Studies of geographic variation aid advanced taxonomy as well as the understanding of microevolutionary forces, speciation, and the nature of species and biodiversity. This emphasizes the need to review the understanding of geographic variation in avian species that was developed primarily from the viewpoint of species classification. The main purpose of this study was to examine geographic variation in the body size of Black-headed Gull Chroicocephalus ridibundus, a monotypic species. Our dataset consisted of measurements of 70 skin specimens of adult Black-headed Gull from museum collections. Morphological measurements of tarsus length, total head length, entire bill length, nalospi length, and bill depth at the gonys were used for statistical analyses. Morphological measurements were compared among breeding areas of the Black-headed Gull across the entire Palearctic region. Those breeding on the Kamchatka Peninsula, Russian Far East, were significantly larger (especially for total head length and nalospi length) than those of other Palearctic breeding populations. A west to east increasing trend in size was also observed. In addition, birds wintering in Japan were revealed to be mainly from the Kamchatka breeding population, based on body size measurements. The recovery records of birds banded in the Russian Far East support this finding.