CALAVERAS WARBLER. Vermivora ruficapilla gutturalis (Ridgway)

Field characters.—Half bulk of Junco. Body coloration yellow beneath, olive green above; head and neck (except throat) gray; eyelids white; male has a chestnut colored crown patch, visible only at close range. No white or black markings whatsoever on wings or tail. (See pl. 9a). Voice: Song of male 4 or 5 rapidly uttered shrill notes followed by 3 or 4 lower ones: tsirp, tsirp, tsirp, tsirp, sup sup sup; call note a tseep, or tsit.

Occurrence.—Common summer visitant to Transition Zone on west slope of Sierra Nevada. Recorded from 3 miles east of Coulterville and from Sequoia eastward to Yosemite Valley and slopes adjacent. Highest stations, at 7400 feet altitude near Mono Meadow and at 6700 feet above Yosemite Falls. Some few individuals wander higher in mountains after nesting (for example, 8000 feet on McClure Fork of Merced River; August 29, 1915). Sings and forages 10 to 70 feet above ground in broad-leaved trees such as black oak and maple, but nests in shaded situations on ground. Solitary.

The Calaveras Warbler is common during the summer months in the black oaks and maples along each side of Yosemite Valley and in similar situations elsewhere on the western flank of the Sierra Nevada. Among all the warblers to be seen in the Yosemite Valley during the summer months the present species is the only one which does not forage and nest in the same niche. The Calaveras seeks its food and does its singing well up in trees, but places its nest immediately upon the ground.

The niche of the Calaveras Warbler is not invaded by any other species of warbler, although other birds of this group may be close around. (See fig. 56.) In the pines and cedars are the Audubon and Hermit warblers, the golden oaks of the talus slopes harbor the Black-throated Gray Warbler, and in the stream-side willows and cottonwoods is the Yellow Warbler; while the tangles of underbrush above moist ground on the Valley floor shelter Tolmie and Golden Pileolated warblers.

During May and June the song of the Calaveras Warbler may be heard frequently, for the males sing at short intervals through most of the day. One bird watched near Columbia Point on June 2, 1915, was singing at intervals of 7 to 12 seconds, each utterance occupying but a second or two. The pitch is high although the notes are not so piercing as those of a Yellow Warbler and the song as a whole suggests that of the Lazuli Bunting. Four or five notes are given sharply and distinctly, then three or four less sharp ones are uttered in more rapid time. Three phrasings of the song written by us in the field are as follows: tsirp, tsirp, tsirp, tsirp, sup sup sup; tsu'-ip, tsu'-ip, tsu'-ip, tsu'-ip, seet-seet-seet-seet; again seit, seit, seit, seit (4 or 5), che-che-che-cha. Sometimes the terminal group of syllables is omitted.

Fig. 56. Diagrammatic cross-section of Yosemite Valley (looking eastward) showing principal vegetational associations and the forage "niches" occupied by the seven species of Warblers which breed in the Valley. The nesting places of some of the species are in different locations from the forage places shown here.

The forage range of this warbler lies chiefly in trees other than conifers. Such trees as the black oak and big-leafed maple renew their foliage every spring and the Calaveras Warblers find excellent forage in the insects and larvae which feed upon this tender new leafage during the spring and summer months. Less often these birds may be found in golden oaks and occasionally in Douglas spruces. They usually forage 25 to 40 feet above the ground, keeping within the stratum of new foliage, but they have been seen as low as 10 feet and as high as 70 feet above the earth. When within the foliage their yellow and green coloration makes it difficult to locate them, especially as the birds do not move about as rapidly as some of the other warblers. At times a Calaveras Warbler will poise on rapidly beating wings to capture some insect otherwise out of reach.

A good view of the male Calaveras Warbler reveals a plainly colored bird, lacking contrasted markings of any sort. (See pl. 9a). The head is clear gray, the throat and lower surface continuously clear yellow, the upper surface olive green. The female differs only in showing less contrast between the dull gray of the head and the olive green of the back. There is lacking in both sexes the brilliant yellow of the Yellow and Pileolated warblers, and there are none of the black and white markings of the Audubon, Hermit, and Black-throated Gray warblers. The Calaveras Warbler bears somewhat of a resemblance to the Tolmie, especially in the immature plumage, but then the difference in habitat and the smaller size of the former are sufficient for distinguishing the two.

A nest of the Calaveras Warbler was discovered in Yosemite Valley near the base of Sentinel Rock on May 26, 1911. The location was only about 75 feet from the much traveled south road on the Valley floor and at the base of the talus pile of huge boulders. The nest was on the face of one of the larger of these boulders, partly in a diagonal fissure. It was on the north side of the rock and so never received any direct rays of sunlight. The whole face of the boulder was covered densely with yellow-green moss which in places was overlaid by olive-gray lichens. The nest was 43 inches from the base of the rock and about 60 inches from the top. The whole vicinity was densely shaded by black oaks and firs and the ground beneath was strewn with dead last year's leaves of the oaks. There were 5 eggs, and incubation was far advanced. When the nest was first discovered, the parent birds acted very shyly, but after a while they began to show much anxiety, coming down as close as 10 feet from the observer who was sitting below the nest. The female was the bolder of the two birds. Their excited tsits attracted other birds for a time, among these being a brilliant male Hermit Warbler, a singing male Golden-crowned Kinglet, and some Ruby-crowned Kinglets and Western Warbling Vireos.

On June 5, 1915, we were shown a nest of the Calaveras Warbler in the vicinity of Smith Creek, east of Coulterville. It was in a hollow of the ground at the base of an azalea bush, near an old road along the hillside. The creek itself was about 50 feet distant. This nest was 3 inches across the outside and about 2 inches high, the cavity being 1-1/4 inches deep. Strips of bark of the incense cedar, plant fibers, and horsehair comprised the building material. When first discovered it had contained five eggs, but prior to our seeing it the nest had been raided and all trace of the eggs was gone. A third nest was discovered near the bridge over Yosemite Creek above Yosemite Falls on July 1, 1915. It was ensconced in a shallow bole in the bank at the side of a well traveled trail. A tuft of grass overhung and nearly concealed the structure. One of the adult birds was flushed from the nest, which, however, contained neither eggs nor young.

Calaveras Warblers continue in the Yosemite region at least throughout August; individuals have been seen along the McClure Fork of the Merced River on August 26 and 29, 1915. The latter is our latest date of noting this species in the region, although Mr. Joseph Mailliard (1918, p. 17) made definite record of an individual in Yosemite Valley as late as September 16 (1917).