12 



G. J. DIVOKY 



ter, 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, Coruus 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 exten- 

 sive 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 consoli- 

 dated 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 im- 

 mediately south of the large islands in the 

 northern Bering Sea. They are formed by the 

 southward movement of ice caused by the pre- 

 vailing 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/m 3 per h). Surface waters di- 

 rectly under the pack ice have much lower pro- 



duction (0.29 mg C/m 3 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/m 3 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 quan- 

 tity of phytoplankton is available to higher 

 levels of the marine food chain. 



The winter distribution of birds in the 

 Bering Sea correlates well with the findings 

 on primary productivity. Densities south of 

 the ice and the continental shelf average less 

 than 10 birds/km 2 . At the ice front during one 

 cruise in March, densities exceeded 500 

 birds/km 2 . Densities at the ice front increase 

 from south to north; they drop in the region 

 where the ice front grades into more consoli- 

 dated pack ice, and are less than 0.1 bird/km 2 

 in the consolidated pack. 



The most numerous species at the ice front 

 are common and thick-billed murres, which 

 constitute more than 90% of all birds seen. 

 Irving et al. (1970) were the first to report on 

 the large number of murres at the ice front. 

 Feeding flocks of 25,000 individuals have 

 been observed at the front, in which densities 

 were as high as 10,000 birds/km 2 . No other 

 diving species is common at the ice front. The 

 parakeet auklet (Cyclorhynchus psittaculus) 

 is seen on most cruises, but only during a 

 small percentage of observation periods and 

 always in low numbers. Black guillemots are 

 common north of the ice front and stragglers 

 are occasionally seen at the front. Pigeon 

 guillemots (Cepphus columbus), least auklets 

 (Aethiapusilla), and crested auklets (A. crista- 

 tella) are irregular visitors to the front. 



Surface feeding species commonly found at 

 the ice front include the northern fulmar (Ful- 

 marus glacialis) and five species of gulls. The 

 fulmar is common south of the ice and is 

 found only in the southern portion of the 

 front. Three species of Larus are found at the 

 ice front. The most common is the glaucous- 

 winged gull (Larus glaucescens); the glau- 

 cous gull is less frequently seen. The slaty- 

 backed gull (L. schistisagus), a species that 

 breeds in Asia, is most common west of St. 

 Matthew Island (McRoy et al. 1971). The 

 black-legged kittiwake (Rissa tridactyla) is 



