Vol 'l9i9 ] Bent, Variation in Black-throated Loons. 239 



the breeding of the two extreme forms in the same or in contiguous 

 areas; but it would be upset by the discovery of more complete 

 intergradation, unless such intergrades could be regarded as hybrids. 

 A final choice between these two theories cannot be made until 

 more material is available showing the distribution and relation- 

 ships of the forms to be found in Siberia, a vast and little known 

 region. 



I will now attempt to state, roughly and in general terms, the 

 main known facts in this complicated case and let the reader judge 

 for himself how they fit in with the above theories. There are 

 apparently three or four fairly well marked subspecies of Black- 

 throated Loons, as follows: — 



1. Gavia arctica pacifica (Lawrence), the smallest of all, in 

 which the hind neck or nape is much lighter gray than the crown 

 or forehead, nearly white in some cases, the black throat patch 

 terminates below in a straight line and the metallic reflections of 

 this patch almost always appear purplish in any light. This 

 form occupies a breeding area which includes the whole of north- 

 ern North America (which need not be more definitely outlined 

 here), the Arctic Islands west of Greenland and the Arctic coast 

 of Siberia for our unknown distance westward. 



2. Gavia arctica suschkini (Sarudny), intermediate in size 

 between arctica and pacifica, but nearer the latter, in which the 

 colors are nearly as in pacifica, but with a slight tendency towards 

 arctica. This form probably has a breeding range somewhere on 

 the northern coast of Asia, but is known only from specimens taken 

 in winter or on migrations in the Ural and Turkestan regions. 



3. Gavia arctica arctica (Linnaeus), intermediate in size, but 

 nearer viridigularis than pacifica, in which the crown and nape 

 are uniform dark gray, the black throat patch terminates below in 

 a point and the reflections of this patch appear either purplish 

 when held away from the light and greenish when held towards 

 it, or wholly purplish in any light, with considerable individual 

 variation. This form inhabits northern Europe, and northern Asia 

 for an unknown distance eastward and southward in Siberia. 



4. Gavia arctica viridigularis (Dwight), the largest of all, but 

 intergrading perfectly with arctica, in which the crown and nape 

 are colored as in arctica, the black throat patch terminates below 



