CALIFORNIA LEAST VIREO. Vireo belli pusillus Coues

Field characters.—Less than half bulk of Junco or Linnet. Plumage appearing light gray above, whitish beneath; a single inconspicuous light bar on wing (pl. 50d). Movements quicker than those of other vireos but less nervous than those of warblers or kinglets. Voice: Song of male, a rapidly uttered series of three or more warbling notes ending in a short questioning note, a pause, then the series repeated with downward inflection.

Occurrence.—Summer visitant locally in Lower Sonoran Zone. Common at Snelling and below Lagrange; one pair found at Pleasant Valley May 22 and 23, 1915. Keeps low (6 feet or less from ground) in willow and other thickets along streams and sloughs. In pairs or solitary.

The California Least Vireo dwells in the dense thickets of willows and other plants which grow along the lower courses of the Merced and Tuolumne rivers, chiefly west of the foothills. In size, coloration, voice, and habits it is well set off from the other three species of vireos in the Yosemite section, so that chance of confusion with them is slight.

The Least Vireo is decidedly smaller than any of the other species of vireos it is less fluffy in appearance than the Hutton Vireo, the next in point of weight. Its grayish tone of coloration above, single wing bar (pl. 50d) scarcely discernible at a distance, and whitish under surface, taken together distinguish it from its relatives.

Near Snelling, on May 29, 1915, eight Least Vireos were observed within a stretch of about 300 yards in the low growths beside the Merced River. In an approximately similar extent of stream-side brush along the Tuolumne River southwest of Lagrange, on May 6, 1919, three pairs of these vireos were found. Each pair had a definite forage beat, within which the two birds could be found at almost any hour of the day.

As regards the niche occupied, the Least Vireo differs from our other vireos in that it chooses a low zone of vegetation in which to search for its food and place its nest. An individual is rarely seen above 6 feet from the ground, and usually it keeps below four feet. Our impression is that all the other vireos habitually forage well above the six-foot level.

A completed nest of this vireo was found near Lagrange on May 8, 1919. (See pl. 51a). It was in deep shade under a thicket of willows and white alders which grew on the lower slope of a pile of gravel left by a gold dredger. The nest was 19 inches above the gravel, and instead of being placed in one of the stout crotches of the adjacent alder it had been lashed to a slender fork on the brittle stem of a weed of the previous season's growth. This was only 7 feet from the margin of a pool of quiet water. In form the nest was a well rounded, deep and rather thin-walled cup with slightly inrolled rim. It was composed of dry shreds of plants felted compactly with down from cottonwoods and willows. Outside, it measured 2 inches in height and 2-1/2 inches in greatest diameter, while the interior was 1-1/2 inches deep at the center and about 1-5/8 inches across the opening.

When first found, this nest was empty, but on the following day, by 6:30 A.M., one egg had been laid. During our second examination of the nest the male came close and sang his song at intervals of 10 or 15 seconds. To one observer the song sounded like this: we-cher, che we, che we-chey? we cher, che we, che we, cheey! Each set of syllables was uttered rapidly, with a distinct rest between the two.

Another pair of these little vireos was watched around one of the other small ponds in the same general locality. The female was foraging and she moved about rather rapidly, occasionally flying upward a short distance to get some particular insect on the leafage. Meanwhile the male traveled along with less frequent change of position, keeping to perches fairly close to his mate, and singing at short intervals. When he was giving the song, his whole body vibrated with the effort, the throat swelling visibly at each syllable, and the tail being depressed at the same time. This song was transcribed on the spot as wretchy, wretchy, wretchy, wretchy, wree? wretchy, wretchy, wretchy, wretcheur, wreer. The r's here indicate a burred or rolling quality; and the whole song was, as usual, hurried in its delivery. The question-and-answer inflection was striking.