The Dawson Leuco 



Taken in Mono County 



Photo by the Author 



MAMMOTH CREST, A NESTING HAUNT OF THE DAWSON LEUCO 



tephrocotis tephrocotis Swainson, of the northern Rocky Mountain region, in British 

 America and western Alaska, general coloration in all plumages grayer toned, less 

 intensely brown, size slightly less, the bill being distinctly less in bulk, and wing averag- 

 ing more rounded; juvenal plumage much grayer, especially anteriorly, both above 

 and below; breeding females less different, breeding males least different, but still 

 perceptibly less vivid in the chestnut about the head." 



Nesting. — Nest: Placed in sheltered niche of mountain cliff or under boulder 

 of rock-slide; a thick-walled, tidy structure, compacted of moss or dried grasses, or 

 weathered vegetable fiber; lined indifferently with finer grass and occasional feathers; 

 diameter outside, 6 or 7 inches (mm 152-177); inside 2.50-2.75 (mm 63.5-69.8); depth 

 outside 3.00 (mm 76.2); inside 1. 50-1. 75 (mm 38.1-44.5). Eggs: 4 or 5, elongate 

 ovate, pure white. Av. of 10 specimens: 22.5 x 15.6 (.88 x .61). Season: June 

 (cliff nesters), July (moraine nesters); one brood. 



Range of Leucosticte tephrocotis. — Western North America, breeding in the 

 higher mountains from the Alaska Peninsula south to the southern Sierras, and in 

 winter deploying over plains east to Saskatchewan (casually to Minnesota) and south 

 to Nevada and Colorado. Eastern limits of breeding range in the United States 

 imperfectly made out. 



Range of L. t. dawsoni. — At least the higher portions of the central and south- 

 ern Sierras from Nevada County south to Olancha Peak; also sparingly about the 

 higher peaks of the White Mountains; retires in winter to lower levels, chiefly easterly. 

 Northward extension imperfectly made out. Examples seen by Vernon Bailey Aug. 

 17 (1898) on Mt. Shasta (N. A. Fauna, no. 16, "Shasta Report," p. 124) are likely 

 to have been transitional; not seen on Shasta in 1916. 



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