THE SUB-ANTARCTIC FORMS OF 



THE GREAT SKUA 



(CATHARACTA SKUA SKUA) 



By J. E. Hamilton, m.sc. 

 (Plate I; text-fig. i) 



INTRODUCTION 



The component forms of the genus Catharacta may be readily divided into four 

 principal groups which are considered by Lowe and Kinnear (1930) to constitute 

 a single species, Catharacta skua (Briinnich), of which the Great Skua is the type. 

 The four groups are : 



(1) The Great Skua, Catharacta skua skua (Briinnich), which extends over the 

 North Atlantic Islands to north-eastern America. Geographically it is completely 

 isolated from all the other groups and there can be no doubt as to its individuality. 

 There is usually, but not always, a strong chestnut cast in the plumage. 



(2) The Chilian Skua, Catharacta skua chilensis (Bonaparte), occupying the east and 

 west coasts of South America from Rio de Janeiro to Callao. This is the characteristic 

 South American bird, but it extends to the Falkland Islands also. On the extreme eastern 

 side of that group I have seen it mixing freely and breeding with the form which is 

 numbered 4 below. The Chilian Skua is suffused with an even deeper chestnut colour 

 than is the Great Skua. 



(3) McCormick's Skua, Catharacta skua maccormicki ( Saunders), is a strictly Antarctic 

 form distinguished by the entire absence at all stages of chestnut colouring, by the 

 vinous tinge of the lower surface, and usually by the presence of a conspicuous pale 

 collar of acuminate feathers many of which are bright gold in colour. 



(4) The sub-Antarctic Skua, Catharacta skua (subspecies), which may conveniently 

 be termed the Brown Skua. This heading comprehends an assortment of birds which 

 are found on all the sub-Antarctic islands, on the Tristan da Cunha group and on Gough 

 Island. These birds may be described as brown in colour, often having on the neck 

 yellow acuminate feathers which are, however, not so brilliant as the corresponding 

 feathers of McCormick's Skua. Chestnut and pale markings are very frequently present, 

 the former on the dorsal and the latter on both dorsal and ventral surfaces. The presence 

 or absence of the chestnut patches on the feathers is quite fortuitous and sometimes 

 they are not bilaterally symmetrical ; a chestnut mark on one side may correspond to 

 a pale mark on the other, so that it appears probable that the pale marks are due to the 

 absence of chestnut pigment. 



