Flocks of Ross's Gulls Larus roseus were observed along the coast of Shiretoko Peninsula, eastern Hokkaido, between 26 and 29 January 1984. On January 26, 24 adults and 8 juveniles in first winter plumage were seen at Maehama, Shari. They were hovering, circling, and occasionally surfacefeeding at sea 10-20 m off the beach. Sometimes they floated on the water, but did not dive. The flock moved from one place to another after foraging for about 10 minutes. Another flock of 67 Ross's Gulls was observed at the Shari Harbor at the same hour by another observer. Thus at least 100 Ross's Gulls were on the Shari coast on that day. Pack ice situated about 2 km off shore and there were brash ice and small ice cakes on the open water between the pack ice and the shore, the condition which seems to be especially favored by the gulls.
Next day (January 27) the flocks of Ross's Gulls disappeared from the Shari coast as the sea was covered by pack ice and no open water became available. However, several smaller flocks consisting of 5-27 birds were seen along the Shiretoko Peninsula between 27th and 29th. After January 30 the sea was closed by ice and no Ross's Gulls were observed. My observation supports those by DIVOKY (1976, 1981) and DENSLEY(1977) that Ross's Gulls chiefly feed at the edge of pack ice.
