At Sea Distribution of Spectacled Eiders: A 120-Year-Old Mystery Resolved

ABSTRACT.-The at-sea distribution of the threatened Spectacled Eider (Somateria fischeri) has
remained largely undocumented. We identified migration corridors, staging and molting areas, and
wintering areas of adult Spectacled Eiders using implanted satellite transmitters in birds from
each of the three extant breeding grounds (North Slope and Yukon-Kuskokwim Delta in Alaska and
arctic Russia). Based on transmitter locations, we conducted aerial sur­ veys to provide visual
confirmation of eider flocks and to estimate numbers of birds. We identified two principal
molting and staging areas off coastal Alaska (Ledyard Bay and east­ ern Norton Sound) and two off
coastal Russia (Mechigmenskiy Bay on the eastern Chukotka Peninsula, and the area between the
Indigirka and Kolyma deltas in the Republic of Sakha). We estimated that >10,000 birds molt and
stage in monospecific flocks at Mechigmenskiy and Ledyard bays, and several thousand molt and
stage in eastern Norton Sound. We further identified eastern Norton Sound as the principal molting
and staging area for females nest­ ing on the Yukon-Kuskokwim Delta, and Ledyard Bay and
Mechigmenskiy Bay as the prin­ cipal molting and staging areas for females nesting on the North
Slope. Males marked at all three breeding grounds molt and stage in Mechigmenskiy Bay, Ledyard
Bay, and the Indi­ girka-Kolyma delta region. Males from the Yukon-Kuskokwim Delta molt and
stage mainly at Mechigmenskiy Bay. Equal numbers of males from the North Slope molt and stage
at all three areas, and most males from arctic Russia molt and stage at the Indigirka-Kolyma
delta region. Postbreeding migration corridors were offshore in the Bering, Chukchi, and Beaufort
seas. In winter, eiders were in the Bering Sea south of St. Lawrence Island. Our estimates
from surveys in late winter and early spring suggest that at least 333,000 birds winter in
single-species flocks in the pack ice in the Bering Sea. Received 14 August 1998, accepted
10 February 1999.