2009 年 41 巻 1 号 p. 18-33
Since 2002, we have observed gull flocks at Chijiwa Beach in Tachibana Bay, Nagasaki Prefecture, Japan. The gull flocks consist mainly of Black-tailed Gulls Larus crassirostris and Vega Gulls L. vegae. These gulls stay at the beach between October and March. The Vega Gull generally has pinkish legs, but within these flocks some gulls, which look like Vega Gulls, have yellowish legs. Between January 2005 and March 2007 we counted the adult gulls, excluding Black-tailed Gulls and Slaty-backed Gulls L. schistisagus, and calculated the ratio of gulls with yellowish legs in the flocks as being 12.4%. In addition, we attempted to identify these gulls with yellowish legs, using the following characters: body size comparison with Black-tailed Gulls, Vega Gulls or Slaty-backed Gulls; colour of the orbital ring and iris; the dark streaking pattern of head plumage; timing of the moult; mantle colour brightness (Kodak Gray Scale: GS); wing projection (Fig. 1: C); and wingtip pattern. In late January 2006 and 2007, and late February 2007, we selected three gulls and photographed them. Based on the respective combination of the characters listed above we identified one as L. heuglini heuglini, another as L. fuscus graellsii/intermedius, and the third as L. glaucoides kumlieni. Given that gulls with yellowish legs comprise 12.4% of the gulls at the beach, it seems probable that gulls related to the Vega Gull, such as L. heuglini and L. fuscus, more frequently migrate in the winter period than had hitherto been believed.