Long-term monitoring of movements by individually-identified Great Cormorants in Southern Kanto, Japan

Abstract

From 1998 to 2018, in four colonies in the southern Kanto region of Japan, 6,807 Great Cormorants Phalacrocorax carbo were banded with color bands for individual identification. During 22 years between 1998 through June 2019, 6,575 sightings of 1,783 individuals were recorded. The number of color-banded cormorants gradually increased from the beginning to the middle of that period, and then decreased in the latter half of the study period. The number of records reported by bird watchers showed a pattern similar to the number of banded cormorants. However, the decrease in the records in the latter half appeared to have been influenced by the decreasing number of reported individuals. The recorded number of cormorants decreased, reflecting the advanced age classes of the banded population. The banded cormorants were reported mainly in Tokyo, Chiba, and Kanagawa prefectures where the birds were initially captured and released, with some reports from distant locations, including Aomori Prefecture to the north and Shiga Prefecture on the Pacific coast of Honshu Island. Therefore, these prefectures were demonstrated to represent the main range of movement for the population of Southern Kanto area. Results gained by these records indicated a need for establishing a strategy to maintain long-term record keeping by observers.