WESTERN WARBLING VIREO. Vireosylva gilva swainsoni (Baird)

Field characters.—Two-thirds size of Junco; tail shorter than body. Plumage grayish green with no highly contrasted markings; a light line over eye; no light bars on wing. (See pl. 50b). Movements slow as compared with warblers; keeps usually within crown foliage of trees. Voice: Song of male a sustained and voluble warble uttered at short intervals; both sexes give a throaty or burred call note, szhee or zree.

Occurrence.—Summer visitant in Upper Sonoran, Transition, and Canadian zones on both sides of Sierra Nevada; commoner on west slope. Observed from Pleasant Valley and near Lagrange eastward to Porcupine Flat and Merced Lake; also at Walker Lake and Mono Lake Post Office. Frequents deciduous trees chiefly, most often near streams, foraging from 10 to 60 feet above ground. Solitary except when pairs are caring for broods.

The Western Warbling Vireo is the most widely distributed and the commonest of the four species of vireo occurring in the Yosemite section. While usually found in deciduous trees and in the general vicinity of streams it is at times observed well away from water and is occasionally to be seen or heard high in tall coniferous trees.

Vireos as a group are birds of deliberate mien. When an individual is discovered, as often happens, in the same tree with some one of the 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 (pl. 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 individuals 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 is 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.

During the spring and early summer months the Western Warbling Vireo is, within its range, one of the principal contributors to the early morning chorus of bird voices. At El Portal on May 31, 1915, one of us rated it as fourth among the various contestants, being exceeded in loudness by only the Western Tanager, Pacific Black-headed Grosbeak, and Western House Wren. The song is a voluble rolling warble sustained for several seconds at a time and repeated at very short intervals. If is more varied and slightly more slowly timed than the roll of the Purple Finch and among all the bird songs of almost continuous production it is, to our way of thinking, most pleasing. Often, in the heat of midday, when, for one reason or another, most other species are stilled, the warbling vireo continues its melodious song with little or no indication of lagging. Indeed, it is a warm weather bird, often being silent in the cool of morning or evening, and singing less on cloudy or foggy days than on those marked by bright sunshine. It is a well known trait of the male of this bird to sing while he is taking a turn in the duty of incubation on the nest. The song season lasts from the time the birds first arrive in the region until about mid-July. A male was heard in broken song in Yosemite Valley on July 23, 1915. The call note of the species is a burred zree or szhee. This note may be repeated over and over again in a very insistent tone, in case a jay has entered the nesting precincts of the vireo.

In a shady spot among some pine trees on the north side of Yosemite Valley, a nest of the Western Warbling Vireo was found on June 17, 1915. It was located 4-1/2 feet above the ground at the forking of two almost leafless branches of a coffee berry bush. The nest was, as usual, strapped to and slung within the crotch between the diverging branches. The cup was about 3 inches in outside diameter at the top and about 6-1/2 inches from rim to rim around the bottom. One of the parent vireos was sitting on the nest, and the color of its back blended well with the gray bark of the bush and the gray nest material, but its bright black-appearing eye was conspicuous. The bird did not flush until the observer was within four feet of the nest. Two of the four eggs in this nest hatched on June 22 and the others were hatched by the 24th. By July 7 this brood had left the nest. Another nest of this species was discovered in a young black oak. It was about 12 feet above the ground and 3 feet out from the trunk. Like the other nest, it was composed of light gray bark fibers and weed stems together with some white egg-cases of spiders. There were 4 tiny young in this nest on June 25 (1915). Upon our visiting the place again on July 7 the then fully feathered young took wing and left the nest as the observer climbed the tree. Some few broods are evidently brought off at later dates, as a family group was seen near Merced Lake on August 23, 1915.