Two Forms of Bush Warbler Cettia diphone Occur on Okinawajima Island: Re-evaluation of C.d. riukiuensis and C.d. restricta by Multivariate Analyses.

It has long been believed that the Ryukyu Bush Warbler Cettia diphone riukiuensis (Kuroda, 1925) is resident and the only subspecies distributed in Ryukyu Archipelago, Japan. However, we found that Bush Warblers on Okinawajima occur in two distinct plumage forms. One form, the brown form, is characterized by having deep rusty brown upper parts, while the other, gray form, has grayish olive upper parts. Our investigations, using mist-nets to trap birds clarified that the brown form is resident and that the gray form winters on the Island. Univariate analysis of 13 measurements taken of the birds, revealed significant morphological differences of the two forms. Likewise, multivariate analysis differentiated the brown form from the gray form. The type series of C. d. riukiuensis (14 specimens, already lost) were identified as being of the gray form by using a linear discriminant function obtained from 4 measurements from the two forms. The measurements and plumage characteristics in the original description of C. d. riukiuensis conform with the gray form. These results indicate that the gray form should be ascribed to C. d. riukiuensis. It has become clear that C. d. riukiuensis is a wintering bird, not a resident on Okinawajima. The measurements and plumage characteristics of the brown form conform with the original description of the Borodino Bush Warbler C. d. restricta (Kuroda, 1923). The type series of C. d. restricta (2 specimens, already lost) were identified as being the same as the brown form by using linear discriminant function. These results indicate that the brown form should be attributed to C. d. restricta. C. d. restricta has been regarded as an extinct, endemic subspecies that occurred only on Minami-Daitojima Island. Our results suggest, however, that C. d. restricta is not extinct and that it inhabits on Okinawajima.