SEA ICE AS A FACTOR IN SEABIRD DISTRIBUTION 



13 



common in open water south of the ice but is 

 also found throughout the entire width of the 

 front. The ivory gull is unique in that it is 

 found only at the ice front in winter. In addi- 

 tion to these species, the fork-tailed storm- 

 petrel (Oceanodroma furcata) is a regular but 

 uncommon visitor to the ice front in winter. 

 Densities of surface feeding species at the ice 

 front are low when compared to the high den- 

 sities of murres, and do not regularly exceed 

 10 birds/km 2 . 



The primary food consumed by birds at the 

 ice front is pollock (Theragra chalco gramma). 

 An amphipod (Parathemisto libellula) and the 

 euphausiids are less important. Examination 

 of the stomach contents of birds and fish 

 show that large feeding flocks are usually as- 

 sociated with schools of pollock feeding on P. 

 libellula and euphausiids. 



The habitat of the consolidated pack in the 

 Bering Sea is markedly different from that at 

 the ice front. Whereas the front is charac- 

 terized by bands of ice interspersed with open 

 water and ice coverage rarely exceeding 4 

 oktas (4/8), the consolidated pack consists pri- 

 marily of large expanses of unbroken ice. 

 Small leads are formed by the shifting of the 

 ice caused by currents and wind. Ice coverage 

 is usually 7 to 8 oktas. The southern part of 

 the consolidated pack, which grades into the 

 ice front, has frequent leads. Most of the 

 species found at the ice front can be found in 

 the southern part of the consolidated pack, 

 but murres are most common. Their numbers 

 decrease, however, in the more northerly 

 pack, where leads are less frequent. Black 

 guillemots, in contrast, increase with increas- 

 ing ice cover, and reach their greatest abun- 

 dance in the small leads constantly forming 

 and refreezing deep within the ice. Because 

 they exploit this habitat, they are dependent 

 on the formation of lead systems. I have often 

 seen leads a quarter mile wide refrozen to the 

 point where new ice covered all but a small 

 patch of open water; black guillemots were 

 frequently crowded into this open water. Be- 

 fore the lead closes completely the guillemots 

 must fly to an open lead. When winds are light 

 and temperatures low, lead systems fail to 

 form as rapidly as usual, and when they do 

 they refreeze quickly, causing a loss of the 

 preferred habitat of wintering black guille- 



mots. A severe winter in the White Sea in 

 1965-66 decreased the amount of open water 

 and caused an increased black guillemot mor- 

 tality (Bianchi and Karpovitsch 1969). On a 

 windless day in March I conducted bird obser- 

 vations in the Bering Sea ice where no leads or 

 open water were encountered. The only bird 

 seen was a black guillemot flying over the ice. 

 In situations such as this, where black guille- 

 mots are prospecting for open water, they 

 may use the "water sky" and steam fog asso- 

 ciated with leads as visual aids. "Water sky" 

 is the reflection of the dark water in the clouds 

 over the lead, and contrasts sharply with the 

 "ice sky." The presence of "water sky" allows 

 birds to detect open water from a distance of 

 many miles. 



Aside from birds found in and near island- 

 associated polynias, only murres and black 

 guillemots are regularly found on the consoli- 

 dated pack ice in winter. 



The polynia associated with islands in the 

 consolidated pack provide refuge(s) for sea- 

 birds. Fay and Cade (1959) found the polynias 

 south of St. Lawrence to be most important to 

 oldsquaws (Clangula hyemalis). King eiders 

 (Somateria spectabilis), common eiders (S. 

 mollisima), and oldsquaws are common in the 

 St. Matthew Island polynias (McRoy et al. 

 1971). Because these polynias are in shallow- 

 water areas, they provide feeding opportu- 

 nities for benthic feeding species. 



Spring 



Chukchi and Beaufort Seas 



In April and May a lead system develops 

 from the Bering Strait north to Cape Lisburne 

 and then northeast to Point Barrow. The lead 

 is a flaw lead that occurs between the shore- 

 fast ice and the free-floating pack. It is a 

 major migration route for a number of species 

 of birds, primarily eiders. East of Point Bar- 

 row in the Beaufort Sea, no similar well-de- 

 fined large lead exists. Consequently, there is 

 a greater chance of bird mortality occurring in 

 the Beaufort Sea than in the Chukchi Sea be- 

 cause the early migrants are unable to find 

 open water. In 1960, 10% of all the king eiders 

 that migrate through the Beaufort Sea died 

 during a late freeze (Barry 1968). Additional 

 records of eider mortality due to late breakup 



