WESTERN GNATCATCHER. Polioptila caerulea obscura Ridgway

Field characters.—Size small, near that of Kinglet but form slenderer; length about 4-1/4 inches; tail long, equal to body; bill slender, nearly as long as head. Top of head and back bluish gray; tail black centrally, with easily seen margin of white; under surface of body plain grayish white. Males have a narrow black band across forehead. Flight wavering, seemingly indecisive; when perched, tail is persistently 'wig-wagged.' Voice: Weak; a series of high-pitched wheezy notes, zeu, zee, zree, or cheu, chee, chree, three to six of such notes in a series; call notes of both sexes a flat chee-y, of similar quality.

Occurrence.—Common summer visitant throughout Upper Sonoran Zone on west slope of Sierra Nevada. Observed at Lagrange and Pleasant Valley and thence east to El Portal and to 6 miles east of Coulterville. Small numbers wander to higher zones after the nesting season; for instance, noted by us at 6300 feet altitude near Glacier Point, August 17, 1915, and at 10,300 feet on ridge near Fletcher Creek, September 4, 1915. Recorded in Yosemite Valley August 22 to September 10, 1920 (C. W. Michael, MS), and on September 25, 1917 (Mailliard, 1918, p. 19). Seen once east of the Sierras, at Williams Butte, September 21, 1915. Forages and nests chiefly in blue oaks, but also in digger pines and greasewood brush. In pairs or family parties, but never in larger assemblages as in the case of the Bush-tit.

The Western Gnatcatcher is a very small bird, of active temperament, which, by habits as well as structure, betrays relationship with the very differently colored kinglets. But instead of living in the mountain coniferous forests inhabited by the latter birds, the gnatcatcher, during the summer season, lives in the warm dry foothills which intervene between the heated San Joaquin Valley and the cooler upper altitudes of the mountains. The traveler who enters the Yosemite region along any of the roads from the west, upon arriving at the frontier outposts of blue oaks which are scattered over the first of the foothills far in advance of the digger pines and chaparral, will almost immediately be apprized of the gnatcatcher's presence there by hearing the fine wheezy notes of the bird. From these outlying foothill oaks on to the margin of the yellow pine belt, at the lower edge of the Transition Zone, the gnatcatcher is, during the summer months, one of the most abundant of birds.

The Western Gnatcatcher is migratory in the Yosemite region, arriving in numbers probably in April and departing in late fall. It was already present at El Portal on April 27 (1916). By early May the birds are busy with nesting duties, and as early as May 30 (1915) young have been noted out of the nest. After the breeding season, a few gnatcatchers wander up into the higher zones, as instanced above, but the bulk of the population remains in the foothills until the time of departure in the fall. None was seen in early December and late February at either Pleasant Valley or El Portal.

During the early spring immediately after their arrival from the south, the gnatcatchers are to be seen in pairs, the male in close attendance upon the female. When the latter engages in the work of nest construction her mate remains in the vicinity, part of the time accompanying her on trips for building material or on foraging sorties. Otherwise he guards the nesting precincts against invasion of any rival male. All the while, in the heat of mid-afternoon as well as at other hours of the day, the male gnatcatcher utters his fine wheezy song at frequent intervals, and the female answers from time to time in similar tone of voice with single notes.

When settled for nesting each pair of gnatcatchers is strongly localized. Each keeps within a radius of not more than a hundred yards from the nest tree. This localization permits an observer to take a more accurate census of nesting pairs than is possible with many other birds. At Black's Creek, near Coulterville, our own counts led to an estimate of 64 breeding pairs of the Western Gnatcatcher to each square mile in that immediate district. Carrying these figures farther, in consideration of the estimated area of the Upper Sonoran Zone included in our Yosemite section, we find a total gnatcatcher population just before the appearance of the new broods, to consist of 50,000 individuals.

On the hills near Lagrange in early May, 1919, we found the Western Gnatcatchers busily engaged in building their nests in the blue oaks. One nest, complete but without eggs, found on May 8, may be regarded as typical for the species. It was situated about 10 feet above the ground near the periphery of the tree, amid small twigs and branchlets, and rested directly on a horizontal branch about 40 millimeters in diameter. Outside, it measured 80 millimeters in diameter and 45 millimeters in height; the cavity inside was 35 millimeters across—just the diameter to admit snugly the body of the brooding bird.

The nests are of deep cup-shape, and are constructed throughout of light-weight materials. A framework of fine grass stems forms the main wall, and this is covered both inside and out with softer substances. The outside is felted with lichens such as abound on the bark of blue oaks, with a few grass seed hulls, some small oak leaves, and occasionally a feather or two, the whole being held together with spider web. The inside of the nest is lined almost entirely with feathers, laid flat-ways of the inner surface. Whatever the purpose of the bird in constructing such a nest, the form and outside appearance are usually such that the structure might easily be mistaken for a weathered stub or a small accumulation of debris.

Other new nests were seen at Blacks Creek, west of Coulterville, on May 10 and 11, 1919. Some of these were on branches in situations similar to that of the one just described; others were in crotches of small blue oaks, and several were found in greasewood (chamisal) bushes, at a height of not more than 3 feet from the ground. The construction and dimensions were, in all of these nests, practically the same as those of the one described in detail above. Only the height above the ground varied.

One of the nests found at the latter locality on May 10 contained 5 eggs which the female had already begun to incubate. She was on the nest but snuggled so far down into the cavity that, seen from the ground, only her bill and tail, both held at steep angle, were visible above the rim. A casual glance would have passed the whole by as being an aggregation of twigs and leaves in the crotch of the tree. The bird's only act, when the observer climbed the slender tree in which the nest was placed, was to crouch even lower, as if she were endeavoring to escape detection. This sitting bird permitted herself to be touched on the back before quitting the nest.

A late instance of nesting was noted at Dudley, 6 miles east of Coulterville, on July 12, 1920. This was definitely accounted for by the fact that the first nest and its contents had been destroyed. The nest of date cited contained 3 eggs, one-third incubated. It was situated 17 feet above the ground on a crooked, dead and pendant branch of a large live black oak.

The eggs have a ground color of delicate green, and on this are many small rounded dots of reddish brown. The eggs usually number 4 or 5 to the set.

When foraging the Western Gnatcatcher is quick and seemingly nervous in its movements, constantly twitching about, so that, whether intentionally or not, the conspicuously marked black and white tail keeps the bird easily in the eye of the observer. Its temperament is quite the opposite of that of the vireos and some of the wood warblers, which act with a decided air of deliberation. The gnatcatcher has not, however, the spasmodic flutter of the wings which characterizes the Ruby- and Golden-crowned kinglets. The flight of the Western Gnatcatcher is wavering and indirect, and carries the bird rather slowly across the short intervals which it is accustomed to traverse between trees.