A team of researchers from the University of Massachusetts Amherst in the United States recently unpacked a migratory-bird mystery with the aid of a wind tunnel and a flock of birds. These birds traverse thousands of kilometers twice each year, moving from breeding grounds to wintering areas, and they do so by burning large amounts of protein and fat that build their bodies, including muscle, during the initial stages of flight.
The discovery challenges the long-standing view that migratory birds rely mainly on fat stores for their long journeys. The researchers propose that birds may ramp up protein use early in flight because muscle power is essential for sustained wingbeats, not just fat reserves storing energy for the trip.
The blue tit, observed during the study, helps illustrate the scale of these small travelers and their remarkable endurance. “Birds are extraordinary creatures”, notes Cory Elowe, the study’s lead author and a postdoctoral biology researcher at UMass Amherst. “They are endurance athletes of the air; a bird weighing only 15 grams can fly hundreds of hours nonstop from Canada to South America. How do they manage such a journey?”
Protein as a starter fuel in flight
For years, biologists believed fat was the main energy source for extensive flights. Fat remains a crucial part of the migratory energy mix. Elowe explains that the birds studied burned fat at a relatively steady rate throughout their flights. Yet the research also uncovered a surprisingly high rate of protein consumption early in flight, with protein use tapering as flight duration increased.
Alexander Gerson, associate professor of biology at UMass Amherst and a co-author of the paper, describes this as a fresh angle on the topic. “No one has measured protein burning in such migratory flights before”, he says.
“We knew birds burn protein, but not at this speed or this early in their journey”, Gerson adds. “Some small species can lose up to roughly 20 percent of their muscle mass and recover it within days.”
To advance the study, Elowe collaborated with avian researchers from the Long Point Bird Observatory in Ontario, along the northern shore of Lake Erie. Each autumn, millions of birds pass through this region on their migration, including the black warbler, a tiny songbird that covers thousands of kilometers each season.
Using a wind tunnel to observe flight
The scientists captured twenty striped warblers and forty-four crowned warblers, then transported them to the Western University’s Advanced Bird Research Center, which houses a specialized wind tunnel. This facility is designed to closely monitor flying birds under controlled conditions.
Before flight, fat and lean body mass were measured. The birds were then placed into the wind tunnel as daylight faded. Since many migratory birds fly by night, the team stayed awake for twenty-eight hours, watching for when a bird chose to rest. At that moment, the birds were weighed again to compare pre-flight and in-flight body composition.
“One of the biggest surprises was that even when each bird ended its flight, there was still a substantial amount of fat left”, says Elowe. “But their muscles had weakened. It appears that protein, not fat, limits how far birds can fly.”
Researchers acknowledge that the precise reason for heavy early use of protein remains to be fully understood, but the results open multiple avenues for future investigation into avian metabolism and energy allocation during migration.
A series of birds were tested in a wind tunnel at Western University under the supervision of the research team.
“How is it possible to burn muscles and internal organs and then rebuild them so quickly”, Gerson asks. “What can these birds reveal about the evolution of metabolism and endurance in nature?”
Elowe also considers the cold-weather dimension. Non-migratory birds often warm themselves by shivering in winter, and as climate patterns shift, researchers wonder which strategy—shivering or migrating—will better support survival in changing conditions.
For readers seeking more technical details, the study appears in the Proceedings of the National Academy of Sciences, a landmark exploration of the timing and extent of protein use during migratory flights. This work is part of a growing effort to understand energy strategies in avian migration.
—
Note: The environment department does not publish contact details within this summary. This article reflects published research and aims to convey findings clearly and accessibly for a broad audience.