1920] Swarth: Revision of Avian Genus Passerella 175 



this elevated region south of the Kings River Caiion the present writer 

 found stephensi in abundance in the late summer, evidently on its 

 breeding ground. This is assumed to be the northernmost point 

 reached by this subspecies in the Sierra Nevada, though no collecting 

 has as yet been done on the high country immediately north of the 

 Kings River Caiion, This gorge evidently acts as a barrier to the 

 continuous distribution of Passerella in this direction. From the head 

 of the Cafion (Transition Zone) steep slopes arise, carrying an 

 extremely limited belt of Canadian Zone (where we saw no Passerella) 

 and leading abruptly to the extensive strip of Hudsonian and Alpine- 

 Arctic along the crest of the Sierras. These latter belts are not 

 occupied by Passerella, hence they act as a barrier between colonies 

 on the east and west slopes of the mountains. 



On the east side, in Kearsarge Pass, niariposae is abundant in 

 summer, the southernmost point at which the subspecies has been 

 found breeding. This station on the east side of the mountains is 

 directly opposite that point on the west side where stephensi occurs 

 at its northern limit, the two being separated by the intervening strip 

 of Alpine-Arctic and Hudsonian. Thus mariposae occurs consider- 

 ably farther south on the east slope of the Sierra Nevada than on the 

 west slope. There are one or two specimens of Passerella at hand from 

 Mammoth, Mono County, on the east slope and close to the habitat of 

 monoensis. These appear to be referable to mariposae, and indicate 

 the probable extent of territory occupied by this subspecies on the 

 east slope of the Sierras, a territory cut off from the western colony 

 for its entire length by the Alpine-Arctic and Hudsonian zone of the 

 summit. It is difficult to understand why this isolated division should 

 not be of the monoensis type. There is no obvious barrier between it 

 and that subspecies, and reasoning a priori one would expect the 

 features of the latter race to be developed over this entire eastern 

 exposure where Passerella occurs. Such is most emphatically not the 

 case at the southern limits of mariposae, however, Kearsarge Pass 

 specimens being unmistakably similar to mariposae of the Yosemite 

 region, and impossible to confuse with their nearer neighbors, 

 stephensi on the one hand and m,onoe7isis on the other. 



Farther north, in Eldorado County, there is what appears to be 

 intergradation toward monoe')isis. Birds from the western part of this 

 county are typical mariposae, but in the series from the immediate 

 vicinity of Lake Tahoe and Mt. Tallac there are some individuals quite 

 as small-billed as selected examples of monoensis. So, too, as regards 



