Over the last 20 years, the wintering distribution of the Rook Corvus frugilegus in Japan has been expanding. Records of Rook observations have been documented showing the expanding front line of their range. However, the process and causes for the range expansion of the Rook in Japan have not been discussed in the literature. A questionnaire survey as well as a literature study was conducted to collect information on their wintering ranges in Japan. Questionnaires were sent by email, inserted with a magazine delivery, and posted on a web site. The literature study was conducted by reviewing published scientific articles, bird check-lists for each prefecture, newsletters published by branches of the Wild Bird Society of Japan and other bird watching groups.
The results provided a time-line of the first sightings of Rooks in 19 prefectures. This information helped outline the process of how the Rooks wintering range has expanded in Japan. The wintering distribution of Rooks contracted during the late 1970's on the island of Kyushu, Yamaguchi and Shimane Prefecture. The wintering range of the Rook began expansion in the mid 1980s, and had extended considerably eastward along the Sea of Japan by the late 1980s. During the 1990s Rooks spread southwards from northern Japan as well, and by December 2006 they were confirmed to winter throughout Japan, except for in Tokyo.
The decrease of snow cover in Eastern and Northern Japan is probably responsible for the range expansion of Rooks wintering in those regions. However, there is not a strong correlation between annual snow coverage and the range expansion of Rooks throughout the remainder of the range. Some changes in farmland management may have allowed the species to spread its wintering range in Japan. The increase of Rooks breeding on the continent is likely have contributed to the expansion of the wintering range in Japan. Rooks were exposed to high levels of chemical compounds, such as organochlorines and organomercury in the past. As these pollutants have been restricted, the populations and ranges of Rooks have expanded in the Russian Far East. Therefore, the increase of the Rook population throughout East Asia due to decreased organochlorines and organomercury compounds is likely a key factor in winter range expansion of Rooks in Japan.