408 BRITISH BIRDS. 



found on open water in winter, consequently the East- Asiatic birds pass 

 through Dauria and the valley of the Amoor on migration to winter in the 

 Japanese seas, whilst the West- Asiatic and North-Russian birds migrate 

 across country to the Black Sea and occasionally wander to the Caspian 

 and Mediterranean Seas. It is distributed in winter throughout the 

 coasts of Western Europe from the North Cape to Portugal, occasionally 

 migrating across country to the Italian lakes and the Mediterranean. 

 The Black-throated Divers breeding in Alaska and the valley of the 

 Mackenzie River vary considerably, some of them having a shorter and 

 more slender bill and a paler crown, nape, and hind neck. The latter 

 are regarded as specifically distinct by Baird, Brewer, and Ridgway, 

 but there can be little doubt that they intergrade and should bear the 

 name of Colymbus arcticus pacificus*. This somewhat anomalous fact 

 may be explained by supposing that during one of the later glacial 

 periods the American Black-throated Divers were isolated and in course 

 of time became slightly differentiated ; but when the areas of distribution 

 again became united by the passing away of the glacial period, we may 

 suppose that the Black-throated Divers of the Old World again crossed 

 Behring Straits and re-inhabited Alaska and the valley of the Mackenzie 

 River, so that now both forms are found in this district. The Pacific form 

 of the Black-throated Diver has occurred in winter as far north as the 

 extremity of Lower California; the typical form winters in Behring Sea; 

 two examples have been obtained on Lake Michigan, and one in the Bay 

 of Fundy. 



The Black-throated Diver differs in a few particulars of its habits from 

 its large ally. It is not quite so exclusively oceanic in the selection of its 

 winter-quarters, occasionally visiting inland lakes; and in the breeding- 

 season it is found at a much greater distance from the ocean, occasionally 

 migrating across country for a thousand miles or more. It arrives in the 

 Arctic regions as soon as the ice breaks up, which on the Arctic circle in 

 Siberia takes place about the 1st of June, but it passes through the Baltic 

 provinces in considerable numbers during the first half of May. It leaves 

 the tundras of the north about the middle of September, and passes through 

 the Baltic provinces on its return journey about a month later. On the 

 ground its motions are quite as awkward as those of its allies ; it is unable 

 to stand upright, and pushes itself along with its belly touching the surface ; 

 but its flight is even more rapid, and its powers of diving almost as 



* The synonymy of the pale-naped form is as follows : 



Colymbus pacificus, Lawr. Baird, B. N. Amer. p. 889 (1858). 



Colymbus arcticus, var. pacificus (Lawr.), Coues, Key N.-Amer. B. p. 335 (1872). 



Urinator pacificus (Later.), Stejn. Proc. U. S. Nat. Mus. v. p. 43 (1882). 



Colymbus arcticus pacificus (Later.), Coues, Key N.-Amer. B. 2nd ed. p. 791 (1884). 



