MOUNTAIN BLUEBIRD. Sialia currucoides (Bechstein)

Field characters.—Size half again that of Junco; wings relatively long, reaching nearly to end of tail. No chestnut color anywhere. Male: Clear light blue above, the same but paler on breast; belly white. Female: Upper surface pale grayish blue; rump and tail clearer blue; under surface pale grayish buff; belly whitish (pl. 56b). Young: Breast mottled. Perches stolidly or else engages in flycatching; often hovers over open ground on rapidly beating wings. Voice: No song heard by us; call note a weak chirp.

Occurrence.—Summer visitant to higher altitudes of Sierra Nevada (chiefly in Hudsonian Zone); also in valleys along east base of mountains. Recorded at Mono Meadow, at Tenaya Lake, on Mount Hoffmann and Mount Clark, and thence eastward to Mono Lake Post Office, near Williams Butte, and Mono Craters. Also winter visitant to San Joaquin Valley, as below Lagrange; transient in Yosemite Valley and at Smith Creek, east of Coulterville. Forages on grasslands; nests in cavities in trees, occasionally about buildings. In pairs at nesting time and in small scattering companies at other seasons.

The Mountain Bluebird, sometimes called Arctic Bluebird, is, during the summer season, chiefly a bird of the Hudsonian Zone. An exception to this statement is found in the occurrence of the species regularly east of the mountains in the neighborhood of Mono Lake, in territory which lies below 7000 feet altitude and is in the arid part of the Transition Zone. The greater part of the population, however, is found at altitudes above 8000 feet and from there it ranges up to the highest meadows found in our mountains short of timber line. The highest points at which we saw the species were near the head of Lyell Cañon, on the slopes of Mount Florence, and on Warren Peak, in each case at close to 10,500 feet altitude. Mountain Bluebirds were observed once (June 21, 1915) at Mono Meadow, which is a few miles southward from Glacier Point and at an altitude of only 7400 feet; but these individuals were probably nesting at some higher locality adjacent to the eastward.

There need be no difficulty in recognizing the Mountain Bluebird in its summer haunts, as there is no other bird of similar size with conspicuously blue coloration in the high mountains or on the east slope. In fall and winter this species and the Western Bluebird sometimes occur on common ground, but then the paleness of the blue and the total lack of chesnut will readily distinguish the Mountain Bluebird. The Mountain Bluebird has a hesitating mode of flight, as with the Western, and the wings appear large and broad in proportion to the size of body. In general behavior the Mountain Bluebird is much like the Western Bluebird. In the matter of forage territory the present species displays greater preference for meadowlands, in large measure avoiding forested areas.

The numbers of these birds present locally at any season of the year are less than the numbers of the Western Bluebird. On Tuolumne Meadows one or two birds per hour of observation was a maximum. On Mount Hoffmann, June 29, 1915, 4 adults were seen near the top of the peak and a few others lower down, scarcity of available forage grounds and nest sites elsewhere in the vicinity having served, perhaps, to concentrate the birds. In the fall months small flocks are formed, 8 having been noted together on one occasion. These loose companies wander about with seeming aimlessness. There is no directness of movement in the fall such as characterizes many migrating birds.

The Mountain Bluebirds had apparently left the territory east of the Sierras before mid-September of 1915; yet they were noted in much higher country on two occasions in the latter part of that same month. On September 26 a flock was observed on the slopes of Warren Peak, and on September 28, 4 were observed in Tioga Pass and a like number on Tuolumne Meadows. Mr. C. W. Michael (MS) reports small numbers in Yosemite Valley on October 30, and November 2, 10, and 13, 1920, the only instances of occurrence we know of there. The species is reported as a transient at Smith Creek, 6 miles east of Coulterville, by Mr. Donald D. McLean, who took a specimen there October 8, 1916. In the winter season Mountain Bluebirds make their appearance in the San Joaquin Valley. Two birds noted along the road below Lagrange on December 16, 1915, constitute our only definite lowland record for the Yosemite section, but there are many reports of the winter occurrence of these birds in numbers on the San Joaquin plains.

The Mountain Bluebird nests in cavities, making use, for the most part, of deserted woodpecker holes. At Tuolumne Meadows one pair was nesting in such a hole, whose orifice was two inches in diameter, and which was situated 7 feet above the ground in a live lodgepole pine. At the Farrington ranch near Williams Butte a Flicker hole in a willow stump was utilized. At Mono Lake Post Office a pair of Mountain Bluebirds had appropriated to their uses a ledge in a woodshed, entrance to which was gained through a hole in the wall. Here at a height of 10 feet from the ground a loosely woven nest had been constructed. This nest was made of shreds of bark many of which showed evidence of having been freshly pulled from the trees for the purpose. There were included also numerous chicken feathers from the nearby farmyard. The dimensions outside were roughly 6 or 7 inches in diameter and 2-1/2 inches in height. The depression for receiving the eggs was 3-1/4 inches wide and 1-1/2 inches deep. After one brood had been reared this nest was re-lined to receive a second set of eggs.

In 1916, nesting activities with the Mountain Bluebirds (pl. 56b) in the neighborhood of Mono Lake were instituted early in May, which makes it readily possible for some at least of the pairs there to rear two broods. Thus the pair observed at Mono Lake Post Office had on May 20, 1916, a nest with 6 fresh eggs. The 5 young reared from this lot were abroad and being cared for by the male on June 30, and by July 3 there was a second set of 5 eggs which the female had commenced to incubate. A pair of birds which arrived at the Farrington ranch about May 1 took possession of an old Flicker hole almost at once. By May 10 the nest lining had been finished and by May 16, 5 eggs had been laid; incubation commenced the next day.

The several Mountain Bluebirds encountered on Mount Hoffmann on June 29 and 30, 1915, fluttered about as though anxious over the security of nests. They probably had eggs or small young in cavities left by the rotting out of branches in the stunted white-barked pines which abounded on the upper slopes of the peak. When we reached Tuolumne Meadows early in July of 1915 the bluebirds were already well started on their nesting program, as by July 8 one nest there had 3 young nearly ready to fly and other pairs were busily engaged in getting food for young. Presumably these high-mountain birds are one-brooded, for we did not see young abroad until well along in July and there would not have been time for them to rear another brood that season.

When the eggs have hatched both parents attend to feeding the young, and if a person approaches a nest at this season the bluebirds will hover in the air before the site and utter remonstrant chirps. After the broods were out, the relative numbers of the species almost reached the status of 'abundant.' The young Mountain Bluebirds have a spotted pattern of markings in the juvenile plumage—a 'family' resemblance to the robins, thrushes, and solitaires. In the bluebird this is due to white center streaks on the feathers of the breast and back. This plumage is worn but a short time, being replaced in early fall by one which is practically identical with that of the adults.

In the nesting season and indeed through most of the year the Mountain Bluebird subsists upon insects. These are captured in two totally different ways, according to the habits of the insects sought. For beetles and others which fly through the air a bluebird will take position on a boulder in a meadow or on the low outswaying branch of some tree and dart after the insects which pass by. For insects which live on the ground, such as grasshoppers, the bird mounts 10 to 20 feet into the air over the grassland and then by fluttering its wings rapidly, hovers in one place for several seconds and intently scans the surface below, like a Sparrow Hawk when similarly engaged. If something is sighted the bird drops quickly to the ground and seizes it; otherwise the bluebird moves a short distance to a new location which is given similar scrutiny. It thus examines the ground in a manner recalling that employed by the robin though from an aerial location where its scope of view is much greater though less thorough. The Mountain Bluebird elsewhere in its range is known to eat berries though not to the extent of its lowland relative; none of the birds which we saw or collected in the Yosemite section gave any indication of having eaten food other than insects.