We studied the dynamics of the colony sites of Grey Herons Ardea cinerea from 1960 to 1999 in Hokkaido, Japan. According to our estimates, 66 colonies existed, 13 of which disappeared. The number of colonies increased rapidly from about 1980 onwards. The rate of increase in the number of colonies per year grew from 0.40 before 1980 to 2.05 after 1980. By contrast, the mean number of nests per colony decreased from 170.5 to 48.4. It was also found that in recent years the proportion of inland colonies has increased. The mean number of nests was 48.8 in inland colonies and 107.0 in coastal colonies. With natural and artificial changes occurring to the environment, new small-scale satellite colonies appear to have been established at inland sites, leaving the old large core colonies in coastal areas. We suggest, therefore, that core colonies should be protected, with the highest priority, for effective conservation of Hokkaido's Grey Heron population.