Now, at the end of summer, when thrush nightingales leave Sweden for Southern Africa, they don’t fly flat out. Instead, they cruise at a steady pace – and according to the study from Lund, that’s no coincidence.
Migratory birds spend hundreds of hours on the wing. But until now, researchers have assumed that the efficiency of turning energy into flight was constant across speeds. Using wind tunnel experiments with thrush nightingales, the researchers at have now shown otherwise.
“We found that thrush nightingales are not equally efficient flying at all speeds. Their efficiency peaks at an intermediate flight speed – around 7–8 metres per second – not at very slow or very fast speeds,” says Pablo Macías Torres biology researcher at Lund University.
Earlier models suggested birds convert about 23 per cent of their metabolic energy – the energy that the body releases from nutrients through digestion – into flight. The new results show the maximum is closer to 15 per cent, and crucially, it depends on how fast the bird is flying.
“Our study shows that energy efficiency varies and reaches its maximum at intermediate speeds – so not all speeds are equal in terms of efficiency,” says Pablo Macías Torres.
A sweet spot in the sky
These new findings not only give scientists their best estimates yet of bird flight energetics. They also help researchers build more accurate models of bird flight and help explain how small birds manage their transcontinental journeys.
“Understanding bird flight efficiency helps us appreciate the remarkable physiological adaptations that enable birds to conquer the air and perform extraordinary long-distance migrations, such as the one thrush nightingales make while winging steadily south,” says Pablo Macías Torres.
