AUDUBON WARBLER. Dendroica auduboni auduboni (Townsend)

Field characters.—Size two-thirds that of Junco. Rump always yellow (except in young newly out of nest), and tail large-appearing and always with a wide bar of white across it near end (fig. 57b). Chin usually distinctly yellow. Adult male in summer: Top of head, chin, rump, and patch on each side of breast yellow; breast black; upper surface bluish gray streaked with black. (See pl. 9e). Adult female in summer: Top of head, chin, and rump yellow; breast mottled with gray and black; upper surface bluish gray. Adults and immatures in winter: More or less brownish both above and below; little or no black on breast; chin usually (but not always) distinctly yellow, though rump always so. All movements quick and nervous; often flies out from foliage of tree in semicircular course. Voice: Song of male a series of mellow notes, run together rapidly, not loud, and of tinkling quality; call note of both sexes a sharp tsip.

Occurrence.—In summer common visitant to Transition, Canadian, and Hudsonian zones on both slopes of Sierra Nevada (most plentiful in Canadian); recorded from 3 miles east of Coulterville eastward clear across the mountains to Mono Lake Post Office. Remains in Transition through October. Keeps chiefly to coniferous trees, foraging 10 to 50 feet or higher above ground, and nests in same situations. In pairs or solitary. In winter common visitant to Lower and Upper Sonoran zones on west side of mountains, as at Snelling, Lagrange, Pleasant Valley, and El Portal. At this season, forages extensively in outer parts of broad-leafed trees. Solitary, or in scattering companies, often with birds of other species.

The Audubon Warbler is the most widely distributed and the most abundant of all the species of wood warblers found in the Yosemite region. It occurs in numbers throughout the main forested districts of the mountains during the summer season, and it frequents the deciduous trees and brush of the foothill and valley country in the winter time.

Altitudinally its summer range extends from the beginning of the Transition Zone yellow pines on the west slope, at 3300 to 3500 feet, up through the lodgepole pines and other conifers of the Canadian and Hudsonian zones to the upper limit of unstunted trees at 10,000 feet or a little higher. It distribution is uninterrupted from near El Portal and the ridge of hills above Coulterville eastwardly across the mountains, through the Tioga and Mono passes, to Williams Butte and Mono Lake.

During the winter months the birds are entirely gone from the high country. In this season the species occupies most of the hill and valley country lying below the level of regular snowfall. The district from El Portal westward to Snelling and Lagrange is tenanted by numerous Audubon Warblers from October until early April. In Mono Valley east of the mountains there are, in all probability, no Audubon Warblers whatever present during the long season of snow and storm there.

The migratory movements of the Audubon Warbler are but imperfectly known. Birds of this species begin to appear in the lower altitudes in late September or early October, but at this time many are still in the mountains where they continue until late in October. Through September there are droves of Audubons in the trees and brush on the east slope of the Sierras. Doubtless these are mostly birds from stations to the north en route to their wintering grounds in southern California and beyond. The return movement of the species is accomplished in April, and by early May the western foothills are practically cleared of the species. It yet remains to be learned whether the birds which leave the Sierras in the fall go directly westward into the foothills, to remain there for the winter, or whether they move southward and individuals from northern localities migrate into the foothill territory.

As to numbers, in the summer time our censuses show one to two singing birds during an hour of ordinary walking through favorable territory. About 10 were noted in 4 hours on the floor of Yosemite Valley May 31, 1915. Eleven were noted in 5 hours in the vicinity of Porcupine Flat on June 27, 1915. And 3 were recorded in 3-1/2 hours at the head of Lyell Cañon on July 16, 1915. After the young are out, better scores are to be made; 13 were noted in 3-1/2 hours along the Tenaya Trail from Tenaya Lake down to Mirror Lake, July 30, 1915. Fully 50 were seen in 6 hours between Vogelsang Lake and Mono Pass on September 7, 1915. At Gem Lake, on September 13, 35 were noted in 2 hours of intensive hunting, this warbler being then the most plentiful bird there. In 1920 Audubon Warblers were abundant in Yosemite Valley during the first three weeks of October; then a sudden decrease in numbers was noted, and the species disappeared on October 28, save for a solitary individual noted on November 3 (C. W. Michael, MS).

More differences in plumage are shown by the Audubon Warbler according to sex, age, or season, than by any other common bird of the Yosemite avifauna. Upon hatching, the young are sparsely clothed with a grayish white natal down. While still in the nest the juvenal plumage is acquired. In this the body feathers are sharply streaked, while the flight feathers are closely similar in color and markings to those of the adult. In these first two plumages the sexes look alike, but upon the replacement of the body feathering, which takes place in August and results in the 'first winter' plumage, the males and females present differences. The males gain considerable clear yellow on the chin, while in the females the chin is only indistinctly yellow. Both males and females then acquire the yellow rump. Thus the young birds, within about three months, have three distinct plumages. The last of these is retained until the following spring (April) and then another molt of the body feathers brings the bluish gray back and bright yellow patches on sides and crown, of the 'first nuptial' plumage. Not until August or September, however, when the birds are about fifteen months old, is there renewal of the flight feathers of the wing and tail, which were acquired with the juvenal plumage. Such long service usually results in these feathers becoming badly worn, and bleached to a pale brown.

In this same molt, the second or adult fall molt, both sexes show more or brighter yellow on the chin, crown, and sides, and in the males the breast and sides are rather heavily mottled with black. These features make possible the distinguishment of birds fifteen months or more old from those only three months of age. It is thus possible to recognize eight different feather assemblages or 'plumages' in this warbler.

The above brief outline of the molt program of the Audubon Warbler will serve to explain why so much variation is apparent among individuals seen in the field at different seasons and even at the same time.

The Audubon Warbler is considerably larger than any of the other common warblers of the Yosemite region. One species, the Alaska Myrtle Warbler, a sparse winter visitant here, is similar in size and general appearance to the Audubon. (See pl. 9e). It has a yellow rump and white-spotted tail, but its chin is always clear white. In the Audubon Warbler ten of the twelve tail feathers (all but the innermost two) are marked with large white patches near the ends, whereas in the Myrtle Warbler only the three outer feathers on each side are so marked and the spots are smaller. (See fig. 57.) The mass effect of the white on the spread tails in the two species is thus quite different. The Audubon and Myrtle warblers are the only warblers with white-spotted tails, yellow rumps, and dark backs to be found in the region.

A feature common to both of these warblers in comparison with the other warblers of the region, is the relatively great length and breadth of the tail feathers. This may be a special adaptation for the twofold purpose of aiding in the short circuitous flights and in the displaying more conspicuously of the white markings, possibly directive in their function.

Through most of the year the only note heard from the Audubon Warbler is a sharp tsip or chit, but this is given frequently both when the birds are engaged in foraging and when they are in flight. This note is distinctive enough in character to serve as a means of recognition once it has been learned by the observer. In March or April, before the birds depart from the lowlands, the males begin to sing, and after the birds arrive on their nesting grounds the full songs are to be heard regularly and frequently through May and June and, especially at the higher altitudes, even well into July.

The song resembles most nearly that of the Hermit Warbler, but is more mellow and tinkling, and lacks the burred or 'z' tones of the latter's utterance. One phrasing of the Audubon's songs goes si-wi, si-wi, si-wi, sissle, sissle, see-see; another turly, urly, urly, urly, urly, i-ci. These are given much as if the syllables were spoken rapidly and in a whispering voice.

Fig. 57. Tails of (a) Alaska Myrtle Warbler and (b) Audubon Warbler; natural size. Note that in the former the white (clear) areas are present on only six feathers, whereas in the latter species ten feathers bear white. This character, under favorable conditions, may be used in field identification.

During the summer season the Audubon Warbler keeps mainly to coniferous trees, foraging from 10 to 50 feet or more above the ground. In the Transition Zone and part of the Canadian Zone it shares this habitat with the Hermit Warbler, but at higher altitudes it is the only warbler present in the evergreen forests. In this same niche its nesting is carried on. After the breeding season this restriction is broken and the Audubons range widely here and there, wherever food offers. In the dry days of autumn when the wind is shaking down the dead leaves of the deciduous trees the birds spend their time about such trees seeking the flying and other insects then available. When they move into the foothills and valleys for the winter they take to a variety of situations, hunting often in the live oaks, again in shrubbery or in low chaparral, and not infrequently alighting on the grass or ground in pursuit of some terrestrial insect. Few other insectivorous birds show such seasonal diversity in forage grounds.

This warbler forages most especially about the peripheral foliage of the trees. In seeking the sedentary insects lodged on the leaves and in their axils a bird is accustomed to change its post of view by flying out beyond the leafage in a semicircular course toward a new location. This is quite the opposite of the habit of the Lutescent Warbler, for example, which works about within the terminal or crown foliage. The special mode of leaf examination, just alluded to, on the part of the Audubon may be accounted for by the greater proneness of this warbler to indulge in fly-catching. The flight out into the open air puts the bird in position to see and seize with least additional expenditure of energy, passing insects, or insects disturbed from the foliage. This fly-catching habit is often practiced toward evening by several of the birds in near proximity to one another. Then insects are active in numbers about the warm sunlit upper portions of the trees. On these short sorties the bird's wing-beats appear rather weak, and a vacillating manner of flight results. The tail is widely spread, and the patches of white in transverse row near the end show forth plainly. In a word, the Audubon Warbler is the most open-acting, above board, and the least reclusive, of all our wood warblers.

We were not fortunate in finding any nests of the Audubon Warbler in the Yosemite country. But nests elsewhere are known to be placed many feet above the ground on branches well out from the main stem of the tree and so located that they cannot be readily made out from below. Save when the site is disclosed by a female going to the place, carrying material for the nest or food for the young, much time and energy will be expended vainly in hunting for the structure. On May 17, 1919, in Yosemite Valley, a female Audubon Warbler was seen in the top of a small yellow pine gathering dry needles. She moved off toward a group of large trees of the same kind and was soon lost to view. A similar fleeting glimpse was obtained of another bird in Little Yosemite Valley the following day. Bob-tailed young already out of the nest were seen in Yosemite Valley on June 23, 1920. A juvenal bird barely able to fly, and so probably just out of the nest, was seen by one of us at Tuolumne Meadows on July 26, 1915. These dates indicate approximately the extent of the nesting season.

In the fall, and to a less extent during the winter months, Audubon Warblers are given to traveling in small open flocks, either along with their own kind or mixed with bluebirds. Such an aggregation was seen in some black oaks near Camp Curry on October 7, 1914, there being in it about a dozen Audubons in all. The 'location note,' tsip, so frequently uttered by each individual in one of these scattering groups, seems to serve well in helping to hold the flock together in its general onward movement. Over El Capitan Meadows eight were seen on October 24, 1915, with an equal number of Western Bluebirds. The Audubon Warbler is thus more sociably inclined than any other member of its family found regularly in the Yosemite section.

In the fall the Audubons range widely, some keeping to the middle altitudes, others dropping to the lowlands or moving south, while a few may range even above timber line. On September 6, 1915, while one of us was traversing the south slope of Parsons Peak at an altitude of 11,500 feet, a lone Audubon Warbler flew past. Others had been seen in the dwarfed pines near Vogelsang Lake at 10,350 feet altitude a few days earlier. Of all our warblers the Audubon is the hardiest as regards ability to stand the cold and storms of the winter season at the lower levels and the variable weather of the summer season at the upper altitudes.