CALIFORNIA YELLOW WARBLER. Dendroica aestiva brewsteri Grinnell

Field characters.—Half size of Junco. Yellow color predominating; no black or white markings whatsoever. Male: Clear yellow beneath (narrowly streaked with chestnut, but this not discernible at a distance); upper surface greenish yellow. (See pl. 9c). Female and young: Pale yellow beneath, unstreaked; upper surface dull greenish yellow. Movements quick and nervous; hops along small branches in zigzag course. Voice: Song of male very high pitched, piercingly shrill, 4 or 5 sharply enunciated notes followed by quick series of shorter ones; call note a sharp tsip.

Occurrence.—Common summer visitant at both bases and on adjacent lower slopes of Sierra Nevada, extending up through the Transition Zone. Recorded from Snelling and near Lagrange eastward to Yosemite Valley; and again about Mono Lake. Chiefly in cottonwoods and willows along streams, foraging up to 40 feet from the ground; nests in same general surroundings, but usually less than 15 feet from ground. Solitary.

Long ago the appellation "summer yellow bird" was given to the Yellow Warbler in recognition of its clear yellow coloration and of the fact that it comes to our latitudes only during the warmer months of the year. The species is well represented in the Yosemite section from late spring until early fall, and is found from Snelling and Lagrange eastward to Yosemite Valley, and again, east of the mountains, near Mono Lake. Everywhere it exhibits a strong preference for deciduous trees near streams.

The California Yellow Warblers which are to nest on the west slope of the Sierra Nevada arrive there some time before those individuals destined to nest east of the mountains reach their particular haunts. Thus yellow warblers were already present in Yosemite Valley on April 28, 1916, while in the same year the species was not noted near Mono Lake until May 19. The fall departure takes place toward the end of August. The birds do not wander to the higher zones, as do the Lutescent Warblers, for example, but leave their nesting haunts rather early for the lowlands to the west, en route to their winter quarters to the south of the United States. Our own latest definite record for Yosemite Valley was made on August 19 (1915), but Mr. Joseph Mailliard (1918, p. 19) states that in 1917 the species was present until somewhat later, disappearing in early September. In 1920 the last seen by Mr. C. W. Michael (MS) in the Valley was noted on September 11.

Numerically the Yellow Warbler is an abundant bird within its restricted environment. Fully 20 were seen or heard during a 4-hour census taken in Yosemite Valley on May 31, 1915. These were practically all in the cottonwoods, alders, willows, and in other deciduous growths near the Merced River or its tributary streams. Similarly, along the lower reaches of the big rivers where these emerge from the foothills, these birds are plentiful in the month of May.

The song notes of the California Yellow Warbler are shriller than those of any of our other warblers, and, indeed, are exceeded in height of pitch by the notes of only a very few birds. This feature alone is often sufficient to identify the song. Syllabification can do little more than indicate the theme of the song, for the notes are well above those of the human voice. Wee, wee, wee, sit, sit, sitsitsit, sieu, is one of our renderings; and chee, chee, chee, chee-e-e-e-e-e-er another. The call note is a loud chip or tsip. Song is not heard often after the first part of July, but there may be a partial revival of singing in the latter part of August just before the birds depart for the season.

Yellow warblers like many other birds have definite forage beats which they traverse repeatedly through the day. This was well illustrated by observations on a bird of this species seen at Chinquapin in mid-May, 1919. A small black-oak sapling had grown up through the sea of chaparral near our camp and from time to time this tree would be occupied, momentarily, by a California Yellow Warbler. The bird always arrived from a certain direction (coming from another similar station) and upon departing went to still another definite tree situated about 50 yards distant.

Nesting with the California Yellow Warbler begins soon after the birds arrive in the region, our earliest record being for May 29 (1911); on this date, in Yosemite Valley, a female was flushed from a nest containing four eggs. Two nests seen on June 5, 1915, at Smith Creek, east of Coulterville, each contained young a few days old. Nest construction in each of these instances must have commenced in the middle or early part of May. June marks the height of the nesting season for the Yellow Warblers here; in this month the greatest number of nests comes to attention. An adult was seen feeding young recently emerged from the nest, on July 24 (1915) in Yosemite Valley. This instance marked about the close of the nesting season. The one nest found east of the mountains (near Williams Butte) was only under construction on June 23 (1916).

Yellow warblers nest abundantly on the floor of Yosemite Valley. Some of the nests are in growths close to water, whereas others are located in brush tangles or other rank growths back some distance from the streams. A nest found June 7, 1915, may be taken as fairly typical. It was 52 inches above the ground in the crotch of a forking stem of a chokecherry which grew in a clump of the same plant, and was shaded by a black oak. As usual it was higher than wide outside, being 3-1/2 inches in height by 3 to 3-1/4 inches in diameter. The cup-like cavity was 1-3/4 inches across at the top and the same in depth at the center. Shreds of bark and flat plant fibers were the principal materials used in construction, the lining being of horsehair and a few feathers.

When found, this nest contained three eggs; the following day a fourth was laid. During our first approach to the nest on the 7th, both male and female were about and voiced their alarm. Returning to the nest later that day we noticed only the female. She departed at once, dropping close to the ground and then speeding off through the underbrush. A nest of this species probably used in the previous year was seen 8 feet away in another cherry bush and 36 inches above the earth. Still another occupied nest was seen 7 feet above the ground in a small incense cedar, close to a well traveled road.

One of the nests found at Smith Creek on June 5, 1915, was 4 feet above ground in a chokecherry bush and near the stream. It was 2-1/2 inches in diameter and 4 inches in outside height, and was made of plant fibers and feathers. There were 3 young, about 2 or 3 days old, and one unhatched (evidently infertile) egg. The other nest was 4 feet above the ground in a mountain lilac (Ceanothus integerrimus). it also contained young, 4 in this instance, and not over 3 days from the shell. The parents of this brood were tsip-ing excitedly about 30 to 50 feet from the nest.

None of the nests just mentioned was watched until the young emerged; but observations on still another nest, in Yosemite Valley, help to complete the story. This nest was found by Miss Margaret W. Wythe (MS), who watched it at intervals from June 13 to 27, 1915. It was placed about 15 feet above the ground in a small pine tree growing at the margin of a pond. It rested on the next to the topmost whorl of branches and one side was against the slender trunk of the tree. On June 23, the male was seen with his bill full of 'green worms.' The young left the nest on or before June 27; on that day they were perched in adjacent shrubbery while being fed at frequent intervals by the parents. No more than three young birds were seen at any one time.

But little is to be seen of the yellow warblers after the young are grown. They then take to foraging, individually and unobtrusively. Soon the molt with its quieting influence comes on, after which the birds slip off southward for the winter.