted by Joseph Montes de Oca on January 01, 1970 at 00:00:00
This came up recently on the Florida Birding and Rarities page. Someone put up a similar picture of an anhinga and the consensus was that it was caused by iron staining. The poster thought it was "Erythrism, a genetic mutation which causes rufous coloring." But others argued against that since that apparently would affect the entire bird and not only part of the bird as pictured here and on the facebook anhinga as well.