Flying high (and low): Bizarre migration habits perplex bird researchers

ThrushA new mystery is unfolding in the migratory behavior of birds.

Billions of songbirds travel long distances during their annual migrations—up to 18,600 miles in a single year. Researchers have long believed that, in order to conserve energy, these birds might pick an altitude with favorable winds and stick with it rather than climbing and descending repeatedly.

A new study by University of Michigan-Dearborn Assistant Professor of Biology Melissa Bowlin shows that’s not necessarily the case. Bowlin and her colleagues recently completed the first full-altitude flight data for migrating songbirds and found that Swainson’s thrushes (Catharus ustulatus) made repeated altitude adjustments of more than 100 meters over the course of their nighttime migratory flights.

“We expected the birds to behave like commercial aircraft, ascending to a particular altitude, leveling off and cruising near that altitude, and then coming down just before they landed,” Bowlin said. “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.”

The team of researchers equipped the birds studied with radio transmitters to measure both air temperature and air pressure. From these two sets of data, the team could determine the altitude of the birds and discovered that thrushes change altitude midflight both dramatically and often: The graphs in Bowlin’s paper, which represent the birds’ altitude, show multiple peaks and valleys over periods of several hours.

Bowlin said the reasons for the altitude changes are not clear, but speculated that a number of factors could be in play—the birds could be responding to the lights of cities or the rising thermals they create, adjusting to small, localized changes in atmospheric conditions, or using a navigation strategy researchers don’t yet understand.

Very little is known about flight altitude at this point because most studies of bird migration focus on routes or behavior at sites where birds stop to refuel, Bowlin said. But studying flight altitude could positively impact bird welfare as well as the airline industry.

“Every year, $1.28 billion is lost globally as a result of bird-plane collisions, and millions of birds die from flying into skyscrapers, communications towers and other structures,” Bowlin said. “Better understanding the altitude patterns of birds’ flights could eventually help us reduce these occurrences.”

Bowlin’s research—which was funded in part by the National Geographic Society—is published in The Auk: Ornithological Advances. The manuscript, “Unexplained altitude changes in a migrating thrush: Long flight altitude data from radiotelemtry,” is available online at The Auk’s website.

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