Monday, July 22, 2024

A warbling vireo on Strawberry Hill in Cambridge on Sept. 8, 2022. (Photo: Richard George)

When I walk along a river or stream and I hear many birds singing, I turn on my Merlin app to identify the songsters. The first bird to pop up on my screen in summer is almost always a warbling vireo (Vireo gilvus). These little birds are often not seen. They are small, greenish-brown and nondescript. But the males sing incessantly from the tops of trees.

According to ornithologist Arthur Cleveland Bent, “The warbling vireo, if it were not for its song, would not be a notable species, for it is a little bird in leaf-green plumage, inconspicuous as it moves about among the foliage on the highest branches of its favorite elms and poplars where it spends the summer days, surrounded by green leaves and almost hidden by them.”

Warbling vireos are nondescript birds with olive above and whitish or yellowish below and on the sides. (Photo: Tom Murray)

In the 1800s, warbling vireos were especially abundant in elms on Boston Common, where they could be heard singing “at almost any hour of the day, from early in the month of May until long after summer has gone.” Warbling vireos especially liked elm trees, where they were fond of nesting.

Bad news for warbling vireos

In 1851, people introduced house sparrows to Brooklyn, New York, to control linden moth populations, which were harming the region’s basswood trees. Within 50 years, house sparrows had spread across the continental United States. They are very aggressive, and defend their nests by attacking other birds. In 1889, it was reported that house sparrows attacked 70 bird species and even evicted some from their nests. House sparrows were bad news for warbling vireos, and the vireo population declined.

The vase shape of the American elm, as seen in Somerville’s Milk Row cemetery, makes it a desirable shade tree. (Photo: Marty Aligata)

More bad news

If house sparrows were not enough, there was still more bad news for warbling vireos. In the 1920s, furniture makers imported elm wood to make cabinets and tables. This wood introduced a fungus that bark beetles spread to native American elms. Marie Beatrice Schwarz, a scientist in the Netherlands, first identified the cause of this disease (thereafter called Dutch elm disease). The disease spread along the East Coast and killed more than 40 million American elm trees. Because elm trees were favorite nesting sites for warbling vireos, vireo populations declined even more. Populations further declined in the mid-20th century when cities and towns sprayed shade trees with pesticides.

Good news for warbling vireos

House sparrows eat seeds and grains, which were abundant in horse manure in the 1800s. With the advent of the internal combustion engine, however, both horse manure piles and house sparrow populations declined. Pesticide use decreased, too. This was all good news for the warbling vireo. Today, the warbling vireo population stands at about 53 million and is increasing at about 0.6 percent per year.

A warbling vireo in North Cambridge on Aug. 16, 2021. (Photo: Richard George)

The best way to tell if a bird is a vireo is to listen to how often it sings. During the summer breeding season, the males sing all day long, thousands of times per day, even in the hottest weather. They continue singing into September when most other birds have stopped singing. 

Warbling vireos eat caterpillars, cocoons, moths and butterflies, as well as insects that range in size from tiny aphids to large dragonflies. In late summer, they eat berries and the fruits of dogwood, pokeberry, sumac and more. They forage mostly in the tops of trees, snatching insects from leaves and branches.

A warbling vireo near Cambridge’s Fresh Pond on May 5, 2022. (Photo: Richard George)

Warbling vireos often nest near streams and rivers in pouch-shaped nests that hang from the fork of a horizontal tree limb. Some people think that before European settlers arrived in North America, the birds nested exclusively along stream borders – but as people chopped down forests and built roads, shade trees along city streets created a similar habitat. According to ornithologist Aretas A. Saunders, this explains warbling vireos’ preference for nesting in elms and silver maples, “trees that originally were found along stream borders.”

The pouch-shaped nest of a warbling vireo in the Great Meadows National Wildlife Refuge of Concord and Sudbury on May 30, 2016. (Photo: Tom Murray)

Warbling vireo nests are wonders of engineering. They build their hanging pouch nests 20 to 90 feet above the ground. They use strips of bark, leaves, grasses and paper, thread and string, which are held together by spider or caterpillar silk. John James Audubon observed one male and female building a nest:

One morning I observed both of them at work; they had already attached some slender blades of grass … Before the end of the second day, bits of hornets’ nests and particles of corn husks had been attached … On the fourth morning, . . . the little creatures were absent nearly an hour at a time, and returned together bringing grass, which I concluded they found at a considerable distance. I followed them as they flew from tree to tree toward the river … They led me quite out of the village, to a large meadow, where stood an old hay-stack. They alighted on it, and in a few minutes each had selected a blade of grass. Returning by the same route, they moved so slowly from one tree to another that my patience was severely tried. Two days were consumed in travelling for the same kind of grass. On the seventh I saw the female at work, using wool and horse-hair. The eighth was almost entirely spent by both in smoothing the inside. They would enter the nest, sit in it, turn round, and press the lining, I should suppose a hundred times or more in the course of an hour. . . .

A warbling vireo alights on a branch in Bolton on May 7, 2021. (Photo: Tom Murray)

During the breeding season, the males spend much of their time singing from the tops of trees, which is their way of patrolling and defending their territory. The female lays about four eggs. After the eggs hatch, one adult always stays with the young, while the other forages for caterpillars or insects to feed the hungry youngsters. As the chicks grow, the parents feed them more often – up to 30 times per hour!

The next time you meander along a stream, listen for a bird that sings incessantly. You might not be able to spot this little camouflaged bird, but you are probably hearing the song of a warbling vireo.


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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.