Wandering Warblers

These featherweight frequent fliers go the distance.

“Prothonotary warbler” is a big mouthful of a name for a tiny handful of a bird. A migratory songbird that can be found in spring and summer nesting in the swampy wetlands of eastern Virginia, it sports a brilliant lemony-yellow plumage on its breast and head that make it a highly visible seasonal resident in the Commonwealth. 

As an adult, a prothonotary warbler weighs an average of only 13 to 14 grams—or about as much as a half-cup of Cheerios. Nevertheless, at summer’s end every year, these warblers take flight for a journey of thousands of miles over land and water to winter in Central and South America—only to turn around and head north again come spring.

The prothonotary warbler is also known more descriptively, if prosaically, as the golden swamp warbler. It got landed with its more multisyllabic moniker supposedly due to its bright yellow color, which (also supposedly)was the color of robes or hoods once worn by a group of official scribes or clerks in the Catholic church (“prothonotary” comes from the Greek and means “first scribe”). Frankly, this widely-repeated origin story strikes the casual researcher as pretty darn esoteric for a swamp-dwelling New World bird. On the other hand, no more convincing story is on offer, and this is the one the Audubon Society is going with, so who are we to argue?

According to Catherine Viverette, an assistant professor in the Virginia Commonwealth University Center for Environmental Studies, the prothonotary warbler’s yellow color is something the birds can’t “manufacture” themselves. Rather, they have to get it from their food—particularly from the aquatic mayflies that the birds feed heavily on during the high-energy-demanding nestlings season. A healthy, well-fed warbler’s bright feather color is known as an “honest signal,” the avian-world equivalent of a Harvard MBA, advertising a bird that’s got the goods as a potential mate.

Researchers at VCU have been studying the prothonotary warbler for some 30 years, since biology faculty members Charles and Leann Blem launched a project to install nesting boxes in tidal freshwater wetlands areas along the James east of Richmond—a favored habitat for the birds. The development of microweight leg-band-attached geolocators have helped researchers gain interesting insights into the habits of these birds, including the truly astonishing distances they travel. Data from VCU research has confirmed that at least some of the birds that nest in Virginia take a direct route south, which carries them across long stretches of open Caribbean water. (One warbler tracked by other researchers in Louisiana traveled nearly 5,000 miles between departing its summer breeding grounds in that state in mid-August and returning in late March, and may have traveled as far south as Colombia.) Data also suggest that while the birds seem to take their dawdling time in the fall, come spring, the males in particular beat it north with remarkable speed to claim the best nesting spots, sometimes making the trip in several weeks, says Viverette.

Perhaps even more impressive, the birds, on returning, will make their home not simply in the general area where they hatched but sometimes in the very same box or an immediately neighboring one. VCU researchers have seen this “natal fidelity” demonstrated for as many as 6-8 successive years in some of the warblers they have tracked.

On other matters of fidelity, however, your prothonotary warbler is not a model of virtue.  Prothonotary warblers may lay two clutches of eggs in the course of a breeding season—and for the first time this year, says Viverette, there were documented triple-brooders in the James River nest boxes. But while mating pairs are “socially monogamous,” raising young birds together, “the birds are not as monogamous as we thought they were,” says Viverette. Not only might the eggs or nestlings in a particular nest have more than one father, but occasionally females are laying their own eggs in another female’s nest, she says. “This had never been reported in a warbler, and we certainly were not expecting to see it.”

Like so many socialites readying for the resort season, prothonotary warblers lay out a new wardrobe in late summer, molting all their feathers before heading back to the tropics, where for the winter they inhabit mangrove forests—“one of the most endangered habitats worldwide,” notes Viverette. 

Because they are so visible, prothonotary warblers serve as a kind of “canary in the coal mine” for problems, such as habitat loss, that might be affecting migrating birds in general. A recently developed collaboration between VCU and researchers in Panama is looking to better understand these warblers across their range, as a means to preserve habitats that not only birds depend upon. “The same things these birds need, we need,” says Viverette. “We can use these birds to link our communities and share our support and challenges and solutions.”

This article originally appeared in our June 2017 issue.

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