From late November to mid-April, ice cover of the Chukchi and Beaufort seas is almost complete. The only areas where birds can be expected to winter in these seas are the chronic lead systems. Such lead systems are found off Wainwright and Point Barrow and south of the Point Hope-Cape Thompson area (Shapiro and Burns 1975). Only the black guillemot (Cepphus grylle) is known to regularly winter offshore from Wainwright and Point Barrow (Gabrielson and Lincoln 1959; Nelson 1969). In the Point Hope-Cape Thompson area, glaucous gulls (Larus hyperboreus), the common murre (Uria aalge), and the thick-billed murre (U. lomvia) occur throughout the winter (Swartz 1967). It is likely that black guillemots are also found in this area.
The lack of chronic lead systems in the Beaufort Sea precludes the presence of wintering seabirds. The one species that may be found wintering in the Beaufort is the Ross' gull (Rhodostethia rosea). Ross' gull is believed to winter primarily in the Arctic Ocean (Bailey 1948). The number of sightings that have been obtained in both the eastern and western Arctic indicate that the species may winter over much of the Arctic Ocean. It may thus be expected to occur in both the Chukchi and Beaufort seas during winter.
Ice cover—not prey abundance—plays the major role in severely limiting bird numbers in the Arctic Ocean in winter. Prey is known to be abundant in parts of the Arctic Ocean during the period of ice cover. In the Chukchi Sea, Eskimos fishing through the ice can catch 23 kg of arctic cod per person per day (D.C. Foote, unpublished data). Eskimos jig for the fish at considerable depths, and the cod do not appear to be as common directly below the ice as they are in summer. The effects of new ice (which forms on the underside of the ice during the winter) on the under-ice fauna are not known. The abundance of amphipods in ice-covered waters in winter is demonstrated by the experience of the Greeley Expedition in the eastern arctic. They discovered that any scrap of food thrown into a lead was quickly consumed by amphipods. Nets were made to catch the amphipods and the availability of this food source played a major part in the survival of the expedition (Schmitt 1965).
Aside from the food found in leads in the ice, the only food available to birds in the Beaufort and Chukchi seas in winter is carrion and the feces of mammals found on the pack ice. The presence of the arctic foxes on the pack ice during the winter demonstrates the availability of scavenging opportunities on the ice. Arctic foxes on the pack ice live on feces and the remains of seals killed by polar bears (Ursus maritimus). Polar bear and seals are both common in the Beaufort and Chukchi seas in winter, but no scavenging seabirds are found there in the winter. It was thought that the ivory gull (Pagaphila eburnea) was associated with marine mammals during the winter, but they are now known to winter at the Bering Sea ice edge, where they feed on fish and crustaceans (Divoky 1976). The only birds associated with polar bear kills in the Chukchi Sea in March are ravens, Corvus corax (T. J. Ely, Jr., personal communication).
Bering Sea
Ice begins to cover the northern Bering Sea in November and reaches its maximum by February, when it usually extends as far south as the edge of the continental shelf, and covers nearly 75% of the surface of the Bering Sea (Lisityn 1969). Coverage can vary greatly from year to year. In certain years Bristol Bay may be completely covered and in others ice is found only in the northern part of the Bay. Almost all ice in the Bering Sea is first-year ice. This ice tends to be flat on the top and underside and in general lacks the extensive keels and pressure ridges found on multi-year ice.
The Bering Sea ice has a number of large-scale features of importance to birds. The "front" is a zone of ice south of the consolidated pack that is composed of small floes, ice pans, and brash ice. This zone is prevented from forming large floes by the action of swells from the open water to the south. The front continually changes in width. When winds are from the south, it is compressed into a narrow band; when winds are from the north, it is a broad zone composed of bands of ice interspersed with open water.
Polynias (areas of open water) are found immediately south of the large islands in the northern Bering Sea. They are formed by the southward movement of ice caused by the prevailing winds. This movement causes ice to be pushed away from the south side of islands, leaving areas of open water. Large polynias are associated with St. Lawrence, St. Matthew, and Nunivak islands and with the south side of the Seward Peninsula (Shapiro and Burns 1975).
The most biologically active area of the Bering Sea in winter is the ice front. Studies of primary productivity in April show that production at the surface in the ice front is high (1.98 mg C/m3 per h). Surface waters directly under the pack ice have much lower production (0.29 mg C/m3 per h), and that in the water south of the ice is lower yet. At this time production within the ice is very high (more than 5 mg C/m3 per h) (McRoy and Goering 1974). Because this phytoplankton bloom is trapped in the ice, it is not available to grazers. Thus, before the spring melt the ice front is the only area where a large quantity of phytoplankton is available to higher levels of the marine food chain.