The Prairie Warbler

A Moveable Feast – and Feaster

by: Virginia Andrews

One of Nantucket’s few species of nesting warblers, the Prairie Warbler is a summer treat for birders. The male sports a greenish back with chestnut accents, a bright yellow breast, a black shoulder patch and streaking along the flanks.

But the most distinctive feature is a bold facial marking, composed of a dark line through the eye, connected to one outlining a half-circle of bright yellow below the eye. It looks a little bit like a fighter whose black eye is in the late technicolor phase of healing.

But aside from a bit of bill-snapping between challenging males, they are not particularly scrappy birds. The bright color and distinctive markings make it a treat to watch the male as he sits teed up on a branch overlooking his territory. Females have essentially the same pattern of eye-liner, but in a more subdued olive green. Another feature bird-watchers look for is behavioral: a frequently wagging tail with mostly white undertail coverts.

But perhaps the quickest way to find one is by listening. Their song, if it can be called a song, is a series of notes described as “zee-zee-zee-zee-zee-zee-zee,” rising in succession. More of a declarative statement than a melody, it is the male’s way of announcing his availability in the breeding season.

A sung scale, it is somewhat thin and buzzy, but carrying. It comes in two similar-sounding but basic varieties. The first is louder, more an advertisement and territorial display. The second one is softer and slower than the other, more conversational, but still a rising scale.

Early 20th century ornithologist Elliot Coues described the Prairie Warbler’s song to his friend, state ornithologist Edward Howe Forbush, as sounding, “like the plaint of a mouse with a toothache.”

One has to wonder about Coues’ experience with rodent dentistry, but perhaps he was only teasing his colleague. Forbush called the song the birds’ most distinguishing feature, the most unique among warblers.

“Once learned it is not forgotten,” he said.

It is a sound that the alert listener can pick out in Nantucket’s open spaces, whether in Head of the Plains or the central moors.

But what would a bird called a “Prairie” anything be doing on an island off the East Coast?

One thing to note is that Nantucket, thankfully, has a lot of land protected from development. But before breaking out into a chorus of “Home, Home on the Range,” it must be said that Prairie Warblers also partake of the usual ornithological misnomer, where names are sometimes the least descriptive thing about the bird.

“Never,” Forbush lamented in his three-volume study of New England birds, “was a bird more ineptly named.”

Prairie Warblers occupy a halfway point, neither open field nor dense forest, but between the two. They utilize transitional areas called “early successional habitat.” This refers to the way plants chase each other through the landscape.

We tend to think of them, particularly if they

happen to be trees, as immutable, firmly rooted. But plants move according to plans of their own, driven by environmental conditions. They have ways of getting started, then other plants come along, move in, and “succeed” the pioneers. The natural tendency in a stable or accreting beach is for dune grasses to share the space with lichens and other hardy species, such as the false heather Hudsonia, or poison ivy. These in turn are overtaken by Rosa rugosa, bayberry, bearberry and eventually by pine and scrub oak, with bayberry hosting the nitrogen-fixing bacteria that nourish larger plants.

Had the Prairie Warbler’s habits been extensively analyzed in the early years it might well have been named the “Shrub” Warbler. But as transitional zones move, so do birds. In ecological terms, Prairie Warblers are quick adaptors.

In the 19th and early 20th centuries, Prairie Warblers were found in very different locations than those where they breed today. They were unknown on Nantucket before the 1940s, with one record in September 1945 and another in September 1946. These were most likely migrating from other locations. They were common on Cape Cod.

It was a puzzling paradox to ornithologists of the day. Fields burned in the 19th century for shorebird hunting were growing up. Pines and

scrub oaks were moving in. Pine Warblers had arrived. Ground-nesting Eastern Towhees were also present on the island. But why so few Prairies?

At that time Massachusetts was near the northern end of their range. Prairie Warblers had moved east from Nebraska in the 19th century as forests were cleared. But they did not thrive in developed areas. Found at Mount Auburn in the 1830s, they had abandoned that locale by the 1860s.

But as agricultural fields were abandoned and the shrub cover began to move in, they spread out, east, south and north. But then, as forests re-grew, they shifted around again. In the 1950s they began breeding in mangrove swamps in Florida. They found their way to Maine and the southern parts of other New England states. They were also becoming more common on Nantucket. Two were banded here in 1958 and four in 1963.

Their numbers rose through the 1970s, with breeding confirmed in one block and considered probable in two more. In 1986 15 were banded in that year alone. But the birds were still on the move, appearing in some spots and absent from others.

They continue to shift around. The second “Breeding Bird Atlas,” completed in 2011, showed them absent from three previous blocks but with data suggesting possible nesting activity in three others.

They now breed across the southeastern United States, from southern Maine to northern Florida, the southern tip of Illinois to east Texas and Louisiana. They spend the winter mainly in the Caribbean, along with a few spots in Central America.

On migration some make a trans-oceanic trip, some a shorter over-land journey. High-flying nocturnal migrants, they are vulnerable to kills by towers and other structures. Like other neotropical migrants they also have to contend with bad weather, pesticides and other modern hazards. Although not considered endangered, they have decreased significantly and are on the watch list.

They arrive in our area in late May and begin nesting in early June. Females pick the territory and build the nests. Both parents forage and feed the young. They are unusually monogamous, unless a nest fails or a second family is started.

Their habitat preference also goes along with their food sources. They are primarily insecteaters. Studies have shown them to be somewhat

generalist about what they eat, consuming many types of insects. On the list of favorites are softbodied beetles, caterpillars of butterflies and moths, flies, spiders and insects in the order Hemiptera, the “true bugs.”

These comprise a large number of different species, but all specialize in sucking plant juices. Tree-hoppers, plant-hoppers, spittlebugs, aphids, plant lice, white fly, mealy bugs and scale insects are among Prairie Warblers’ menu items. This makes them welcome in orchards and gardens, where they glean bark and foliage for damaging pests.

As long as they are here, moving through the landscape, they help to maintain an always precarious balance. ///

Virginia Andrews writes the weekly “Island Bird Sightings” column for The Inquirer and Mirror, Nantucket’s newspaper since 1821, and is a regular writer for Nantucket Today.