SHORT-TAILED MOUNTAIN CHICKADEE Penthestes gambeli abbreviatus Grinnell

Field characters.—About two-thirds size of Junco. Top and back of head, and whole chin and throat, black; side of head below eye and short stripe above eye, white; rest of body chiefly plain gray. (See pl. 10d). Manner active and alert; when foraging, often hangs inverted from smaller twigs. Voice: Commonest call a wheezy rendering of the syllables chick-a-dee-dee; this often shortened to simply chee-chee-chee; also an alarm note, tsick-a. Song a clearly whistled tee-tee, too-too.

Occurrence.—Common resident in Canadian and Hudsonian zones, less numerous in Transition; present on both slopes of Sierra Nevada. Recorded from Smith Creek, 6 miles east of Coulterville, and from Feliciana Mountain eastward across mountains to Williams Butte and Mono Craters. Lives in trees, mostly conifers, foraging up to 50 feet above the ground. In pairs at nesting time; flocking loosely at other seasons, sometimes in company with other small birds.

The Short-tailed Mountain Chickadee inhabits the whole of the main forested portion of the Yosemite region, beginning at the lower margin of the Transition Zone and ranging up to the highest unstunted trees, at 10,000 feet or higher. It occurs on both slopes of the mountains, down on the west to Feliciana Mountain, and to Smith Creek, east of Coulterville, and on the east slope to Williams Butte and Mono Craters. Within this range the species seems to be strictly resident. We found no indication of an up-mountain migration in summer, nor were chickadees noted in winter at any station in the western foothill country. The birds are as fixed in this respect as the Plain Titmouse is in its range.

During the winter season individual chickadees are not closely restricted as to immediate neighborhood, the birds being then associated in bands which rove through the woods for considerable distances in their daily search for food. But in summer the forage range of individuals is restricted to a small area which has for its center the site chosen for, or occupied by, the nest. Chickadees forage at various heights in the trees, sometimes at 6 feet or less from the ground, again at 50 feet or even higher. On several occasions chickadees have been seen to descend to the surface of the ground and to forage there for a time; but the birds then hop about with seeming awkwardness, as if out of their proper niche.

The chickadee population is greatest in the Canadian Zone. Only a few of the birds are found in the Transition Zone and but moderate numbers occur in the Hudsonian. During June, 1915, before the young were abroad, our censuses in Indian Cañon and at Porcupine Flat gave between 2 and 3 of these birds per hour of observation. A 4-hour census in Yosemite Valley on May 31, 1915, revealed only 2 birds, and the same number were noted during a similar trip there on April 28, 1916. In the Hudsonian Zone, between Young Lake and Tuolumne Meadows, 8 were recorded in 4 hours on July 9, 1915. But by the end of July young were out everywhere and "the woods were full of chickadees." No less than 36 were counted in 3-1/2 hours by one of us going from Tenaya Lake to Mirror Lake on July 31, 1915.

Mountain Chickadees are sociable creatures and spend most of their time in small flocks. The individuals comprising these flocks are continually calling, with the probable purpose of keeping within hearing distance of one another. The company travels along intact, yet spread out through adjacent trees, so that the individuals do not cover each other's ground in their search for food. Apprehension of danger by one is at once communicated to all members of the company, so that each may be instantly on the alert. In the spring, by the end of April if not earlier, these bands break up and pairs are formed as a first step in the nesting program. When the young are fledged enough to leave the nest the family, both parents and young, goes forth and remains as a group at least until time for the fall molt. In early autumn flocks numbering 10 to a dozen individuals are formed, and in this manner the chickadees spend the time until the impulse comes to prepare for nesting once more.

In Yosemite Valley on April 30, 1916, a pair of Mountain Chickadees was followed for about twenty minutes as the birds flew from tree to tree gleaning insect food of various sorts. Black oaks seemed to be preferred and the birds would explore one tree fairly well before going on to another. Terminal twigs, newly opened foliage and moss-covered trunks seemed to be inspected with equal interest. If the upper side of a twig did not yield food the bird would hang inverted to scrutinize the lower surface, seeming to be quite as much at ease in this reversed position. Most interesting was the way in which the chickadees explored cavities. A bird would disappear into some hollow left by the rotting out of a stub, scan the interior for a few seconds, come to the opening and look about for possible danger, then disappear inside again. It would continue this performance until the cavity had been examined throughout. Occasionally one of the chickadees would drop into a manzanita bush, and twice the birds alighted on the ground for a short while.

The chickadee is ceaselessly active, in large part from necessity, we suppose, for a bird must cover much territory each day to get the requisite amount of daily bread. The birds, although intent in their search, are alert for anything out of the ordinary. They have a large bump of curiosity, and any unusual sight or sound will bring them to the point of interest in a hurry. If a person will sit down at the foot of a fir tree, well screened by the canopy of drooping boughs, and 'screep' with lips to back of hand, he will soon have, literally at arm's length, a number of chickadees. The birds at first are excited and call frequently as they hop and flutter from branch to branch. Then, after some moments of intent peering, they move away quietly, and resume their usual occupation.

The Mountain Chickadee shows considerable range in vocal powers, hardly to be guessed on first acquaintance. The usual call note is the one popularly rendered as chck-a-dee-dee, but with an asthmatic, wheezy intonation; often it is reduced to simply chee-chee-chee. Sudden surprise is evinced by an explosive tsick'-a, repeated several times. On sharp mornings when the birds congregate on the sunny sides of the columnar fir trees, they often indulge in a chorus of cheerful notes, each group of sounds a jumble of sibilants impossible to syllabify and interspersed with the ordinary calls. In addition there is what we consider the real song, a strikingly clear, far-carrying, whistled, tee-tee, too-too. The two pairs of notes are on different pitches, higher and lower, respectively. Frequently the second note of the first pair is omitted and the remaining note prolonged: tee; too-too. The song is given in winter as well as in spring and summer. It is one of the easiest remembered of Sierran voices, and one that a person can readily imitate by whistling.

In the spring months the chickadees, then foraging in pairs, will be seen to indulge in some of the features of courting behavior exhibited by other birds. At times one of a pair (probably the female) will quiver her wings, whereupon the other will feed her, at the same time holding his tail slightly spread. Now and then a bird which has been calling will be seen while perched on a twig to pound with his bill on the part of the wood between his feet, producing an intermittent tattoo audible for some distance. This possible exhibition of 'spring vigor,' recalls the much louder, rolling tattoo which the flicker and certain other woodpeckers are wont to beat in the spring.

Nesting activities among the Mountain Chickadees are commenced in late April or early May. After the pairs separate off from the winter flocks, the two mates go about in close companionship and begin inspection of old woodpecker holes and of natural cavities either in living trees or in dead and weathered stubs. The nest holes of the White-headed Woodpecker abound in a good part of the chickadee's range, and it is to this bird more than to any other one agency that the chickadee is indebted for suitable nesting places. The White-head digs a new nest hole for itself each season, and as the cavities persist for several years in condition suitable for use by chickadees the latter may even have opportunity for exercising choice. Indeed the chickadees do investigate many holes before settling upon one as the location for the nest of the season. The manner of the birds' work in gathering material was not observed. By the time the nest is ready for eggs there has been accumulated a felted mass of soft material some five to six inches below the entrance. This material consists chiefly of hair. In some nests the soft hair of chipmunks is the principal material, while in others the coarser hair of the Mule Deer predominates. In one nest examined at Porcupine Flat there was hair from meadow mouse, California Ground Squirrel, and chipmunk. In another nest we found down feathers of some bird; and in still another, bones of small mammals were mixed with the hair in such a manner as to suggest that the chickadees had chanced upon some owl pellets and used parts of these in building.

The material composing the nest is closely compacted so as to make a thick felt carpet, and as such, must be of considerable value in conserving warmth when the female chickadee begins to incubate her relatively large clutch of eggs. In some instances this 'carpet' rests directly upon the layer of small chips which served as flooring for the woodpecker tenants of an earlier year. But if the hole is unusually deep, grass and similar coarse material is piled in first so that the nest proper will not be too far below the entrance.

The number of eggs varies, 5 and 8 being the extremes for the complete sets found by us; probably 7 is the most usual number. During the period of incubation, which lasts about 2 weeks, the female occupies herself almost or quite exclusively in covering the eggs. The male, on the other hand, continues to forage actively, and may be seen to visit the nest from time to time, supplying food to his mate. Confirmation of this division of labor is found upon examining in hand representatives of the two sexes at this season. The breast and abdomen of the female are bare, the skin rather rough-surfaced and thickened in the manner characteristic of incubating birds generally; while the male lacks any indication of such special modifications. In the first part of the incubatory period the female may flush from the nest when disturbed; but when the eggs are about ready to hatch she will often remain upon them, even if the nest be chopped open. She will then make a hissing sound and whir her wings in a remonstrant manner from time to time whenever the disturbance becomes acute. In one instance the female, fighting valiantly, maintained her place until finally we picked her off from the eggs. In decided contrast is the behavior of the male; he may come around at the beginning of the disturbance but never exhibits much concern, and may wander away again after a little while. After the young are hatched, both parents share equally in the feeding of the brood.

NESTS OF SHORT-TAILED MOUNTAIN CHICKADEE IN THE YOSEMITE REGION

A most striking departure from the usual location for a chickadee's nest was come upon in Yosemite Valley on June 23, 1920. A pair of the birds had placed their nest far back in a crevice in the stone retaining wall of the north approach to the new Sentinel Bridge. The entrance was 5 feet above ground, yet at the same time, 2 feet below the level of the elevated roadway at that place. The young could be heard calling whenever a parent bird brought food. The rumbling of heavy vehicles overhead did not seem to disturb the birds in the least. Choice of such an unique site may have been prompted by the birds failing to find a nest location of the usual sort in the neighborhood—a condition due to the removal of dead trees by the Park authorities, and therefore, in final analysis, one that was man-wrought.

After the nesting season the chickadees and several others of the smaller birds are wont to associate with one another in flocks of varying size. Such a gathering was seen in Yosemite Valley on July 30, 1915. Included in the openly formed yet coherent aggregation were the following species: Mountain Chickadee, Black-throated Gray Warbler, Western Chipping Sparrow, Sierra Creeper, Warbling Vireo, and Cassin Vireo. The birds were foraging through black oaks, incense cedars, and young yellow pines, each kind of bird of course adhering to its own particular niche and own method of getting food.