Tree-Dwellers

black-throated green warbler

Warbler-watching is even more frustrating in late summer and early autumn than it is in spring. Not only do most warbler-watchers suffer from “warbler neck” as they look up at flitting birds foraging in the treetops, but they are faced with identifying what the late, great Roger Tory Peterson labeled “confusing fall warblers” in his Birds of Eastern and Central North America field guide.

They are “confusing” because, in many warbler species, most of the juveniles, along with the females and sometimes even the males, have donned dull plumages for their fall and winter sojourns in southern climes. In addition, they aren’t singing as they do in spring. At least then, if a warbler is hidden by tree leaves, I can often identify the bird by its song.

But one warbler species that is easy to identify, no matter what the season, is the black-throated green warbler. All but immature females of this obliging species show at least some of its signature black on its throat. It also has a green back, a gold and green face, white wing bars, a pale yellow breast and belly, and black streaks on its sides and flanks.

Coincidentally, the black-throated green warbler is, by far, the most common species here during fall migration. Or maybe it only seems to be the most common because even in a flock of mixed, dull-plumaged warblers that I can’t identify before they fly off, the black-throated greens are always conspicuous. They often come closer for a look and have been described by warbler-watchers and authors Jon Dunn and Kimball Garrett in their field guide Warblers as “confiding” and “rather tame.”

Black-throated greens take their time during fall migration and except for yellow-rumped warblers, which winter farther north than any other warbler species, are the last warbler species to migrate through our forest. From the latter part of August until early to mid-October, they move southward in the eastern United States usually in mixed species flocks, including both other migrant species and local species such as black-capped chickadees and tufted titmice. Taking particular advantage of strong, northwesterly winds during periods of high pressure systems, they migrate by night and forage by day.

At Hawk Mountain on September 21, 1996, hawk-watchers counted 100 black-throated green warblers, but that doesn’t even compare with a count made in September 1899 in Scioto County, Ohio, where warbler-watcher Rev. W.F. Henninger counted a flock of 2000 warblers, which included 200 black-throated greens and enormous numbers of bay-breasted and blackpoll warblers.

“It was,” he wrote, “like a regular army as it moved up a long sloping hillside… Lisping, chipping, whirling, driving,” they raced up the hill with the Reverend in hot pursuit. Then they stopped, and he had time to identify and count them.

Eventually, black-throated greens reach their wintering grounds, which may be as far south as northern South America but are primarily in the mountainous regions of Mexico and Central America, Cuba and Jamaica in a variety of habitats from tropical conifer forests and cloud forests to oak and pine forests. At higher elevations they mingle with other members of the so-called virens (meaning “growing green”) superspecies–the golden-cheeked (of the Texas hill country), and the hermit and Townsend’s warblers of western North America, all of which have black throat patches.More broadly, they all belong to the Dendroica (meaning “tree-dweller”) genus, the most diverse and, at 27 species, the most numerous of the wood-warbler genuses. Of these, 16 species spend their breeding months in eastern North America, including yellow, chestnut-sided, magnolia, Cape May, black-throated blue, black-throated green, yellow-rumped, yellow-throated, pine, Kirtland’s, prairie, palm, bay-breasted, blackpoll, blackburnian, and cerulean warblers. Furthermore, 12 of those species–yellow, chestnut-sided, magnolia, black-throated blue, yellow-rumped, blackburnian, yellow-throated, pine, blackpoll, prairie, cerulean, and, of course, black-throated green warblers — breed somewhere in Pennsylvania.

Of all those species, Dendroica virens, the black-throated green warbler, is the most abundant breeder on our mountain. Yet it was not always so. When our forest was younger, we only saw this warbler in migration. But now that our forest has reached 100 years of age, it rings with the singing of male black-throated green warblers from the last week in April, when they arrive, until the end of July or early August, although their singing is most intense during the first couple weeks of their arrival. In fact, ornithologist Margaret Morse Nice and her husband Leonard, back in 1932, counted 466 songs by one male in an hour and 14,000 songs in 94 hours by another male.

Like several other warbler species, black-throated green warblers sing two distinct songs. Unlike the undistinguished buzzy trills of some warbler species, the two songs of black-throated greens are easy to learn, once you identify them. Ornithologists refer to them as the accented–“see, see, see, su-zee”–and unaccented–“zoo, zee, zoo, zoo, zee.” The unaccented is often referred to as “trees, trees, murm’ring trees,” or as ornithologist Bradford Torrey interpreted it–“sleep, sleep, pretty one, sleep.” The accented, according to F. Schuyler Mathews in his Fieldbook of Wild Birds and Their Music, resembles a “bar of the familiar old sea song “Larboard Watch” — “Lar-board watch a-hoy!” Now I don’t know that song, but knowing the accented song as I do, I can understand that analogy.

The accented “lar-board” song is sung mostly by unmated males and mated males when females are around. The unaccented “trees” song is sung while defending territory from other males or after males are mated. They are also sung almost exclusively at dawn and dusk. So even if I can’t see the singer, I can get some idea of what is going on by what song is being sung.

Ideal, black-throated green warbler breeding habitat in Pennsylvania consists of large, unbroken mixed conifer and northern hardwoods at or above 1,000 feet in the Allegheny High Plateau Section of the Appalachian Plateau. Yet, in the Ridge and Valley Province, where we live, black-throated greens live from 600 to 1200 feet in elevation and in both our lower, hemlock/hardwood forest beside our stream and in our dry, oak forests at higher elevations. But then black-throated green warblers are adaptable creatures that breed from British Columbia to Newfoundland in the north and south to New Jersey along the Atlantic seaboard and in the Appalachians as far south as Georgia and Alabama. They prefer a variety of habitats, such as hardwoods in West Virginia, pine forests farther north, and even forested wetlands.They feed in the middle and upper canopy of middle-aged and mature forests and eat a wide variety of insects and spiders, especially hairless caterpillars during the breeding season, although they will eat hairy caterpillars, particularly during a gypsy moth caterpillar outbreak. Mostly they pluck their prey from the upper sides of leaves, but they also hover and feed on the undersides. They even occasionally dart out to catch insects on the wing. During migration they like poison ivy and other berries in addition to insects.

Although nominally monogamous, the females are not above “sneak” copulations with quiet second males because males are always in excess in this species. But their male partners guard them as they build their nests and before they lay their eggs. They also fluff out their feathers in courtship displays and assist in nest-building.

Bird photographer Cordelia Stanwood spent many hours in the beginning of the twentieth century observing nest-building black-throated green warblers in the forest outside Ellsworth, Maine. She wrote, “I never saw a more substantial looking little nest. It was also one of the most beautiful I have ever found, a perfect harmony of grays.” Bound together with spider silk, the cup-shaped nests are made of twigs, grass, lichens, weed stems, and bark, lined with moss, fur, feathers, hair, and fine stems and attached to a branch or fork of a conifer or hardwood tree anywhere from three to 80 feet aboveground. In it, a female lays four to five, olive-speckled, grayish-white, oval eggs.

She alone incubates the eggs for about 12 days and chases off other birds, such as red-eyed vireos and blackburnian warblers, as well as black-throated greens that stray too close to her nest. Once the eggs hatch, she broods her young from four to six days and does most, if not all, of the feeding the first couple of days while the male sings. But the male soon assists her, and both mainly feed their nestlings small insects and spiders.

The young fledge at 10 to 11 days of age, staying close to their parents the first two days, and then, as their flying skills improve, following their parents and begging loudly. At that point, each parent takes off with a part of the brood, and they may stay together as long as a month, feeding in mixed-flocks that include black-capped chickadees. Year after year, the parents may return to the same breeding area. Certainly, they do sing in the same places on our mountain every spring.

Like most small birds, they provide food for raptors such as Cooper’s and sharp-shinned hawks. Their eggs and young are eaten by blue jays, red and gray squirrels, and probably other tree-climbing predators, for instance, black rat snakes and raccoons. Other threats include collisions with manmade objects–television and cell phone towers, windmills on ridgetops, windows–during migration, and habitat loss on both their breeding and wintering grounds.

Still, because they live in a broad range of habitats throughout the year and eat a wide variety of foods, they are not as threatened as more habitat-specific warbler species such as Kirtland’s and golden-cheeked warblers. In fact, Robert Mulvihill, who directs bird research at the Powdermill Nature Reserve, the 2,200-acre biological field station of the Carnegie Museum in southwestern Pennsylvania, says that over the 45 years that fall bird-banding has been done at the Reserve, numbers of black-throated green warblers have risen. In 2004, they hit an all time high of 208, which eclipsed their previous high of 130 in 2001. In comparison, from 1961 until 1999, the average number of banded black-throated greens was 104.

On the other hand, Mulvihill says that, “A real concern is the progression of hemlock woolly adelgid and its effect on the black-throated green warblers and other, even more hemlock-specialized bird species such as blackburnian warblers and blue-headed vireos. Only time will tell whether this will lead to range and/or habitat shifts and how these will affect overall statewide population levels.”

I can only hope that the versatile black-throated green warblers will continue to find plenty of nesting habitat on our mountain. I would sorely miss their accented and unaccented songs, their confiding ways, and their striking good looks.
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Click on the photos to view the original photo pages on Flickr, including details on location and the photographer’s notes, if any. Top photo by Stewart Ho (used by permission); second photo of immature black-throated green by mcormier7781@rogers.com (used by permission); third and fourth photos by BlackGum Swamper (used by permission); fifth photo by Gavatron (used by permission)