CASSIN VIREO. Lanivireo solitarius cassini (Xantus)

Field characters.—Three-fourths bulk of Junco; tail shorter than body. Plumage grayish green above, olive gray on head; under surface whitish; eye encircled by white (pl. 50a); two light bars on wing; bill black. Movements deliberate. Voice: Song of male a series of detached notes, now rising, now falling in inflection, quee-up, tseer, etc.; call note a harsh che.

Occurrence.—Common summer visitant to Transition Zone (sparing in lower Canadian) on west slope of Sierra Nevada; recorded in nesting season from 3 miles east of Coulterville and from El Portal, eastward to east fork of Indian Cañon at 7300 feet and to near Merced Lake at 7500 feet altitude. In spring migration passes through lowland and foothill country, as at Snelling, Lagrange, and Pleasant Valley. In fall small numbers wander to higher levels, as along McClure Fork to 8300 feet and along course of Rafferty Creek. Not observed on east slope. Frequents chiefly incense cedars and golden oaks. Solitary except when pairs are caring for broods.

The Cassin Vireo is a summer visitant at middle altitudes along the west flank of the Sierra Nevada. Its distribution at nesting time closely parallels the ranges of the golden oak and incense cedar, though the bird does not restrict itself exclusively to these two trees. In and around Yosemite Valley this species and the Western Warbling Vireo are often to be found together, although the Cassin shows preference for the drier portions of the Valley, for example, near and upon the talus slopes along the north and south walls. During the spring migration the Cassin Vireo is a common transient in the western foothill country where, during its passage, it is to be seen in blue oaks and chaparral on the dry hillsides. In early fall after the young are grown a few of these vireos wander up into the Hudsonian Zone before taking final leave of the country for the winter.

The first of the Cassin Vireos probably arrive in the Yosemite region early in April. On our visit to El Portal on April 27, 1916, the species was already well established there, and the same was found to be true in Yosemite Valley the day following. Near Lagrange, in 1919, Cassin Vireos were passing through the blue oak belt in numbers on May 7, and a few transients were observed near Coulterville on May 9 and 10, while in 1915, migration was still in progress at Bullion Mountain on May 26, at Pleasant Valley on May 23 to 28, and at Snelling on May 27. In 1919, however, nesting was already under way in Yosemite Valley on May 22. It seems likely that the late migrants seen in the foothills in 1915 were bound to some much more northerly station rather than that they were going to swell the number in the Transition Zone of the region immediately to the east. The species continued in evidence through August; single birds were seen as late as September 1 near Echo Creek, September 2 in Yosemite Valley, and September 7 along Rafferty Creek, all in the year 1915. Mr. Joseph Mailliard (1918, p. 19) states that a few were still in Yosemite Valley on September 28, 1917.

The Cassin Vireo is the largest of the four species of vireos in the Yosemite section. (See pl. 50.) In general, the bird gives the impression of having an abnormally large head and short tall, and of being big-eyed, the latter obviously by reason of the conspicuous circlet of white around the eye. In good light the head appears an olive slate, the back greenish, and the under surface ashy white, with a yellowish tinge on the sides.

The movements of this vireo are like those of the Warbling Vireo, but they are even more slow and deliberate. It perches stolidly, and when insects are spied captures them by direct thrusts of the bill. Occasionally a bird will poise on fluttering wings to seize some object not otherwise obtainable. But even then, there is little suggestion of the nervous activity of, for example, the Audubon Warbler.

The Cassin Vireo is a slow but persistent singer; the syllables of its song are set off from one another by long rests. With one bird which was kept under observation for some time these breaks varied from about one to ten seconds. Another, similarly studied, sang at intervals which, by the watch, ranged from one to three seconds. After a long series of these closely spaced notes the latter bird was quiet for ten minutes or more save for two series of five or six notes each. Each note is clear cut and loud so that the song rings out, and may be heard for a considerable distance. Successive notes are variously inflected, some rising, others falling; at times a bird will give a regular alternation of rising and falling inflections. Hence the name "question-and-answer bird" has been suggested for the Cassin Vireo. Some of the notes were syllabified by one of us as tseer', pee'rit, pee'-o-wup, syrup, que'-up, tseer, etc. Another series was written as che'weh, cheweuh', che wer, occasionally wee'cha. The notes suggest the words "to eat? to cheer!" The bird has also a scolding or alarm note che, che, che, and the two members of a pair when together may indulge in low conversational notes. The Cassin Vireo continues in song through much of the summer, one in song being heard in Yosemite Valley on July 23, 1915. There is a revival of song after the molt; on September 2, 1915, one was heard in the Valley giving a song almost as full and persistent as that ordinarily to be heard in the spring.

A nest of the Cassin Vireo was found in Yosemite Valley on May 22, 1919. It was placed in an incense cedar at the edge of the Merced River. The nest was on a branch which extended out over the rushing stream and was about 18 feet above the surface of the water. The nest was a deep cup lashed by the rim to two forking branchlets forming a crotch. The following day another nest, in an early stage of construction, was found near the road along the north side of the Valley. It was 7-1/2 feet above the ground at a fork in an outswaying branch of a young black oak beneath a larger tree of the same kind. The bird came to the nest singing loudly and, while still singing, proceeded to add material around the rim, standing on one of the supporting twigs while it worked. Two automobiles passed unheeded by the bird, which sang again before departing. Two minutes later the vireo came again with material in its bill, sang, added material to the nest, sang, and departed into the golden oaks across the road. The bird seemed not at all inconvenienced by having its bill laden with supplies, and, indeed, this is generally true of our song birds; movement of the bill is not a necessity in singing.