THE GREAT AND ARCTIC-SKUAS 205 







establish a long graded series of intermediate varieties. 1 On the other 

 hand, these varieties can as a rule be classed as light or dark, so that, 

 subject to the reservation here stated, the term dimorphic is not out 

 of place. The various forms interbreed freely. The nestling young, 

 though said to differ slightly in shade, according as they have dark or 

 light coloured parents, 2 are all of a dusky or sooty brown hue, the char- 

 acteristic colour of the all-dusky adult type. In the fledgling stage the 

 plumage is also of the dusky type, i.e. sooty brown, but Collett made 

 the observation that a marked difference becomes apparent if the 

 feathers are raised and their covered parts exposed, some individuals 

 showing more white, others more brown ; but whether there are 

 variations at this stage corresponding to all the variations of the 

 adult stage has yet to be seen. It may be noted that sooty brown is 

 the characteristic hue of the down and first feathering of the two 

 other species of Stercorarius the Buffon's and Pomatorhine-skuas, 

 and that in the Pomatorhine species there are also dark and light 

 adult forms, with all degrees of intermediate variation. 3 



What makes these unusually wide variations of peculiar interest 

 is that they occur within one and the same species, and are of fairly 

 regular though varying occurrence throughout its breeding range.* 

 They do not correspond therefore to the exceptional and local hybrid 

 forms occurring between different species, such, for example, as those 

 between the hooded and carrion-crows in localities where the breed- 

 ing ranges of these overlap. And, further, they do not correspond 

 to the variations within the species that rank as subspecific, for the 

 latter are local in their nature, and by no means so marked. If, 

 indeed, a local race of a given species, e.g. Corvus corax hispanus, 

 Parus ater hibernicus, differed from another local race or from the 

 typical form of the same species, e.g. Corvus corax corax, Parus ater 



1 Collett quoted in Naumann's Vogel Mitteleuropas, xi. 319 ; E. Selous, Bird Watcher in the 

 Shetlands, pp. 15-20. 

 1 Collett, loc. ait. 



3 According to Kolthoff's observations in N.-W. Greenland, quoted in Naumann's Vogel 

 Mitteleuropas, xi. 313. 



4 The light form of the Arctic-skua is much the commoner in northern latitudes. 



