Migratory Birds Energy Use Related To Physical Characteristics And Climate

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Main Category: Veterinary
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Article Date: 13 May 2008 - 17:00 PDT

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In a new study published in the open-access journal PLoS ONE, researchers from Princeton University, the University of Montana, and the German Max Planck Institute for Ornithology shed some light on the energy-saving abilities of migratory birds and how climate change may affect their energy usage. It was found that pointed wings, a smaller weight to wing area ratio, and systematic avoidance of high winds and atmospheric turbulence enable migratory bird to save a lot of energy during their long-haul flights. This is the first time that ornithologists have demonstrated this phenomenon in free-flying wild birds. If climate change causes the high winds and atmospheric instability that climatologists expect, it could have a crucial effect on the energy usage of small migratory birds.

Biologists Melissa Bowlin and Martin Wikelski found that migration requires an enormous expense of energy. There is a 50% increase in energy expenditure in small songbirds during migration. The authors' conclusions came from an investigation of energy use in Swainson's thrushes (Catharus ustulatus), a common bird used to study migration. They are small songbirds, weighing about 30 grams, which can travel 4800 km from their winter home in Panama to their breeding grounds in Canada. It has been shown that the one-way trip requires some 3.2 million wing beats and 1300 kJ of extra energy. Because the songbirds do not have the ability to store much energy and cannot take long stops along the way, their trip must be extremely efficient.

Heart rates were measured during migration, and thrushes that had rounder wingtips and greater body mass compared to wing area were found to have higher heart rates. Thus, birds with these characteristic used more energy during the flight. Bowlin and Wikelski also found that as wind speed increased, a thrush's heart rate would increase; this was noted for both tailwinds and headwinds. The birds demonstrated heart rate increases as atmospheric instability increased as well. The researchers argue that if small migratory birds avoid high winds and turbulence, they should be able to reduce the energy expenditure during migration.

Wikelski says that, "We think that climate change may have severe consequences for small intercontinental migrants."

Researchers believe that even without considering the impact of a changing climate, migration is the most perilous event of the year for small songbirds. It is possible that the frequency and severity of dangerous high winds and turbulence increases as birds pass over continents. "As this affects the energy budgets of the birds, the need for and locations of stopover sites would change," notes Wikelski. The potential for birds to arrive too early or too late can lead to a disruption of the breeding and wintering system. If they run out of energy while flying over the Sahara Desert or the Atlantic Ocean, the birds may never reach their destinations.

An evolutionary explanation is suggested to explain the pointed wingtips of migrating bat, bird, and insect species since these traits would reduce energy expenditure during flight. This study, testing free-flying migrants, helped to clarify the role of these traits in survival.

Lead author Melissa Bowlin explains that, "Until now, research on the energetics of small birds in flight was largely confined to individuals flying in artificial wind tunnels. Now, we can study small, free-flying birds making actual migratory lights in the wild." The thrushes were tracked during a spring migratory flight over the central USA using a temporary radio transmitter fastened to the bird's back. The instrument, weighing less than one gram, relayed heart beat information to an antenna on the ground.

Pointed Wings, Low Wingloading and Calm Air Reduce Migratory Flight Costs in Songbirds
Bowlin MS, Wikelski M
PLoS ONE (2008). 3(5): e2154.
doi:10.1371/journal.pone.0002154
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About PLoS ONE

PLoS ONE is the first journal of primary research from all areas of science to employ both pre- and post-publication peer review to maximize the impact of every report it publishes. PLoS ONE is published by the Public Library of Science (PLoS), the open-access publisher whose goal is to make the world's scientific and medical literature a public resource.

About the Public Library of Science

The Public Library of Science (PLoS) is a non-profit organization of scientists and physicians committed to making the world's scientific and medical literature a freely available public resource. For more information, visit http://www.plos.org

Written by: Peter M Crosta
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