The largest study of flight costs in the wild finds that riding the air can make flying cheaper than swimming, overturning a textbook ranking
Two of the world’s heaviest birds, the trumpeter swan (left) and the Andean condor (right) and have the same mass but very different costs of transport.
© Brett Blocker / Alvaro Moya Riffo
To the Point
- Largest study of flight cost: 17 years of tracking data from 850 individuals is combined with atmospheric conditions to create the most accurate picture yet of transport cost for wild birds and bats.
- Revision of a classic rule: Real-world data overturn the classic laboratory ranking of swimming, flying, and running costs.
- Atmosphere is habitat: Wind and uplift help determine the energetic cost of flight, which is relevant for predicting how climate change could affect migration, distributions and conservation.
An Andean condor and a trumpeter swan both weigh about 11 kilograms. Yet the condor covers each kilometer of flight for about a fifth of the swan’s energy bill. The difference is the air: the condor rides rising currents and flaps for 0.1 percent of its flight time, while the swan flaps for 99 percent. A new 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.
“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,” says first author Martina Scacco.
Textbooks rank the ways animals travel by cost: swimming is the cheapest, flying comes next, and running is the most expensive. That ranking comes from laboratory measurements of animals in swim tanks, in flight tunnels, and on treadmills. Outside the lab, the study found that this ranking changes.
“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,” says senior author Emily Shepard from Swansea University.
Soaring flight is cheaper than swimming
The study put a price on that difference: the condor’s cost of transport was 79 percent lower than the swan’s, making soaring flight cheaper than swimming.
“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,” adds Shepard.
The work led by scientists from the Max Planck Institute of Animal Behavior and Swansea University is the largest study yet 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 range in size from about 12 grams to more than 11 kilograms. An international team of 68 researchers contributed data to the project.
The study 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.
“If atmospheric conditions change, so too will the cost of flight, and therefore how animals decide to migrate and establish communities,” says senior author Kamran Safi from the Max Planck Institute of Animal Behavior.
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.
Measuring cost of transport in the wild
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. This enabled them to calculate the amount of time individual animals spent flapping or soaring along a stretch of flight, and to approximate how much energy they spent per meter travelled-their cost of transport.
“We boiled the differences down to fuel economy, nature’s equivalent of miles per gallon,” says Scacco. This allowed the scientists to compare transport costs across species and flight modes, “like we compare freight costs of sea mail or air mail,” she adds.
Indeed, 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.
How to fly a hundred laps of the planet
Soaring vultures offer 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. Remarkably they achieved that while flapping only 0.1 percent to 7 percent of flight time. And the heaviest vulture, the Andean condor, flapped least of all.
They did this by exploiting atmospheric energy. Soaring birds rode rising columns of air, which lowered their cost of transport the most, “like us riding the elevator to the top floor,” says Shepard. “Soarers are masters at extracting energy from the environment. When conditions are right, they can drive their costs down to very little.”
But conditions are not always right. The larger the soaring bird, the more restricted they were in terms of when and where they could fly.
Costly, but flexible flight
Flapping birds, on the other hand, use high-energy movement to cope with more variable conditions. Wind had the most significant effect on their transport costs, with tail winds providing support and headwinds presenting resistance.
Says Safi: “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. 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.”
In future work, the authors aim to build on their findings by measuring animals’ energy use directly, such as through heart rate monitoring. Doing so will capture differences in individual physiology, providing an even more realistic measure of the energetic cost of flight.
