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Swainson’s Thrushes confound expectations, change altitude while migrating at night

Swainson's Thrush in North Dakota, May 8, 2009, by Matt Reinbold, Bismarck, ND, USA (Wikimedia Commons).
Swainson’s Thrush in North Dakota, May 8, 2009, by Matt Reinbold, Bismarck, ND, USA (Wikimedia Commons).

Researchers who study long-distance migrations have long assumed that songbirds conserve energy by flying like passenger jets — that is, that they ascend to an altitude with favorable winds and then remain there until it’s time to descend.

To test that assumption, Melissa Bowlin, an assistant professor of biology at the University of Michigan at Dearborn, led a team that attached tiny pressure and temperature sensors to seven Swainson’s Thrushes captured in a forest fragment south of Urbana, Illinois. Then, driving in a car outfitted with a radio receiver, they tracked the birds as they made nine flights through the night sky.

Swainson's Thrush wearing transmitter and antenna. Photo by J. Craves.
Swainson’s Thrush wearing transmitter and antenna. Photo by J. Craves.

The results, published online in the journal The Auk: Ornithological Advances, were surprising.

Rather than travel at a single altitude, the thrushes made repeated altitude adjustments of more than 100 meters over the course of their flights. The reasons for the altitude changes are not clear.

Bowlin speculates that the birds could have been responding to city lights or to thermals, rising columns of warm air, created by urban areas. The thrushes may also have been adjusting to small localized changes in atmospheric conditions or using some navigation strategy that we don’t understand yet. In any case, something more was going on than simply minimizing energy use.

“I really thought that the birds would mostly behave like commercial aircraft, ascending to a particular altitude, leveling off and cruising near that altitude, and then coming down just before they landed,” says Bowlin. “I was shocked when I made the first graph for the first bird, and thought it was an anomaly — maybe the transmitters weren’t working correctly! The more data we obtained, however, the more often we saw the up-and-down pattern to the birds’ flight.”

Bowlin says she became interested in flight altitudes while tracking birds equipped with heart-rate transmitters as part of her graduate research. “One of the things you notice as you track birds,” she says, “is that sometimes they seem to be flying quite high — you can get a clear signal even when you’re driving through an area with tall trees. Other times, the signal is blocked by trees and buildings, meaning that the bird is flying low.”

Understanding the altitude patterns of birds’ flights, she says, could eventually help conservationists reduce collisions with skyscrapers and communications towers and mitigate other threats.

The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

Read the paper

Melissa S. Bowlin, David A. Enstrom, Brian J. Murphy, Edward Plaza, Peter Jurich, and James Cochran. 2015. Unexplained Altitude Changes in a Migrating Thrush: Long-Flight Altitude Data from Radiotelemetry. The Auk: Ornithological Advances, Volume 132. DOI: 10.1642/AUK-15-33.1.

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