Mrs. Harding showed me some half dozen nests of this warbler in the locality near her camp at Asquam Lake, N. H. All were in low bushes of mountain-laurel (Kalmia latifolia) from 12 to 18 inches above the ground and were not very well concealed. They were well made of strips of inner bark, canoe birch bark, straws, fern fronds, and dry leaves, and were lined with black horsehair and fine black rootlets. Altogether, Mrs. Harding (1931) found 15 nests similarly placed in low mountain-laurels, from 9 to 15 inches up, and all made of similar materials, but she says that “skunk fur is used freely as a substitute and sometimes pine needles or bits of moss,” in the lining. So far as I know, she has not found pieces of rotten wood in the nests, as commonly reported by others.
Miss Cordelia J. Stanwood, of Ellsworth, Maine, tells me that the nests she finds near her home are placed in small firs or spruces. Frederic H. Kennard mentions in his notes a Maine nest wonderfully well hidden in a clump of little spruces about one foot from the ground. He also reports two Vermont nests, one about 2 feet from the ground in a tangle of raspberry vines beside a logging road, the other about 8 or 10 inches up in a little thicket of low-growing mountain maple. Robie W. Tufts tells me that the few nests he has examined in Nova Scotia were all built in “small spruce or fir seedlings two or three feet from the ground in heavy woods of mixed or coniferous growth.”
Francis H. Allen writes to me of a nest he found in an unusual situation in Waterville, N. H.: “It was placed about a foot from the ground in the small twigs of a fallen beech, on which were the dead leaves of last season. * * * I collected the nest July 3 after the young had left it. The measurements were: Diameter, outside, 31⁄2 inches; inside, 2 inches; depth, outside, 21⁄4 inches; inside, 11⁄8 inches. It was composed mainly of fragments and shreds of dead wood, apparently stuck together by some glutinous substance, and in one place it had what seemed to be a web of some kind binding it. A few beech buds and bud scales were worked in, and a bleached leaf fragment, a shred of yellow birch-bark, and a small dangling strip of canoe-birch paper—the last perhaps for ornament—completed the body of the nest. The lining was of fine black rootlets. The general effect of the outside was a light yellow or bright straw-color. It was an interesting and a beautiful nest.”
Dr. Chapman (1907) says that “nests found by Burtch (MS.) at Branchport, New York, were built in birch saplings eighteen and twenty inches from the ground, and in a blackberry bush fourteen inches from the ground.” He quotes from the manuscript of Egbert Bagg, of Utica, N. Y., who found nests very similar to the one described above by F. H. Allen. But he says that “one nest had some of the finer quills of our common porcupine (even large enough for their barbs to be visible to the naked eye). This sort of lining might be satisfactory to the old bird, protected by her coat of feathers, but would seem to be somewhat dangerous to her naked fledglings.” One of his nests, evidently built in an upright fork, measured “diameter, outside, 31⁄2 inches, inside, 21⁄4 inches; height, outside, 5 inches; depth, inside, 11⁄2 inches.”
T. E. McMullen has sent me the data for 22 nests, found in the Pocono Mountains, Pa. All of these were built in rhododendrons in woods, two on hillsides, one on the edge of a road, one on the edge of the woods, one near a creek, and three along a creek bank. Most of Mr. Brewster’s (1938) Lake Umbagog nests were placed low down in yews (Taxus canadensis). Apparently, the favorite nesting sites of the black-throated blue warbler are in the broadleaf evergreens, mountain-laurels and rhododendrons, where these are available; next in popularity come the other evergreens (spruces, firs, and hemlocks) of small size; but nests have been found in many places in deciduous seedlings, saplings and sprouts, mainly maple and beech, or in various other bushes or tangles.
Mrs. Harding gave me an account of the building of a nest, which she watched during a period of four days. Most of the work was done by the female, but the male helped shape the nest occasionally. The beginning of the nest and much of the main part of it was made of thin strips of the paperlike bark of the white, or canoe, birch firmly bound in place with great quantities of cobwebs; during the early stages of building the rim was anchored with several strands of cobweb to the surrounding leaves and twigs to secure it while the nest was being shaped; this the bird did by sitting in the nest and turning around in all directions, molding it inside with her feet and breast and pressing her tail down over the edge to smooth the exterior. The male sang in the vicinity and brought some of the material, and once he drove away another male. The nest was finished on the fourth day. This process is described in more detail in Mrs. Harding’s (1931) paper, where she notes “there is usually an interim of at least twenty-four hours before the first egg is laid. The female lays the eggs at intervals of twenty-four hours—frequently early in the morning. * * * On the morning of the fourth day when the clutch is complete the female commences incubating.”
Eggs.—The black-throated blue warbler lays normally four eggs, three are not a rare complement, but five are seldom found. Richard C. Harlow tells me that in over 200 nests that he has examined he has found only 4 sets of five.
The eggs vary in shape from ovate to short ovate, rarely tending to elongate ovate, and are only slightly lustrous. They are white or creamy white, speckled, blotched, or clouded with tones of “pecan brown,” “russet,” “Mars brown,” “cinnamon-brown,” “chestnut-brown,” “bay,” or “auburn,” with undertones of “benzo brown,” “light brownish drab,” “light violet-gray,” or “pale Quaker drab.” There is quite a little variation in the markings, ranging from spots and undertones that are distinct and clearly defined to spots clouded together and undertones only faintly discernible. The markings are usually concentrated at the large end, often forming a loose wreath, or sometimes a solid cap of brown. Occasionally, markings are well scattered over the entire egg. There seem to be two distinct types, one having spots of two or three shades of brown, with gray undertones, the other with tones of only one shade of brown, with drab undertones. The measurements of 50 eggs average 16.9 by 12.8 millimeters; the eggs showing the four extremes measure 18.9 by 13.0, 16.7 by 13.5, 15.2 by 12.2, and 17.0 by 11.8 millimeters (Harris).
Young.—The period of incubation for the black-throated blue warbler, according to Miss Stanwood’s notes, is about 12 days; and the young remain in the nest for about 10 days. Incubation of the eggs and brooding of the young is done by the female only, but feeding the young and cleaning the nest is shared about equally with the male. She saw the young fed with daddy-long-legs, white moths, caterpillars, crane-flies, mosquitoes, and many other insects.
Quoting from the notes of J. A. Farley, Mr. Forbush (1929) gives the following picture of a brooding female: “She had spread the white feathers of her lower parts out so completely over her young that there was not a vestige now visible of the four young that I had found a short time previously filling the nest so full. She ‘fluffed’ herself out so as to hide all traces of the young. * * * She made a beautiful picture. The whole effect was wonderful. The bird seemed to be sitting in a billowy mass of eider down, or cotton wool, that swelled, or rather bulged, up all around her, a regular bed of down.”