The High Speed Chase to Find Why Birds Change Altitudes

By Joshua Rapp Learn

Swainson's Thrushes have been found to make mysterious altitude changes during nighttime migratory flights.

Image credit: J. Craves

It was generally around 4 a.m. when the car chase would start to get old. Melissa Bowlin would be thinking, “Please just land, bird — that spot looks great over there” as she operated the equipment in the backseat.

Bowlin, an assistant professor at University of Michigan-Dearborn, was chasing down birds at unearthly hours as part of a study to determine why migratory songbirds such as the Swainson’s thrush (Catharus ustulatus) made sudden changes in altitude, dropping or climbing hundreds of meters during flight. The study was recently published in The Auk: Ornithological Advances.

“We expected the birds to fly like aircraft, going up to a certain height then staying there,” Bowlin said.

To conduct the study, she and the coauthors, many of them her students, captured Swainson’s thrushes, which generally migrate at night, attached analog radio transmitters and released them again where they caught them in Illinois. The devices sent out binary codes that told researchers the temperature and air pressure, which they could then use to calculate altitude. The researchers would then stake out nearby in their vehicle— often in a Dairy Queen parking lot — and wait for the night flight.

Researchers used vehicles outfitted with antenna to track songbirds as they migrated northwards. Melissa Bowlin said the vehicles, such as this one in Illinois, looked like a “classic suspicious vehicle” to police, who stopped to talk to them several times while they were parked at the side of the road with antenna and lights on. Image credit: Melissa Bowlin

Researchers used vehicles outfitted with antenna to track songbirds as they migrated northwards. Melissa Bowlin said the vehicles, such as this one in Illinois, looked like a “classic suspicious vehicle” to police, who stopped to talk to them several times while they were parked at the side of the road with antenna and lights on.Image credit: Melissa Bowlin 

As soon as the thrushes began their migration — inevitably when one of the students was getting ice cream — the researchers would tear off in their car in pursuit — the antenna they’d fixed to gather the blips of data had a limited range, only 10 kilometers when the birds were flying high, so they needed to stay reasonably close to the birds as they moved around Illinois, Wisconsin and sometimes Michigan.

The thrushes averaged around 50 kph and accelerated greatly when catching tailwinds, making them so difficult to chase that the team amassed a few speeding tickets in their night pursuits.

“We are often mistaken for tornado chasers,” Bowlin said, partly due to the antenna that stuck out of the roof of the various vehicles they used during the study period. “We do try our best not to speed, but occasionally we have.”

Method in the Motion

The way the birds moved up and down created new questions, for which Bowlin said there are several hypotheses. The most common idea is that they are responding to fluctuations in the atmosphere — things like wind speeds and thermals left over from the daytime. Or it could have something to do with the way they navigate during their migration from Mexico and Central America to Canada during the summer.

Another reason could be problematic though, as it may have implications for so-called tower strike — the term used for birds that are killed running into skyscrapers or other tall structures.

“Millions and millions of birds are killed every year from tower strike,” Bowlin said. Lights in oil rigs, communications towers and lighthouses are known to attract birds, and could make them descend toward the bright spots. “We know they hit skyscrapers much more often when the skyscrapers are lit than when they are dark.”

She said that finding out why the birds change altitudes could potentially give conservationists a clue toward helping to reduce these deaths in declining songbird populations.

“If they are being attracted by lights, then maybe we need to use different lights that won’t attract them,” Bowlin said. Possible solutions could also include moving or planning for better placement of communication towers.

“All of those things are necessary if we want to reduce tower strike in different birds.”

Joshua LearnJoshua Rapp Learn is a science writer at The Wildlife Society.

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