In western Pennsylvania, “its local breeding range is correlated rather closely with the distribution of the white pine and the hemlock. Where these conifers prevail, the Black-throated Green appears, although in the mountains it is by no means averse to hardwood timber, if high and dense” (Todd, 1940). And, in the Pymatuning Swamp region, “wherever tall black birches and equally tall, slender hemlocks grew side by side, the Black-throated Green Warblers were almost sure to be found, and no less than twenty pairs were located” (Sutton, 1928). Referring to the central Allegheny Mountain region, Prof. Maurice Brooks (1940) says that “this species, in its distribution within our area, presents one of the most puzzling problems with which we have to deal. It occurs everywhere at high elevations, in spruce, hemlock, northern hardwoods, white pine, oak-pine scrub, and oak-hickory.”
Still farther south, on Mount Mitchell, in western North Carolina, Thomas D. Burleigh (1941) found it to be “a plentiful breeding bird in the thick fir and spruce woods at the top of the mountain, appearing in April when the ground is frequently still covered with snow and lingering in the fall until early October.”
Spring.—From its winter home in Mexico and Central America, the black-throated green warbler, starting early in March, migrates northward through eastern Texas and up the Mississippi Valley, mainly in the forested areas. I noted it in the great wave of warblers migrating along the Texas coastal islands early in May. The fact that it is so rare in southern Florida, and still rarer in Cuba, suggests that many individuals must make the perilous flight from Yucatan across the Gulf of Mexico to the Gulf States. From Louisiana it takes a more northeasterly route, mainly along the Alleghenies, to New England and beyond. It is one of the earlier warblers to arrive in Massachusetts, often during the last week in April. The birds come along in waves, the first wave consisting mainly of males and later waves containing the females in larger numbers. The passage of individuals seems to be fairly rapid, but the species may be present for nearly a month at any point along its migration route. While migrating it may be seen, like other warblers, almost anywhere—in the tops of woodland trees, in roadside trees and shrubbery, in gardens and in parks, before it settles down in its favorite breeding haunts. There must be a very heavy migration through Ohio, for Milton B. Trautman (1940) says that in the “larger flight 50 to 125 were daily recorded, and it was evident that there were several thousands present.”
Nesting.—Although the black-throated green warbler is one of our commonest breeding warblers, I have never found its nest in my home territory, though I have spent many hours hunting for it in its favorite pine woods. While hunting through a somewhat open tract of pitch pines on Martha’s Vineyard, Mass., on June 8, 1919, with Frank C. Willard, we found a nest with four fresh eggs 8 feet from the ground in a small pine; it was saddled on an upward-slanting limb and partially supported by a whorl of three small branches. It was a pretty nest, made of grasses, seaweed, and strips of inner bark, and was lined with fine grasses, cowhair, horsehair, and a few white feathers. The male was incubating and was very tame, coming within a few feet of us; he also returned and sat on the empty nest after Mr. Willard had removed the eggs.
On June 4, 1910, Herbert K. Job showed me a nest near New Haven, Conn., in mixed deciduous woods; it was about 11 feet from the ground, built against the trunk of a large chestnut sprout and supported by a small dead branch and two live twigs; the leaves on this twig screened the nest from above, one leaf forming a complete canopy over the nest, the tip of it being tucked into the rim. It was made largely of materials similar to those in the one previously described, there being three large feathers on the rim and many small feathers in the lining.
The only other nest I have ever seen was found on the island of Grand Manan, New Brunswick, on June 11, 1891; it was placed only 3 feet from the ground between two horizontal branches and against the trunk of a small spruce beside a cowpath in coniferous woods. It was a compact, deeply hollowed, structure made of fine twigs, mosses, birch bark, strips of inner bark, and weed stems, and it was lined with white cowhair and a few feathers.
There is a set in my collection, given to me by Fred H. Carpenter, said to have been taken from a nest only 8 inches from the ground in a small red cedar in an old neglected pasture in Rehoboth, Mass. The nest, now before me, seems to be typical of the species.
The nests mentioned in some notes sent to me by Miss Cordelia J. Stanwood, of Ellsworth, Maine, were in spruces or hemlocks at low or moderate heights, but Knight (1908) says that “near Bangor the species builds fifty to seventy feet up in the larger, taller pine trees.” Robie W. Tufts tells me that, of some 20 or 30 nests that he has seen in Nova Scotia, “all have been built in conifers, including hemlock, spruce, and pine.” In New York and Pennsylvania, hemlocks seem to be the favorite nesting trees, but nests are sometimes placed in beeches or yellow birches; the nests in hemlocks are usually placed on horizontal branches at a considerable height from the ground and generally well hidden in the foliage. A nest examined by Dr. George M. Sutton (1928) at Pymatuning Swamp “was saddled on a horizontal bough only about twenty-five feet from the ground, in a comparatively small hemlock. The nest was very deep and beautifully constructed, its lining including bits of hair, fur, and soft feathers, and its foundational material consisting chiefly of slender and uniform twigs of dead hemlock.”
The two nests studied by F. A. Pitelka (1940), in northern lower Michigan, were on horizontal branches of Norway pines (Pinus resinosa), 23 and 12 feet from the ground, respectively. The materials used in the nests were largely similar to those mentioned above, with the addition of woollike plant fibers and short pine twigs in the lining, and with “a considerable quantity of hypnaceous mosses and bits of birch bark” used as trimmings.