Adult male condor in Patagonia showing the soaring flight that allows birds to flap remarkably little despite their large size. (Facundo Vital via SWNS)
By Stephen Beech
Flying really is the most energy-efficient form of travel for birds, reveals new research.
The largest ever study of flight costs in the wild finds that riding the air can make flying cheaper than swimming — overturning a textbook ranking.
Study first author Martina Scacco explained that an Andean condor and a trumpeter swan both weigh about 11 kilograms (24.2 pounds), yet the condor covers each mile of flight for about a fifth of the swan’s energy expenditure.
She says the difference is the air as the condor rides rising currents and flaps for 0.1% of its flight time, while the swan flaps for 99%.
The study of 50 bird and bat species in the wild showed that once the atmosphere is taken into account, flying can be the cheapest form of travel.
Scacco, from the Max Planck Institute of Animal Behavior (MPI-AB), Germany, said: “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.”
Textbooks rank the ways animals travel by cost with swimming the cheapest, followed by flying, while running is the most expensive.
That ranking comes from lab measurements of animals in swim tanks, in flight tunnels and on treadmills.
(Photo by Veronika Andrews via Pexels)
But the new research, published in the journal Science, found that the rankings change outside the lab.
The study is the largest to examine the cost of long-distance travel for wild animals.
It combines 17 years of GPS and accelerometer tracking from 850 birds and bats of 38 species with four years of radar data covering 12 size classes of smaller flyers.
The animals ranged in size from about 12 grams to more than 11 kilograms.
An international team of 68 scientista contributed data to the project.
Study senior author professor Emily Shepard said: “The laboratory rules tell us that condors and swans must have the same energy bill because they have the same body mass.
“Yet seeing how condors soar and swans flap, we know that clearly there must be a difference.”
She said the study put a price on that difference with the condor’s cost of transport 79% lower than the swan’s, making soaring flight cheaper than swimming.
Shepard, from Swansea University, said: “When flying animals have access to natural atmospheric flows, they have free fuel from the sky.
“This study exposes the mistakes we make when we ignore the real-world conditions that animals have been responding to for millennia.”
The research is the first to factor in the role of atmosphere in the movement of so many wild animals at a global scale, offering a key to conservation and management of aerial species.
Senior author Kamran Safi, of MPI-AB, said: “If atmospheric conditions change, so too will the cost of flight, and therefore how animals decide to migrate and establish communities.”
Adult male condor in Patagonia showing the soaring flight that allows birds to flap remarkably little despite their large size. (Alvaro Moya Riffo via SWNS)
He says the findings offer a new framework for predicting the energetic cost of flight, and how climatic changes might affect where and when animals travel in the future.
The team used accelerometers to tell apart high-cost flapping from low-cost soaring.
With GPS data, they matched each stretch of flight with the wind and rising air the animals met on the way.
That enabled them to calculate the time individual animals spent flapping or soaring along a stretch of flight, and to approximate how much energy they spent per meter traveled — their cost of transport.
Scacco said: “We boiled the differences down to fuel economy, nature’s equivalent of miles per gallon.”
She said that allowed the scientists to compare transport costs across species and flight modes, “like we compare freight costs of sea mail or air mail.”
When compared, the 50 species fell into two main groups: soarers and flappers.
Among the soaring birds, size had a marked effect on transport cost: the heavier a soaring bird, the less it flapped and therefore the lower its cost of transport.
Soaring vultures were a vivid example.
The seven vulture species in the study flew for a combined 128,000 days and more than 4.3 million kilometers, enough to circle the planet more than 100 times.
They achieved that while flapping only 0.1% to 7% of flight time.
And the heaviest vulture, the Andean condor, flapped least of all.
Shepard says they do so exploiting atmospheric energy.
(Photo by Veronika Andrews via Pexels)
She said: “Soaring birds rode rising columns of air, which lowered their cost of transport the most like us riding the elevator to the top floor.
“Soarers are masters at extracting energy from the environment. When conditions are right, they can drive their costs down to very little.”
But she said conditions are not always right.
The larger the soaring bird, the more restricted they were in terms of when and where they could fly.
However, flapping birds use high-energy movement to cope with more variable conditions.
Wind had the most significant effect on their transport costs, with tailwinds providing support and headwinds presenting resistance.
Safi said: “Air can drive flight costs up as well as down for flappers.
“What surprised us was just how much flappers operate in costly conditions where their fuel economy is really low.”
He added: “We hope that our framework will drive further investigations of the different trade-offs these two groups face, and of potential energy saving mechanisms we might still be missing.”