Late last June I sat next to our mountaintop vernal pond that has become a permanent pond the last couple wet years. For an instant, I glimpsed the white spectacles of a blue-headed vireo as it foraged on a large red maple tree across the pond. Then I heard a singing yellow-throated vireo, followed by the droning song of a red-eyed vireo.
I was used to hearing red-eyed and blue-headed vireos singing throughout our forest. But I was not as familiar with yellow-throated vireos. Our son Mark, who has keener ears than I, heard the same singing yellow-throated vireo at the ponds the following day and then another one singing outside the guesthouse day after day in late July and early August.
The vireo family, which includes 14 species in North America, is one of the few songbird families that is thriving. Blue-headed vireo numbers have doubled since the 1970s. Red-eyed vireo numbers have been increasing slowly but steadily from 1966 to 2014, and, here on our property during the various bird counts we’ve done, red-eyed vireos are always the most abundant songbird species, numbering 40 or more.
The yellow-throated vireo is more elusive than the blue-headed and red-eyed vireos. However, the Partners in Flight survey reported that they are common birds with a population that increased by 62% from 1970 to 2014. In Pennsylvania the Breeding Bird Survey recorded a 1.2% yearly increase since the 1960s with an estimated 48,000 yellow-throated males in our state.
Still, the blue-headed and red-eyed vireos have been studied far more than the yellow-throated vireo. Researchers have found that all three species nest in mature forests and spend their winters in a variety of forests mostly south of our borders.
The red-eyed travels the farthest to large forests in the Amazon basin. The blue-headed is a medium-distant migrant that spends its winters in shaded coffee plantations, rain forests, cloud forests, or coastal swamps from the southeastern United States coast, Mexico, and south through Central America as far as northern Honduras. Most yellow-throated vireos winter in Central and South America as far south as the mountains of western Colombia and northern Venezuela.
The yellow-throated vireo, sporting a bright yellow throat, breast, and eye-ring, is easily the flashiest of this mostly olive or gray-backed family of birds. In addition, it has a large bill, olive green upperparts with a contrasting gray rump, and bluish legs and feet. The yellow-throated and the blue-headed have white wing-bars, but the red-eyed vireo’s wings and back are a plain olive/brown.
All three species are easier to hear than see because they forage high in the trees, but telling their songs apart can be challenging. The blue-headed vireo has the highest pitched song with deliberate pauses in between, both slower than the red-eyed and with fewer notes per phrase, while the red-eyed vireo’s pitch is a little lower, droning, and monotonous, hence its nickname The Preacher Bird.
The yellow-throated vireo’s song is similar to but lower pitched than the blue-headed vireo. In the words of ornithologist Aretas A. Saunders, “The song is long continued, consisting of short phrases separated by pauses like the songs of the red-eyed and blue-headed vireos but the yellow-throated vireo’s song is slower, the pauses between phrases longer. The quality of the sound is rather reedy and less clear than the others [and] the phrases are usually slurred together.”
The yellow-throated vireo returns to Pennsylvania in late April or early May. Its arrival records at our central Pennsylvania mountain farm range from April 29 to May 13. This vireo is the most southern of the three species that breed on our property and is found from northcentral and southern New England south to the Gulf Coast and west to the eastern edge of the Great Plains.
It likes to breed in a mature deciduous forest and prefers large oaks and maples, although any tall tree will do. Back in the early twentieth century it was also a bird that nested in towns, suburban areas and cities as large as New York and Boston, but then they disappeared, possibly because of the heavy spraying of insecticides on the large shade trees to combat Dutch elm disease.
In Pennsylvania the habitat of the yellow-throateds consists of tall, old trees widely or closely spaced but without an understory, often near a water source. In our Ridge and Valley Province it builds its nest near a river or stream as high as 1300 feet above sea level. Other parts of the state where it breeds are in the northeast, northwest and southwest corners of the state, with low densities in the Piedmont and the High Plateau regions.
The male arrives first and looks for nest sites that he uses to tempt an arriving female. He sings and calls until a female comes close, and then stands over as many as three or four nest sites, each with a small amount of nest material, his head lowered as if he is building a nest. Sometimes he performs a pre-mating swaying display after he quivers his wings in response to the female’s wing-quivering.
Once they mate, she either accepts one of his nest sites or they may find still another. But unlike both the red-eyed and blue-headed vireos, which nest in shrubs or saplings no higher than 15 feet or as low as five feet from the ground, yellow-throated vireos build their nests in the crowns of the tallest trees, 20 to 50 feet above the ground, making this part of their lives difficult for researchers to observe.
It takes the pair about eight days to construct what ornithologist Arthur Cleveland Bent, in his Life Histories of North American Wagtails, Shrikes, Vireos, and Their Allies, described back in 1950 as “the handsomest nests of any of the vireos, …the whole body of the nest is almost completely covered with small bits of variously colored tree lichens, all held securely in place by numerous fine strands of spider silk, the deep cup, with its thick walls and incurving rim above it, is neatly lined with fine grass tops…” and attached to a forked limb.
Then Bent refers to the three to five, creamy or pinkish white eggs with a scattering of dark reddish to brown and black spots on their larger ends as “the handsomest and most heavily marked of any of the eggs of the vireos.”
Both parents incubate those eggs, although the female covers them at night and half the day while the male alternates with his mate during the day. They vigorously protest, chase and sometimes attack any jays or crows that venture near their nest, but the only predators ever recorded on yellow-throated vireos and their offspring are long-eared owls, Cooper’s hawks, and blue jays. Like other vireo species they also contend with brown-headed cowbird parasitism and usually accept those eggs with their own and feed the nestling cowbirds that compete with their nestling vireos. Researchers have found that in nests without cowbirds, there is at least three to four vireo young, but in those with cowbirds, there is less than one vireo nestling.
The yellow-throated vireo eggs hatch into naked young in 13 days and take a further 13 days to develop into feathered fledglings that have been tended and fed by both parents the usual insect and spider fare such as caterpillars, butterflies, moths, stinkbugs, scale insects, leaf hoppers, beetles, flies and bees, most of which they forage from the interior parts of leaves and branches in the middle and upper levels of the forest canopy.
The parents continue feeding the fledglings for at least two more weeks, but the second week each parent with half the young, go their separate ways, although they may reunite at a later period for a short time. For instance, our son, Mark, last July 24, found a family group of six yellow-throated vireos in an area of young black locusts between the spruce grove and the pond area.
But eventually they go their separate ways in late August, beginning their slow migration south. Our latest date is October 17, but more normally they leave around the middle of September, which is true for most of the commonwealth. During migration they use open woodlands and brushy woodland understory and edges and are not observed as often in the fall as they are in the spring, even though both sexes still display their bright yellow throats.
Some researchers have evidence to suggest that they may use a more eastern route during fall than spring migration and migrate in a southeasterly direction. That may explain why they are rarely seen during fall migration in Pennsylvania and then no more than one or two birds in any one place. Usually, they forage singly with mixed-species flocks of wood-warblers, chickadees, titmice, and other small songbirds.
We can only hope that the yellow-throated vireo and all its relatives continue to thrive in their nesting grounds.