C. W. Bowles (1902) mentions a nest in southern Oregon that “was six feet up in a manzanita bush in a patch of bushes of the same variety about three acres in extent.” But he adds that—
the nests were from three feet and three inches to twenty-five feet from the ground, oaks seeming the favorite in southern Oregon and fir near Tacoma. The usual situation is in a small clump of leaves that is just large enough to almost completely conceal the nest, and yet so very small that a crow or jay would never think of anything being concealed in them. * * * The nests externally are about 3 × 23⁄4 inches and internally 13⁄4 × 13⁄4 inches in diameter and depth. They are composed externally of grass and weed-stalks, that must be several seasons old, (being bleached and very soft) moss and feathers; and lined with feathers (one had evidently been lined from a dead Stellar jay), horse, cow and rabbit hair or fur, and sometimes the very fine stems of the flowers of some kind of moss. The male has never been seen to assist either at nest-building or incubation.
In the Yosemite region, where Grinnell and Storer (1924) found the black-throated gray warbler in fair numbers among the golden oaks on the north walls of the Valley, they found a nest “placed 5 feet 6 inches above ground in a mountain lilac (Ceanothus integerrimus) bush against a main stem.”
From southern California, James B. Dixon writes to me: “This bird breeds sparingly from 2,500 feet to the tops of our mountain ranges in San Diego, Riverside and San Bernardino Counties. During my observations since 1898, I have seen but five nests. One was in a live oak tree, two in manzanita bushes and two in golden oak saplings.” A nest in Riverside County, at 5,500 feet elevation, was in “a scrub growth area which was well wooded with sapling golden oak and manzanita, buck thorn, and other sparsely growing bushes.” The nest was “located 12 feet from the ground in a deep, vertical crotch of a golden oak sapling, and could be seen from only one angle, much like the nest of a gnatcatcher or wood pewee.” Another nest was found “in the dense growth of a young manzanita bush. * * * The locations of the two nests were extremely different, one was carefully concealed in a comparatively bare oak sapling, and the other in the dense foliage of a rank-growing young manzanita bush.”
In the Huachuca Mountains of Arizona, I found but one nest of the black-throated gray warbler. It was 5 feet up in the main crotch of a small oak growing on a steep slope on the side of a branch of Ramsey Canyon; the slope was sparsely covered with scrub oaks and other bushes, with a scattering of tall pines; the nest was so well concealed that I could not get a clear photograph of it. Howard (1899) found three nests in these mountains in upright forks of oak saplings, and says: “I found other nests, some placed in large white oaks and some in sycamores and have known the birds to build high up in pines.” One of his nests from these mountains, in the Thayer collection, was found only 18 inches up in a young fir tree in a thicket; lying against the main stem, it was supported, surrounded, and well concealed by live twigs. Four other nests in this collection, were all taken in the Huachuca and Chiricahua mountains from oaks at heights ranging from 6 to 16 feet above ground. All much alike, their decidedly gray appearance makes them less visible among the gray branches. They are made of light gray, old, shredded stems of dead weeds and grasses, very fine gray plant fibers and a few dead leaves, bits of string, and thread, all firmly bound with spider’s web and decorated with numerous bits of spider cocoons. They are lined with fine brown and white hairs and small, soft feathers.
In New Mexico, Jensen (1923) reports two nests in piñon pines; one was 3 feet and the other 5 feet above ground.
Eggs.—From 3 to 5 eggs, usually 4, constitute a full set for the black-throated gray warbler. These are ovate to short ovate and are only slightly glossy. The ground color is white or creamy white and is speckled, spotted, and sometimes blotched with “chestnut,” “auburn,” “bay,” or “russet,” occasionally with “mummy brown,” with underlying spots of “light brownish drab,” or “light vinaceous drab.” The spots are usually concentrated at the large end, forming a loose wreath, with the drab markings frequently in the majority. Some eggs are only lightly speckled, while others are boldly marked. The measurements of 50 eggs average 16.5 by 12.5 millimeters; the eggs showing the four extremes measure 18.2 by 12.3, 18.1 by 13.1, 14.6 by 12.9, and 16.2 by 11.6 millimeters (Harris).
Young.—The period of incubation does not seem to have been recorded for this warbler. It is probably performed by the female entirely, but both parents share in the feeding of the young. Information on this subject is scanty.
Plumages.—The young black-throated gray warbler in juvenal plumage shows the characters of the species more than do the young of other wood warblers; the black and white areas about the head and throat are strongly indicated in a duller pattern and there are two broad white bars tipping the median and greater wing coverts (see pl. 35); these markings are more subdued in the female than in the male, thus making a slight sexual difference. The back is brownish gray and the underparts grayish white, faintly streaked with black.
I have not been able to trace the postjuvenal molt, but it is perhaps less extensive than in most other warblers. In first winter plumage the young male is much like the adult male at that season, but it is more strongly washed with brown above and with yellowish beneath, the chin is white, the black throat is mottled with white, and the streaking above and below is duller and more obscured. The young female differs from the adult female in about the same way.