Flying is extremely energy-intensive for birds, which is why they use various saving techniques. One of them is wave flight, in which fluttering and gliding alternate. What the energy balance of this flight technique looks like has now been examined more closely by biologists in northern bald ibises. In fact, the results confirmed that the northern bald ibis significantly reduced their energy requirements with this flight technique. However, how efficient wave flight is depends on the duration of the gliding phase.
Many birds fly long distances during their seasonal migrations in fall and spring. They require large amounts of energy to do this, because the constant up and down of the wings burns more calories than any other form of locomotion. Many bird species therefore try to save energy by using special flight techniques. Albatrosses, for example, can soar through the air for hours and days without needing to land or rest, through sophisticated use of air currents and thermals. Flying in a formation or in a swarm can also help conserve energy.
Northern bald ibis as test objects
Another technique is to alternate between flapping and gliding. During this flight, the birds move through the air in a kind of wave: they save energy during the gliding phases, but sink slightly. In the subsequent fluttering phase, it gains altitude again, but also uses more energy than when gliding. “After just one second of gliding, the heart rate drops significantly. On the other hand, the heart rate increases steadily during the first 30 seconds during the flapping of the wings, before stabilizing afterwards,” explains Ortal Mizrahy-Rewald from the Research Institute for Wildlife Ecology at the University of Veterinary Medicine, Vienna.
In order to find out how much energy a bird saves with this type of wave flight, the biologists working with Mizrahy-Rewald examined human-reared northern bald ibis (Geronticus eremita) while flying. The northern bald ibis is a species of ibis, about the size of a goose, that was once common in Europe. Due to intensive hunting, however, it became extinct in Central Europe in the 17th century. As part of a European project, the northern bald ibis is to be reintroduced as a true migratory bird in Central Europe, Spain and Italy. For this purpose, the birds are raised, prepared for independent life and then released into the wild.
Gliding too long is also not good
For their study, the researchers equipped 29 of these northern bald ibises, which were trained to follow an ultralight aircraft, with data loggers. The small sensor backpacks registered the position of the birds, the flapping of their wings, the dynamic acceleration of the whole body and the heart rate as a measure of energy consumption. The evaluation showed that the northern bald ibis saves a lot of energy by alternating between flapping and gliding: “With a gliding percentage of around 20 percent, we measured a maximum saving of eleven percent based on the heart rate,” explains senior author Thomas Ruf from the Vetmeduni Vienna.
According to the scientists, this confirms that intermittent flight is energetically advantageous and can significantly reduce the high effort involved in flying. However, the decisive factor here is the duration of the individual flight phases and their relationship to one another. “With higher gliding percentages, however, the additional energy savings were negligible.” If the gliding phases are too long, the flight speed also drops, so that the bird has to remain in the air longer to reach its destination. The birds therefore have to find out where the optimal change rhythm lies for their size, mass and situation.
Source: University of Veterinary Medicine, Vienna; Article: Frontiers in Ecology and Evolution, doi: 10.3389/fevo.2022.891079