Robie W. Tufts tells me that he has seen the male at the nest; he saw a male come to a partly finished nest sit in it for over a minute as though testing the workmanship and sing twice while sitting there. The male is always very attentive during nest-building, following his mate back and forth on her trips for material and keeping close to her most of the time. His interest in the nest is so keen that it would be strange if he did not sometimes help.
The eastern yellow warbler builds a neat, strong nest, the materials being firmly and smoothly interwoven and the lining compactly felted. Five local nests before me show quite a variation in the materials used and in their arrangement. The most obvious material, occurring more or less in all of the nests, consists of the silvery-gray strands from the last-year’s stalks of milkweed, Indian hemp, or other similar dead weeds. One nest has a great mass of such material below it on one side, evidently to fill in space in the fork that supported it; mixed with this material are a few strands of grasses, other shredded weed stems, bits of wool, and gray fur. Although this nest is far from neat externally, the cup of the nest is well and firmly made of finer silvery fibers and fine grasses, cinnamon-fern down, with which it is profusely lined, and a few fine white hairs. The rim is strongly reinforced with horsehair and decorated with the cinnamon down. This nest, the largest of the lot, measures nearly 5 inches in height and 3 inches in diameter, externally. The smallest and the neatest of the five is made of finer strands of similar materials, without a trace of cinnamon-fern down, the whole being very firmly and smoothly woven into a compact little nest; the rim is neatly made of very fine grasses, and it is smoothly lined with white plant-down; it measures only 2 inches in height and 21⁄4 inches in diameter, externally. Grasses enter largely into the construction of all the nests. One in particular is lined with both white and buff plant-down and a little very fine grass, and has a solidly built rim of strong grasses very firmly interwoven; the foundation consists of dry brown and gray lichens, or mosses, and a lot of cotton waste, such as is used to clean machinery. A two-story nest, which measures 4 inches in total height, is profusely lined with white cotton in both stories. There is little difference in the internal measurements, which vary from 13⁄4 to 2 inches in diameter, and from 11⁄4 to 11⁄2 inches in depth.
None of my nests contain any feathers, but Dr. Roberts (1936) tells of a nest that was made entirely of chicken feathers, with “not a bit of material of any other kind.” It was built in a jewelweed (Impatiens capensis), but after a brisk wind and a sharp shower both nest and weed were completely wrecked. He shows a photograph of a nest built almost entirely of sheep wool, and speaks also of the use of fine strips of inner tree bark, which probably occur in many nests, of quantities of fine, white, silky pappus from various plants, and of a few feathers. Du Bois mentions in his notes a nest in which five soft, white chicken feathers were woven into the lining, the largest one when stretched out measuring 33⁄4 inches; there were also two or three feathers in the body of the nest. In my collection is a beautifully camouflaged nest that was built in the upright crotch of a small poplar and seems to be made very largely of white cotton mixed with fine, light-colored fibers. It is lined with cotton, and with a few green poplar leaves fastened to the exterior, the whole being firmly bound with some of the finest fibers and with spider silk, the light-colored material matches the bark of the tree so closely that it might easily be overlooked.
T. E. McMullen has sent me his data for over 40 nests of the eastern yellow warbler found in New Jersey and Pennsylvania. The lowest nest was only 1 foot from the ground in a small bush, and the highest was 30 feet up in an elm. In addition to the shrubs and trees mentioned above he lists arrowroot, blackberry briers, elder, holly, Osage-orange and button-bushes, birch, wild cherry and oak saplings, and a pear tree.
The well-known habit of building nests one or more stories over cowbirds’ eggs will be discussed under the enemies of the yellow warbler.
Eggs.—Four or five eggs made up the usual set for the eastern yellow warbler; sometimes as many as six are found, or as few as three. In shape, they vary from ovate to short ovate, or rarely show a tendency to elongate ovate. They are only slightly glossy. These handsome eggs show a great variation, both in ground color and in markings. The most common ground colors are grayish white or greenish white but some eggs have a bluish white or even a soft, pale green ground color. The spots and blotches show an even greater variety of colors. Shades of “fuscous,” “olive-brown,” “citrine drab,” “buffy brown,” “buffy olive,” “light brownish olive,” “raw umber,” “metal bronze,” or “tawny olive” are intermingled with undertones of “deep gull gray,” “neutral gray,” “purplish gray,” “pale purplish gray,” “mouse gray,” or “buffy brown.” The markings tend to form a wreath around the large end where, on the heavily-marked types, the blotches overlap the undertones and an almost endless number of shades are formed. Sometimes a few spots or scrawls of dark “mummy brown” or “olivaceous black” stand out in sharp contrast to the other markings. Although the eggs are usually well marked, sometimes with blotches a quarter of an inch in diameter, often they are only finely speckled with the gray undertones. The measurements of 50 eggs average 16.6 by 12.6 millimeters; the eggs showing the four extremes measure 17.8 by 13.2, 17.8 by 13.7, 15.2 by 12.7, and 15.8 by 11.7 millimeters (Harris).
Young.—The incubation period for the eastern yellow warbler has been recorded as from 8 to 11 days (most observers place it as about 11 days for each individual egg); often, but not always, it begins before the set is complete, making the period appear shorter for the first egg laid. Eggs are generally, though not always, laid on successive days, but at times 1 or 2 days intervene between layings. Incubation is performed wholly by the female. The male stands guard near the nest and feeds the female while she is sitting, but she also leaves occasionally to feed herself. The young remain in the nest from 8 to 15 days, according to several observers, but here again the normal time is probably between 9 and 12 days, if they are undisturbed.
Harry C. Bigglestone (1913) describes the hatching process as follows:
At about 5:30 a.m. on July 3 the writer was attracted by a peculiar rolling motion of the egg in the nest, and noticed upon closer observation, that the shell bulged out in a ring around the middle or a little nearer the smaller end; and soon it began to crack at this place. The egg raised on the small end, leaning against the side of the nest, and the young bird freed himself from the shell by a series of pushes and kicks by the head and feet, respectively. The head escaped from the larger part of the shell and the lower part of the body from the smaller end. The crown of the head and the median line of the back of the nestling were downy. This entire process covered a period of less than four minutes.
The empty shells were broken up and eaten by the parents. He says that brooding was carried on entirely by the female, except that he once saw the male brooding for 7 minutes, and adds: