Jul

Tiny Tornado Chasers: Why Rare Songbirds Rely on Destructive Winds

A new Smithsonian paper posits that Swainson’s warblers have tornadoes to thank for ideal habitats.

Despite being found throughout the southeastern United States, Swainson’s warbler is a rare and secretive songbird that is tough to spot. (Gary Graves, NMNH)

With a drab palette of feathers, Swainson’s warblers (Limnothlypis swainsonii) may not look like the bell of the birder’s ball. But these songbirds are rare. And even when a fortunate birder picks up on the warbler’s whistling song in a southeastern swamp or young pine plantation, the bird often remains nearly invisible amongst dense thickets.

Research zoologist Gary Graves first spotted the elusive warbler as a teenager in the 1960s and has been captivated ever since. Graves, who is the curator of birds at the National Museum of Natural History, has spent the better part of three decades studying the songbird. He has trudged through swamps and canebrakes and scoured nearly every county and parish across the fifteen southern states where the warbler is known to breed.

Swainson’s warblers present a biological riddle — how could such a widely distributed songbird be so scarce? “I’ve been searching for the Rosetta Stone that explains both the strange distribution and the enduring rarity of this bird,” Graves said.

Gary Graves holds specimen above a tray of preserved birds

Research zoologist Gary Graves examines the type specimen that John James Audubon used to scientifically describe Swainson’s warbler in 1834. (Donald Hurlbert, Smithsonian Institution)

After sifting through decades of data, Graves noted that the warbler’s distribution and abundance was tied to catastrophic storms. His findings, published in March in the journal Ecology and Evolution, reveal that Swainson’s warblers are tiny tornado chasers who rely on the regenerating forests left in the wake of destructive winds.

A Tale of Two Warblers

In 1834, naturalist and artist John James Audubon described two new species of warblers based on specimens collected by John Bachman, a reverend in Charleston, South Carolina. Audubon named one of the new species after Bachman and the other new species after his friend and fellow ornithologist, William Swainson. The type specimens of both species used by Audubon to formally describe the species now reside in the museum’s Division of Birds.

Swanson's warbler specimen photographed along with color chart and labels

The type specimen of the Swainson’s warbler was collected in 1833 in Charleston, South Carolina, and described a year later by John James Audubon as part of his groundbreaking tome, Birds of America. Katie Sayers, Smithsonian Institution

For eons, Bachman’s and Swainson’s warblers co-existed in the swamps and thickets of giant cane that once covered the southeast. Over the last few centuries, many of these habitats have been replaced by settlements and agricultural fields. Deforestation driven by the sugar industry in Cuba greatly reduced the wintering ground available to Bachman’s warblers. The warblers faced the opposite problem in their breeding habitat. As old-growth forests were logged across the southeastern U.S., millions of acres of regenerated forest opened. While this habitat was optimal for Bachman’s warblers, the birds had trouble locating one another in this vast tract of territory.

Bachman’s warbler populations continued to decline, and the last singing males were observed in the 1960s. The species was declared extinct by the Fish and Wildlife Service in 2023. “That’s shocking,” said Graves, who remembers purported sightings of Bachman warblers taking on a mythic quality amongst birders in his native Arkansas. “It’s the only songbird species that has actually been declared extinct in the continental United States.”

Swainson’s Warbler has a larger wintering range in the Caribbean basin than its extinct brethren, but Graves still dreads the same thing happening to it. This is why he has devoted so much of his career to “figuring out what makes the bird tick.”

A Welcome Whirlwind

Graves has learned a lot by closely observing Swainson’s warblers in the field. For instance, these birds are extremely territorial. Despite their diminutive size (most weigh around half an ounce), the males defend large breeding territories that typically cover 15-20 acres. And they will ruthlessly protect their territory if another male encroaches. “They’re little Napoleons,” Graves said.

Do not be fooled by the diminutive size of a Swainson’s warbler. These songbirds will aggressively safeguard their dominions, which can cover many acres. (Gary Graves, NMNH)

Zooming out has revealed even more about the birds. As he mapped out the various locations where he observed Swainson’s warblers across the southeast, a geographic pattern took shape: the birds were clustered throughout a region known for destructive tornadoes.

Most people associate tornadoes with the southern plains of Texas and Oklahoma, where cool winds descending from the Rocky Mountains mingle with warm, low-lying winds blowing in from the Gulf of Mexico. This mixture sparks volatile winds throughout a region that is popularly known as “Tornado Alley.”

But this alley appears to be moving eastward. In recent decades, high-powered tornadoes have become more common in the southeastern United States, especially in an area extending from eastern Texas to Alabama. This tornado-battered region is a stronghold for Swainson’s warblers, who are adept at adapting to environmental upheaval.

Side-by-side comparison of two maps of the southeastern United States

A map depicting known Swainson’s warbler breeding territories between 1988 and 2014 (left) and a map of tornadoes (depicted in red) that have occurred in the southeastern United States since 1950 (right). Graves noticed an overlap between breeding sites and elevated tornado activity in states like Mississippi. (Graves, Ecology and Evolution)

As tornadoes bulldoze through southeastern forests, they blow down trees and trigger regeneration of dense thickets of shrubs, vines, and young trees. As the birds fly back from the Caribbean in the spring, they encounter the regenerating tracks of past tornadoes, many of which are several miles long. “There’s a shifting mosaic of disturbance and regeneration, and these bird populations are tracking this through time,” Graves said. The natural history of Swainson’s warbler seems inextricably linked to catastrophic storms.

States bordering the Gulf of Mexico are prime real estate for Swainson’s warblers. While tornadoes rip through the regions north of the coastal plain from Texas to Alabama, hurricanes hammer the coasts. This clears even more space for the birds to colonize.

It is likely that other species of birds also rely on forest disturbance generated by tornadoes and hurricanes. But these storms are far from ideal for some other species, particularly our own. “The Gulf Coast region is one of the world’s great places to be a Swainson’s warbler,” Graves said. “But it’s much less ideal for humans in the paths of tornadoes and hurricanes.”

Jack Tamisiea is a Science Communications Specialist at the Smithsonian’s National Museum of Natural History. In addition to covering all things natural history for the museum’s blog, Smithsonian Voices, he tracks media coverage and coordinates filming activities for the museum’s Office of Communications and Public Affairs. Jack recently completed his masters in science writing at Johns Hopkins University and his writing has appeared in the New York Times, Scientific American, National Geographic and other science-focused publications. In his free time, he loves exploring the outdoors with a sketchbook and camera. You can read more of Jack’s work at https://jacktamisiea.com.

This post was originally published by the Smithsonian magazine blog, Smithsonian Voices. Copyright 2024 Smithsonian Institution. Reprinted with permission from Smithsonian Enterprises. All rights reserved. Reproduction in any medium is strictly prohibited without permission from Smithsonian Institution.