Soaring can make flight cheaper than swimming, wild bird study finds
A condor’s travel costs were 79% lower than a swan’s of similar weight, but dependence on rising air limits when and where soaring birds can fly.
An Andean condor can travel at an energy cost 79% lower than a trumpeter swan of roughly the same weight by exploiting rising air, according to research showing that soaring flight can be cheaper than swimming.
The findings challenge the familiar textbook ranking of animal travel, which places swimming as the least energy-intensive option, followed by flying and then running. That ranking draws on laboratory measurements in swim tanks, flight tunnels and on treadmills; outside the laboratory, the atmosphere can transform the calculation.
Both the condor and the swan weigh about 11kg (24.2lb). Yet the condor flaps for just 0.1% of its flight time, compared with 99% for the swan, allowing it to cover each mile for about a fifth of the energy expenditure.
“To us, the sky might just look like background for flying animals, but it is an essential resource that can dramatically change the cost of a journey,” said first author Dr Martina Scacco, of Germany’s Max Planck Institute of Animal Behaviour.
Published in Science, the research is described as the largest study of flight costs in the wild and of the cost of long-distance travel in wild animals. It examines 50 bird and bat species, ranging in weight from about 12g to more than 11kg.
An international team of 68 scientists contributed data. The analysis combined 17 years of GPS and accelerometer tracking from 850 birds and bats belonging to 38 species with four years of radar data covering 12 size classes of smaller flying animals.
Accelerometers distinguished energy-intensive flapping from lower-cost soaring. The researchers then used GPS records to match stretches of flight with the winds and rising air encountered along the way. This allowed them to estimate the time each animal spent flapping or soaring and the energy used per metre travelled, known as its cost of transport.
Dr Scacco described the measure as “nature’s equivalent of miles per gallon”, providing a basis for comparing species and flight modes.
The animals fell into two broad groups: soarers and flappers. Among soaring birds, greater weight was associated with less flapping and a lower cost of transport.
The seven vulture species studied illustrated the scale of those savings. Together, they flew for 128,000 days and covered more than 4.3 million kilometres, enough to circle the Earth more than 100 times. They flapped for only 0.1% to 7% of their flight time. The heaviest, the Andean condor, flapped least.
Senior author Professor Emily Shepard, of Swansea University, said laboratory rules would assign condors and swans the same energy bill because they have the same body mass. Their different use of the atmosphere showed why that assumption did not hold in the wild.
“When flying animals have access to natural atmospheric flows, they have free fuel from the sky,” she said.
Those savings come with a constraint. Soaring birds depend on suitable rising air, and the larger they were, the more restricted they were in when and where they could fly.
Flapping birds, by contrast, used more energy but could cope with a wider range of conditions. Wind had the greatest effect on their transport costs: tailwinds helped, while headwinds increased resistance.
“Air can drive flight costs up as well as down for flappers,” said senior author Dr Kamran Safi, of the Max Planck Institute of Animal Behaviour. “What surprised us was just how much flappers operate in costly conditions where their fuel economy is really low.”
The researchers say theirs is the first study to account for the atmosphere’s role in the movement of so many wild animals on a global scale. They argue that the resulting framework could help conservation and management by improving predictions of flight costs and how climate change might affect where and when animals travel.
“If atmospheric conditions change, so too will the cost of flight, and therefore how animals decide to migrate and establish communities,” Dr Safi said. He hopes further work will clarify the trade-offs facing soarers and flappers, including energy-saving mechanisms that may not yet have been identified.