WESTERN BLUEBIED 617 



and 9, 1915, In Yosemite Valley the species appears re^larly during 

 October. It was seen, for example, first on the 7th in 1914, on the 15th 

 in 1915, and on the 23d in 1920. In 1917 Mr. Joseph Mailliard (MS) 

 saw Western Bluebirds in the Valley on September 17 and 27. The birds 

 continue in the Valley, especially in the mistletoe-laden golden oaks along 

 the sunny north wall (pi. 16a) through November and into December. 

 In 1914, they were present even as late as December 28. How much longer 

 they may remain is not known, but on a visit to the Valley at the end of 

 February, in 1916, none was noted. The attraction for these birds at these 

 higher altitudes is the abundant supply of food in the form of mistletoe 

 berries. This food supply, rather than weather, short of extremely severe 

 storms, seems to be the factor regulating the stay of the bluebirds in the 

 mountains. That snow alone is no particular deterrent to the birds' stay 

 is shown by our observations made on the stormy morning of December 10, 

 1914, at Mirror Lake, when bluebirds were flying about actively, now and 

 again alighting on the snow-weighted mistletoe clumps. Masses of the 

 snow would be dislodged and shower the observer beneath, but the birds 

 themselves seemed in nowise discommoded. 



Like other species such as the Golden-crowned Sparrow and Alaska 

 Hermit Thrush which come into the higher zones in the fall, the Western 

 Bluebirds, when once driven out of the higher mountains, do not return 

 again until the following autumn. It has not been possible to ascertain 

 whether this invasion of the mountains is the result of arrival of winter 

 visitants from the Northwest (Oregon, Washington, and British Colum- 

 bia) or due to temporary expansion of range by birds reared in the nearby 

 foothills. The appearance of bluebirds in the higher country here agrees 

 with the time given for departure of the species from the northwestern 

 states. Furthermore, both adults and young are to be found among the 

 birds occurring in Yosemite Valley. 



With foothill species such as the wren-tit, bush-tit, and Bewick wren, 

 which invade the higher mountains, this movement occurs in late summer, 

 in July or early August, and is indulged in chiefly if not exclusively by 

 birds-of-the-year. On the other hand, the bluebird is like the robin and 

 varied thrush in that its autumnal movements about the country are gov- 

 erned by food supply, and this food supply is of a nature (berries) which 

 fluctuates in quantity from year to year and place to place. The reason 

 that the bluebirds appear with regularity in Yosemite Valley during 

 October might well be in the fact that the crop of mistletoe berries there 

 normally begins to ripen abotit that time. In such case the birds might 

 come, in part at least, from the adjacent Upper Sonoran districts, return- 

 ing there when the food source has been exhausted. 



