A new mystery has been discovered in the migratory behavior of birds! Many songbirds travel long distances during their annual migrations, and it makes sense for them to do everything they can to conserve their energy during these journeys. Researchers have guessed that, for this reason, they might pick an altitude with favorable winds and stick with it rather than climbing and descending repeatedly, but there has been little data to back this up. In a study published online on August 12, 2015 in an open-access article in The Auk: Ornithological Advances, presenting the first full-altitude flight data for migrating songbirds, Dr. Melissa Bowlin of the University of Michigan-Dearborn and colleagues used radio transmitters to track the altitudes of migrating Swainson's thrushes (Catharus ustulatus)(image) and were surprised to find that the thrushes actually made repeated altitude adjustments of more than 100 meters over the course of their nighttime migratory flights. The reasons for these altitude changes are not clear, but the researchers have a few theories. The new article is titled “"Unexplained Altitude Changes in a Migrating Thrush: Long-Flight Altitude Data from Radiotelemetry.” Funded in part by the National Geographic Society, Dr. Bowlin and her colleagues captured 9 Swainson's thrushes in a small forest fragment in Illinois during spring migration season in 2011-13, outfitted them with transmitters, and followed them with a radio-tracking vehicle to gather altitude data once they took off on a migratory flight. "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," explains Dr. Bowlin.
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