Dr. Paul Harrington writes to me: “I have found this bird nesting in pure deciduous forests on two occasions.” One nest was 40 feet up in the crotch of an ironwood, and the other was 20 feet from the ground in a small elm, both in Ontario. Edward R. Ford has sent me the following note: “On Gull Island, about ten acres in extent, which lies in northwestern Lake Michigan, we found the black-throated green warbler in an unusual nesting niche. About half of the island’s area is northern hardwood forest, whose floor cover is largely of American yew (Taxus canadensis). At a height of but two or three feet, among the sprays of this ground-hemlock, we discovered two nests of the species named. Each of these, July 12, 1918, held four eggs. There was a third nest, empty but evidently used that season.”
Nests have also been found in maples, in white, gray, and black birches, in alders and probably in other deciduous trees and bushes. And the following unusual nesting sites are of interest: William Brewster (1906) mentions a nest that he found “in a barberry bush growing in an open pasture at Arlington Heights, one hundred yards or more from the nearest woods.” He also has a nest, taken by C. H. Watrous in Connecticut, that was on the ground “among a large clump of ferns in a very low and damp place under a heavy growth of hemlocks” (Brewster, 1895). John C. Brown (1889), of Portland, Maine, mentions a nest that was built in a grapevine growing luxuriantly about a pagoda at some distance from any woods; it was well hidden from the outside by the foliage, but in plain sight from inside the pagoda. And B. S. Bowdish (1906) records a New Jersey nest that “was built between the stems of a ‘skunk cabbage’ plant, and fastened to a catbriar and the twigs of a dead bush, and was about fourteen inches from the ground, in a very wet part of the swamp.”
Miss Stanwood (1910) watched a pair of black-throated green warblers building a nest in a fir tree, of which she writes:
First they laid knots of spider’s silk and little curls of white birch bark in the shape of the nest, on the horizontal fork about midway of a branch six feet long. Next bits of fine grass, a little usnea moss, and cedar bark fibre. Both the male and female worked on the nest, until observed, the female shaping it with the breast each time they added a bit of material. Around the top were carefully laid the finest gray spruce twigs. These were bound together with masses of white spider’s silk. The white curls of birch bark, the much weathered twigs, the fluffy shining bands and knots of spider’s silk, made a very dainty looking structure. After the first morning, I did not see the male about the nest. As a general thing, I find that, if birds are observed building, the male usually leaves his part of the work to the female. The lady bird continued to shape the nest with her breast, turning around and around, as if swinging on a central pivot, just her beak and tail showing above the rim. If I came too near, she stood up in the nest as if to fly. If I withdrew to a respectful distance, say three yards, she went on with her work of shaping the nest. On the second day the rim of the nest seemed about completed. It was narrower than the rest of the cup and beautifully turned. Nothing to speak of had been done to the bottom. On the fourth day, by touching the inside of the nest with the tips of my fingers, I judged that the lining was about finished. It consisted of rabbit-hair and horse-hair, felted or woven together so as to be very thick and firm. Between the foundation of twigs and bark and the hair lining was a layer of fine hay of which the mouth of the nest was chiefly shaped. I never saw a more substantial looking little nest. It was also one of the most beautiful I have ever found, a perfect harmony in grays.
A very pretty nest in my collection is largely made, externally, of usnea and is profusely decorated with masses of the curly outer bark of the yellow birch. The larger of two nests before me measures about 4 by 31⁄2 inches in outer diameter, the smaller about 3 inches; both are about 2 inches high, nearly 2 inches wide and 11⁄2 deep inside.
Eggs.—The black-throated green warbler usually lays 4 eggs to a set, but quite often 5. These are ovate to short ovate and slightly glossy. The ground color is grayish white or creamy white. The markings consist of specks, spots, blotches, or small scrawls of reddish browns, such as “auburn,” “chestnut,” “bay,” “Mars brown,” or “russet,” with underlying spots of “light brownish drab,” “deep brownish drab,” or “light purplish drab.” Generally the markings are concentrated at the larger end, where they usually form a wreath, but occasionally the spots are well scattered over the entire egg. There is considerable variation. Some eggs have a faint wreath of the pale drab coloring which is relieved with a few bold spots or scrawls of dark “bay” or “Mars brown.” Others are richly spotted and blotched equally with browns and drabs, or they may have a solid ring of “russet” blotches which completely covers and conceals the drab undertones. The measurements of 50 eggs average 17.0 by 12.7 millimeters; the eggs showing the four extremes measure 18.8 by 12.8, 17.2, by 13.4, 15.5 by 12.2, and 18.0 by 12.0 millimeters (Harris).
Young.—It is generally conceded that the period of incubation is about 12 days and that the young remain in the nest from 8 to 10 days, depending on the amount of disturbance. Probably the female does most, or all, of the incubating and brooding, but both sexes assist in feeding the young and in swallowing or removing the fecal sacs. Miss Stanwood (1910b) refers to the development of the young as follows: “On the third day the young birds grow rapidly, burnt-orange in color, covered with an abundant supply of burnt-umber down. The quills and pin feathers showed blue-gray through the skin, and the eyes were just beginning to open.” At another nest, “on the eighth day, the nest was simply stuffed full of little green-gray birds, strikingly like the color of the nest.
* * * On the eleventh day, quite early in the morning, as I neared the nesting place, I heard the fledglings calling from the tree-tops. Soon I caught a glimpse of the Black-throated Green Warblers marshalling their little band away.”
Margaret M. and L. B. Nice (1932) made detailed studies of two nests of this warbler, to which the reader is referred. I quote from their summary: