For Thrush, Flight Less Taxing Than "Rest," Study Says

By John Roach
for National Geographic News
June 11, 2003

Over the course of their migration from Panama to Canada, New World Catharus thrushes spend twice as much energy slurping worms, munching snails, and heating their bodies than they do actually flapping their wings in flight, according to new research.

Henk Visser, a zoologist at the University of Groningen in The Netherlands, said that although this seems counterintuitive, it makes sense.

"It is known that birds stop on one site and stay there for a couple of days or weeks to accumulate body reserves and then migrate to another area and land where they sit and accumulate body resources again," he said.

Although birds almost always spend more energy per hour in flight than they do keeping themselves fed and warm, they spend much more time over the course of the entire migration on the ground than they do in the air.

Calculated for the entire migration of free-flying Swainson's (Catharus ustulatus) and hermit (C. guttatus) thrushes, Visser and his colleagues found that actual flight only represents 29 percent of total energy expenditure.

The rest of their energy is spent trying to keep warm as they build up the energy reserves required for a night of flight.

The researchers report their findings in the June 12 issue of Nature. Similar energy costs of migration have been calculated from theoretical models and wind tunnel experiments, but this is the first time it has been calculated in free-flying individuals.

"This is the first confirmation of these models and our values indicate the models were accurate within 10 percent," said Visser.

John Rappole, a research scientist with the Smithsonian Conservation and Research Center in Front Royal, Virginia, said that he is surprised songbirds spend twice as much energy at rest than they do in flight over the course of migration, but he is not surprised that stopover is energetically costly.

"As the authors note, their findings are comparable both to wind tunnel studies and models based on earlier work," he said. "What is different is that they were able to recapture birds after a migratory flight."

Hanging with Birds

Continued on Next Page >>




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