RUSSET-BACKED THRUSH. Hylocichla ustulata ustulata (Nuttall)

Field characters.—Size nearly twice that of Junco; bill short and slender. Whole upper surface plain brown; wing with a concealed band of buffy which shows in flight; breast buffy with scattered triangular spots of brown; sides brownish, not streaked; belly white; eye surrounded by a narrow ring of buff. (See pl. 11b). Movements deliberate. Voice: Song of male a set theme, 2 to 4 deep clear notes, then an equal number of slurred ones, ascending in pitch, the last one finely attenuated; call note a soft whistled what or whoit; less often a harsher chee-ur-r.

Occurrence.—Summer visitant in Transition Zone on west slope of Sierra Nevada. Recorded as nesting in Yosemite Valley and in vicinity of Bower Cave, and as a transient at Snelling and Pleasant Valley, and near Coulterville. Keeps to low growths near streams, males going up into trees to sing. Solitary.

The Russet-backed Thrush is extremely local in its occurrence in the Yosemite region. In fact we found it at nesting time only on the floor of Yosemite Valley and in the vicinity of Bower Cave. In the Valley it is usually present in some numbers and during the early part of the season may be heard in song about many of the camps.

This species resembles the Hermit Thrush in general appearance though it can be told easily from that bird. (See pl. 11.) The Russet-back is uniformly colored above, having no rufous tinge on the upper surface of the tail; it does not twitch its wings or raise and lower its tail in the manner of the Hermit Thrush. In point of size the present species is the larger, in the ratio of about 32 to 25, the average weight in grams for the two; but this difference in bulk is not enough for separating the two thrushes out of hand. The songs and calls are totally different and whenever heard are the most certain means of identifying their possessors.

In the month of May Russet-backed Thrushes are likely to be encountered as transients at almost any station in the foothill country. We noted the species at Snelling on May 26 to 29, 1915, and at Pleasant Valley on May 24 and 25 the same year. In 1919 it was observed near Coulterville on May 10 and 11. All the birds seen by us at these places acted as though they were merely resting and foraging while en route to more northern localities. Through much of May and on until some time in July Russet-backed Thrushes are much in evidence in Yosemite Valley, but they begin to disappear toward the end of the latter month and are seen but seldom thereafter. It is not known whether the bulk of the population migrates early or not; more likely their added reclusiveness at the season of molt tends to remove them from human view. Mr. C. W. Michael (MS) who kept continuous record of the birds in Yosemite Valley during the season of 1920 observed the Russet-backed Thrush almost daily until July 24. Thereafter it was noted upon but three occasions, namely, August 27, and September 1 and 11. And each of these records was for only a single bird.

The demeanor of this thrush is quiet and reserved. The bird inhabits by preference the rank growth of stream-side vegetation, departing from this only when the male happens to go up some distance in a nearby tree to sing. In migration time when passing through the lowlands the Russet-backs keep to the same sort of cover as is chosen by them for their summer haunts.

By early June, and sometimes sooner, the Russet-backed Thrushes in Yosemite Valley are in full song and may be heard during the day as well as in the morning and evening hours. The song is set in character and each individual thrush begins his song on about the same key—not changing from song to song as does the Hermit. The first syllables of any individual's song are always of the same pitch, and full, clear, and deep; the remainder are more wiry, ascending, and sometimes the last one goes up so high in pitch as to become almost a squeal: wheer, wheer, wheer, whee-ia, whee-ia, whee-ia, or quer, quer, quer, quee-ia, quee-ia, quee-ia. The call note oftenest heard is a soft liquid whistle, what or whoit, sounding much like the drip of water into a barrel. An imitation of this note by the observer will often bring a thrush into close range. Now and then a thrush will give an abrupt burred cry, chee-ur-r; and again there may be a single whistle, louder and higher than the usual call. The song season lasts until early July, after which the birds become quiet. By the end of the month not even the call note is to be heard.

The nest is located in some bush in the vicinity of a stream. One found in Yosemite Valley on June 24, 1915, may be described as typical in structure though it was placed a little higher than usual. This particular nest was at a height of 7 feet above the ground, settled firmly in the crotch of a willow where three branches about one-fourth of an inch in diameter gave ample support. The structure was bulky, 6 inches in height and about 4 in diameter on the outside, with the cavity about two and a half inches deep and the same in diameter. The material used was chiefly dry willow bark, with some grass stems and blades, a few pine needles, leaf 'skeletons' and weed stems. There was no moss on the outside, nor was there horsehair within, the lining being of loosely placed fine stems and roots. Just without the lining of the nest cup was a layer of dried sandy mud, "as firm as though cement were mixed with it" the notebook states (Mrs. Joseph Grinnell, MS). On the 24th the four young were nearly ready to leave and this they had done by the 26th, though they were still in the vicinity on the latter date, for the anxious calls of the parent birds were heard in adjacent trees.

Two nests of the Russet-backed Thrush were found in Yosemite Valley on June 28, 1915, by Miss Margaret W. Wythe (MS). One of these contained 3 young birds about half-grown. The other nest held 2 young birds just out of their shells, and 2 eggs which hatched on the 30th.