622 ANIMAL LIFE IN THE ¥0 SEMITE 



impossible — to exterminate the bluebirds. Even were all the locally 

 wintering individuals in the Yosemite region killed off — and that in itself 

 would be extremely difficult to accomplish — subsequent years would witness 

 a gradual infiltration of individuals and reestablishment of a normal popu- 

 lation. Nor would cutting out mistletoe be economical except in the case 

 of individual trees which it might be desirable to save. Interference by 

 man with the 'natural balance,' save where direct and rather complete 

 control is possible locally, as with ground-dwelling rodents, is never pro- 

 ductive of the favorable results which some persons hope for. 



Mountain Bluebird. Sialia currucoides (Bechstein) 



Field characters. — Size half again that of Junco; wings relatively long, reaching 

 nearly to end of tail. No chestnut color anywhere. Male: Clear light blue above, the 

 same but paler on breast; belly white. Female: Upper surface pale grayish blue; rump 

 and tail clearer blue; under surface pale grayish buff; belly whitish (pi. 56&). Young: 

 Breast mottled. Perches stolidly or else engages in fly catching; often hovers over open 

 ground on rapidly beating wings. Voice: No song heard by us; call note a weak chirp. 



Occurrence. — Summer visitant to higher altitudes of Sierra Nevada (chiefly in Hud- 

 sonian Zone) ; also in valleys along east base of mountains. Eecorded at Mono Meadow, 

 at Tenaya Lake, on Mount Hoffmann and Mount Clark, and thence eastward to Mono 

 Lake Post Oflice, near Williams Butte, and Mono Craters. Also winter visitant to San 

 Joaquin Valley, as below Lagrange; transient in Yosemite Valley and at Smith Creek, 

 east of Coulterville. Forages on grasslands; nests in cavities in trees, occasionally about 

 buildings. In pairs at nesting time and in small scattering companies at other seasons. 



The Mountain Bluebird, sometimes called Arctic Bluebird, is, during 

 the summer season, chiefly a bird of the Hudsonian Zone. An exception 

 to this statement is found in the occurrence of the species regularly east 

 of the mountains in the neighborhood of Mono Lake, in territory which 

 lies below 7000 feet altitude and is in the arid part of the Transition Zone. 

 The greater part of the population, however, is found at altitudes above 

 8000 feet and from there it ranges up to the highest meadows found in our 

 mountains short of timber line. The highest points at which we saw the 

 species were near the head of Lyell Caiion, on the slopes of Mount Flor- 

 ence, and on Warren Peak, in each case at close to 10,500 feet altitude. 

 Mountain Bluebirds were observed once (June 21, 1915) at Mono Meadow, 

 which is a few miles southward from Glacier Point and at an altitude of 

 only 7400 feet; but these individuals were probably nesting at some higher 

 locality adjacent to the eastward. 



There need be no difficulty in recognizing the Mountain Bluebird in its 

 summer haunts, as there is no other bird of similar size with conspicuously 

 blue coloration in the high mountains or on the east slope. In fall and 

 winter this species and the Western Bluebird sometimes occur on common 

 ground, but then the paleness of the blue and the total lack of chesnut 



