36o DISCOVERY REPORTS 



plumage between the dark form and the white are too varied to be classed all together 

 as intermediate as Matthews has done. 



The true plumage in the dark phase is a rich, very dark brown, which appears almost 

 black in some lights. Lowe and Kinnear are of the opinion that "brown" birds are 

 merely faded specimens of the true dark phase and with this I am in entire agreement. 

 A large series of observations was made during the first two voyages of the ' Discovery 

 II ', and it was repeatedly noted that very dark birds were always in excellent plumage. 

 "All-brown" birds were always drab brown or greyish brown, with a dingy, faded ap- 

 pearance, and in no case was such a bird in good trim. The plumage was invariably to 

 some degree ragged and often there were feathers missing. Probably dark phase birds 

 only remain in good condition for a few months after the moult, after which fading 

 and weathering take place. The same fading occurs in several brown Southern Ocean 

 birds, notably in the case of Thalassoica antarctica. After the moult the brown 

 plumage of these birds is a rich chocolate colour, but later it acquires a drab and faded 

 appearance. 



A correlation of colour phases with latitude over the South Atlantic Ocean, at any 

 rate in the oceanic range of the species, is clearly indicated by the observations of the 

 ' Discovery II '. North of about latitude 52° S no white birds were seen, and the great 

 majority were dark. Between this latitude and the Antarctic circle, the percentage of 

 white birds increases progressively with at first dark and then light intermediate forms 

 predominating. In summer it would not be exaggerating to say that every degree of 

 latitude southward from the northerly limit of range of the species shows collectively a 

 progressive lightening in the plumage of the birds. In winter there is likely to be little 

 material alteration, for the latitude range of the birds is merely moved bodily northward 

 some five degrees. Observations in winter over the South Indian and South Pacific 

 Oceans did not give such definite results, although more light-plumaged birds were seen 

 in the southerly part of the range. In the Falklands, dark and intermediate phases are 

 equally numerous, the majority of the latter being dark intermediates. 



Of the South Orkney birds 10 per cent were white, about 75 per cent were light inter- 

 mediates, and the remaining 15 per cent dark intermediates. A considerable number 

 of the light intermediates were very light, being mainly white with light grey-brown 

 specklings. 



In view of Eagle Clarke's account, it seems probable that in the South Orkneys a 

 white race is evolving. Mr R. A. Falla is engaged on an account of the birds observed 

 on Sir Douglas Mawson's 1929-30 Expedition, and it will be interesting to note his 

 conclusions with regard to the Giant Petrels of Kerguelen and Heard Island. 



An explanation of the increase in white and light intermediate birds in the far south 

 is suggested by the development of southern whaling during the last twenty-five years. 

 Where whaling is in progress food is abundant and easily obtained, and it is probable 

 that this has brought about an increase in the Giant Petrel population of high southern 

 latitudes. Since colour is correlated with latitude the increase will be mainly among 

 white and intermediate birds, with the result that higher percentages of these forms are 



