Â鶹ÒùÔº

Now, at the end of the summer, when the thrush nightingales leave Europe for southern Africa, they do not fly at full speed. Instead, they maintain an even pace—and according to a new in Current Biology from Lund University, this is no coincidence.

Migratory birds spend hundreds of hours in the air. Up to now, researchers have assumed that the efficiency of converting energy into flight power was constant regardless of speed. Using wind tunnel experiments involving nightingales, the researchers have now shown that this is not the case.

"We have discovered that the nightingales are not equally efficient at all speeds. Their efficiency is highest at an intermediate flying speed—approximately 7–8 meters per second—thus neither at very low nor very high speeds," says Pablo Macías Torres, biology researcher at Lund University.

Previous models suggested that birds convert approximately 23% of their metabolic energy—the energy the body releases from nutrients through the metabolic process—into flight. The new results show that the maximum value is closer to 15%, and above all, it depends on how fast the bird flies.

"Our study shows that the energy efficiency varies and reaches a maximum at an intermediate speed—so all speeds are quite simply not equally efficient," says Macías Torres.

The new results not only give the researchers the best estimates yet of birds' flight energetics but also help the researchers to construct more exact models of bird flight and explain how small birds are able to make their transcontinental journeys.

"Understanding birds' flight efficiency helps us to assess the remarkable physiological adaptations that make it possible for birds to conquer the air and complete extraordinary long-distance flights, such as the nightingales' impressive flight south," says Macías Torres.