Geographical Variation of the Plumage Polymorphism in the Eastern Reef Heron (Egretta sacra)

I investigated the distribution of white and dark morphs of the Eastern Reef Heron (Egretta sacra) on several coasts and islands in central to southwestern Japan, eastern Australia, and French Polynesia. Both morphs preferred white beaches to dark beaches. Every local population contained some dark herons, however, the range of the white morph was restricted. The white morph population increased in proportion to the distance from the equator in Japan. It was plentiful in the Great Barrier Reef in Australia and was common in Polynesian islands. The distribution of the white morph coincides with the presence of coral reefs, from which white beaches are derived. Only the dark morph was seen outside this range. None of several hypotheses proposed so far explains the observed polymorphism and its geographic variation.