SEA-BIRD MOVEMENTS I43 



From the British Isles the arctic tern is totally absent from Decem- 

 ber (usually November) to March, while it is engaged on this great 

 southward visit. When on passage through Britain it moves usually 

 by coastal routes; and some Baltic and Frisian birds (by ringing 

 records) join up with the passage on the coast of eastern England. 

 In some years arctic terns join the marked inland passage of common 

 terns along the English river- valleys. But beyond Britain most of the 

 passage becomes oceanic, and the records in inland Europe and the 

 eastern Mediterranean are very scanty. 



The four skuas, which in the winter are generally parasitic on other 

 sea-birds, also (like the arctic tern) make long journeys which involve 

 entering and crossing the open oceans. To understand their migrations 

 it is necessary to consider also their breeding-distribution in some 

 detail. The largest of them, the bonxie or great skua, Catharacta skua, 

 is one of the few birds with a bipolar distribution. We think it likely 

 that this interesting species (or superspecies) originated in the north, 

 and colonised the south where it evolved into a bird of more gull-like 

 form and habits than the other skuas; and that the presence of a 

 breeding-outpost in the North Atlantic is possibly quite a recent 

 development, and probably derived from the main subantarctic and 

 antarctic population. 



Most authorities consider all the bonxies to be of one species; 

 but it is possible, as Murphy and others point out, that two of the 

 southern forms, hitherto classed as the subspecies C. skua maccormicki 

 and C skua lonnbergi, may breed on the same islands in the South 

 Shetlands, in which case they should be regarded as separate species, 

 even if they are overlapping end-members of a subspecies chain (see 

 p. 39). If the world's bonxies do not all belong to one species, they 

 certainly form one superspecies; and the differences between the 

 northern C. skua skua and the Falkland Islands and Chilean races of 

 the antarctic skua, C. skua antarctica, and C, skua chilensis are not great. 

 The distribution of the bonxies, on their breeding-grounds and at sea, 

 is shown in Fig. 22 (p. 136). It will be seen that southern skuas cross 

 the equator in the Pacific, and nearly reach it in the Indian Ocean: 

 those which travel up the eastern shore of the Pacific as far as the 

 United States and even British Columbia are C. skua chilensis; one of 

 the great skuas taken in Japan has been positively identified as the 

 south polar skua C. skua maccormicki — not of the race lonnbergi which 

 breeds south of New Zealand and thus much nearer Japan. This 

 form {lonnbergi) has not so far actually been found in the Pacific north 



