Birds sensed deadly storm coming

In Mark Twain's classic book "The Adventures of Huckleberry Finn," the hero and his slave friend Jim are on the run. Rafting down the Mississippi River, they've come ashore to seek a place to be hidden and dry.

Jim tells Huck that "the little birds had said it was going to rain. … "

Henry Streby, who earned a Ph.D. at the University of Minnesota, has evidence in spades that Jim was correct. Birds do react to approaching weather, more so than anyone knew or believed.

Golden-winged warblers being studied by Streby and two graduate students apparently recognized an approaching severe weather system two days before it struck their study area in the Cumberland Mountains of eastern Tennessee last April.

Spring migrants, the birds reversed course upon sensing the storm, which was then about 600 miles to their west, fleeing several hundred miles southeast to avoid it. Interestingly, they roughly followed their usual fall and spring migration routes, individual for each bird.

When the weather cleared, the birds returned to the particular nesting territories they had abandoned. The round trips covered about 900 miles in five days.

Dr. Streby believes that the birds could sense infrasound produced by storms, sound at a level below the normal limit of our hearing, 20 cycles per second. (The infrasound frequency range is used to monitor earthquakes, among other things.)

"Avian sensitivity to infrasound is well documented in the scientific literature," Streby said in an interview. "The reasons they hear infrasound are not entirely understood, but its use during migration is one leading hypothesis."

The discovery of the birds' apparent sensitivity to weather that was days away was incidental to a tracking project involving the warblers. Geolocators had been strapped to the backs of the half-ounce birds to track their flights.

The birds that returned went from Tennessee to North Carolina and Georgia initially, then to the Florida panhandle. Tracking information indicates that one went as far as Cuba.

And how did the birds know when it was safe to return to Tennessee? Good question, said Streby. He assumes it's possible they could hear the storm moving.

For the Tennessee project, Streby and his associates put geolocators on 20 birds in the fall of 2013. Ten of them returned last spring, a normal 50 percent return rate, said Streby. Five of those 10, all males, made the evacuation migration.

Did other bird species also flee the approaching storm? "There is some evidence of this," Streby said, "but we can only speculate. There appeared to be fewer birds of many species during our surveys the day before and after the storm came through.

"We couldn't conduct surveys during the main storm because we also evacuated the area, hunkering down in a Waffle House and Holiday Inn during the worst of it," he said.

The paper, titled "Tornadic Storm Avoidance Behavior in Breeding Songbirds," has been published in the journal Current Biology. It quickly hit the news cycle online and off, here and in England.

Birds "predicting" tornadoes caught the eyes of many editors.

Streby estimates he had about three dozen telephone or e-mail contacts from journalists the first day or two after the paper appeared, plus one live radio interview with the BBC.

Currently, Streby is working with three researchers: graduate assistants Gunnar Kramer and Shawn Peterson, both involved in the Tennessee project and co-authors of the paper, and David Anderson, who supervised Streby's doctoral studies at the university.

The men are tracking the migration of golden-winged warblers that breed here. (Upwards of 40 percent of the world population of this species breeds in Minnesota.) They are using the same geolocators carried by the Tennessee birds.

Work was done with small numbers of birds in 2013 and 2014. This coming summer, a "few hundred" birds will be tagged, Streby said. Geolocators cost $220 each. The $70,000 in funding will come from many sources, he said.

It's an expensive project, he said, but it's important.

"Our broader goal is to link the breeding and wintering sites for all of these populations," Streby said. He seeks data that will inform conservation plans for both breeding and wintering sites.

He hopes his work with Minnesota's golden-wings will provide a model for studying other species.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.