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Migratory birds burn unexpected amounts of protein

A research team led by the University of Massachusetts at Amherst has recently made a surprising discovery. The experts found that songbirds which migrate over long distances from their breeding to their wintering range fuel their flights by burning not only a massive amount of fat, but also a significant amount of the protein that makes up lean body mass such as muscle.

Scientists previously assumed that migratory birds only ramp up protein consumption at the end of their journeys, since they would need to use their muscles for wind-flapping. The current study reveals that protein is burned very early on in the migration, which suggests that birds’ metabolism is more complex and surprising than experts thought.

“Birds are amazing animals,” said study lead author Cory Elowe, a postdoctoral fellow in Biology at UMass Amherst. “They are extreme endurance athletes; a bird that weighs half an ounce can fly, non-stop, flapping for 100 hours at a time, from Canada to South America. How is this possible? How do they fuel their flight?”

Since a long time, biologists have assumed that birds fuel such extraordinary feats of endurance by burning fat reserves. Yet, although they do burn fat at a consistent rate throughout their flights, they also burn protein at an extraordinary rate quite early in their flights. Moreover, the rate at which they burned protein appeared to taper off as the duration of the flight increased.

“We knew that birds burned protein, but not at this rate, and not so early in their flights. What’s more, these small songbirds can burn 20 percent of their muscle mass and then build it all back in a matter of days,” said senior author Alexander Gerson, an associate professor of Biology at UMass Amherst.

The experts collaborated with the bird banding operators at the Long Point Bird Observatory in Ontario, along the northern shore of Lake Erie, where millions of birds gather each fall on their journey to their wintering grounds. Here, the researchers captured 20 blackpolls (small songbirds that travel thousands of miles during their migrations) and 44 yellow-rumpled warblers (a species of shorter distance migratory bird) and took them to the Advanced Facility for Avian Research at Western University, which has a wind tunnel constructed specifically for observing birds in flight.

After measuring their fat and lean body mass before flight, the researchers led the birds free in the wind tunnel at sunset (since these birds naturally migrate at night). When the birds decided to rest – sometimes after as long as 28 hours – the scientists measured once more their fat and lean body mass content and compared the results with those obtained before the flight.

“One of the biggest surprises was that every bird still had plenty of fat left when it chose to end its flight. But their muscles were emaciated. Protein, not fat, seems to be a limiting factor in determining how far birds can fly,” Elowe reported.

Further research is needed to better understand why the birds are burning such vast amounts of protein so early in their flight. “How exactly is it possible to burn up your muscles and internal organs, and then rebuild them as quickly as these birds do?” Gerson asked. “What insights into the evolution of metabolism might these birds yield?”

Finally, these findings could also shed more light on another feat of endurance nonmigratory birds which overwinter in cold areas engage in: shivering – a behavior that helps them to keep warm during winter. “Do birds fuel their winter shivering spells the same way? And as the world warms, which method of coping with the cold – shivering or migrating – might be the better option for survival?” said Elowe.

The study is published in the journal Proceedings of the National Academy of Sciences.

By Andrei Ionescu, Staff Writer

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