WABBLING VIEEO 509 



wood warblers, there is little likelihood of the two being confused. The 

 vireos are more sluggish of movement and never hunt over the trees with 

 the nervous, zigzag movements so characteristic of the warblers. 



Each of the four vireos of the Yosemite section offers good clues for 

 field identification (pi. 50) by both coloration and voice, and in general 

 the species may be separated on the basis of local distribution as well. 

 The Western Warbling Vireo, as compared with the other vireos, exhibits 

 a white stripe over the eye, and no light bars on the wing. Its song is a 

 voluble rolling warble, and is of more nearly continuous production than 

 that of almost any other bird to be heard in the region. The Cassin Vireo 

 is of slightly larger size than the warbling vireo, it has a white circlet 

 around the eye, two light bars on the wing, and a more clearly white under 

 surface, while its song consists of bars of alternately rising and falling 

 inflection, separated by rests. In some places, these two species of vireo 

 inhabit much the same sort of territory, yet the warbling vireo usually 

 shows preference for the deciduous growths along streams, while the Cassin 

 is more inclined to frequent the incense cedars and golden oaks in drier 

 situations. The Hutton Vireo is slightly smaller in size than the Western 

 Warbling Vireo and is decidedly more greenish in tone of color than any 

 of the other three species. It has, by way of contrast, a partial ring of 

 buffy white around the eye and two bars of light color across the wing. 

 The niche of this species is in oak trees of which the evergreen live and 

 golden oaks seem to be preferred. The California Least Vireo, as its 

 name implies, is smaller than any of the preceding species. In general 

 tone of coloration it is light grayish and when seen in spring and summer 

 it lacks contrasted markings of any sort. The song is set in character, and 

 rapidly delivered, with first a rising, then a falling inflection. The bird 

 keeps low in the dense thickets which margin the water courses in the 

 San Joaquin Valley. 



The Western Warbling Vireo probably arrives in the Yosemite region 

 during April, although we have no exact data on this point. It was well 

 established at El Portal on April 27, 1916, and in Yosemite Valley on 

 April 28 the same year. It continues in the region until the end of 

 summer. Several were seen at Merced Lake on August 23, 1915, and single 

 indi\'iduals were noted at Walker Lake on September 10 and 14, 1915. 

 A single bird was noted in Yosemite Valley on September 5, 1920 (C. W. 

 Michael, MS). The greatest numbers are to be found in the vicinity of 

 streams in the Transition Zone where three or four will ordinarily be noted 

 in an hour of observation. Above and below this zone the population ?s 

 somewhat sparser. East of the mountains the species is represented in 

 small numbers. It was seen there on only a few occasions in spring, at 

 Mono Lake Post Office and near Walker Lake. 



