Wild birds rewrite rules for travel

For nearly a century, biology textbooks have ranked the ways animals travel by energy cost: swimming is the cheapest, flying comes next, and running is the most expensive. That ranking rests on laboratory measurements of animals in swim tanks, flight tunnels, and on treadmills. A new study published in Science, led by scientists from the Max Planck Institute of Animal Behavior and Swansea University, shows that outside the lab this ranking collapses. Once the atmosphere itself is taken into account, flying can be the cheapest form of travel of all.

The scale of the project is unprecedented. 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, and an international team of 68 researchers contributed data, making this the largest study yet to examine the cost of long-distance travel for wild animals. It is also the first to factor in the role of the atmosphere in the movement of so many wild animals at a global scale.

The contrast that inspired the work is striking. An Andean condor and a trumpeter swan both weigh about 11 kilograms, so laboratory rules would predict identical energy bills per kilometer. Yet the condor covers each kilometer of flight for about a fifth of the swan’s energy cost. The difference is the air: the condor rides rising currents and flaps for only 0.1 percent of its flight time, while the swan flaps for 99 percent. The study put a price on that difference, finding the condor’s cost of transport was 79 percent lower than the swan’s, making soaring flight cheaper than swimming.

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 Dr. Martina Scacco of the Max Planck Institute of Animal Behavior. The team boiled the differences down to fuel economy, nature’s equivalent of miles per gallon, which allowed them to compare transport costs across species and flight modes, much like comparing the freight costs of sea mail or air mail.

The method relied on accelerometers to distinguish high-cost flapping from low-cost soaring. With GPS data, the researchers matched each stretch of flight with the wind and rising air the animals met along the way. This enabled them to calculate the time individual animals spent flapping or soaring on each stretch, and to approximate how much energy they spent per meter travelled, their cost of transport. 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 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 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 senior author Prof. Emily Shepard from Swansea University. Soarers are masters at extracting energy from the environment, and 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 it was in terms of when and where it could fly, a trade-off between cheap travel and flexible scheduling.

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 tailwinds providing support and headwinds presenting resistance. Air can drive flight costs up as well as down for flappers, says senior author Dr. Kamran Safi of the Max Planck Institute of Animal Behavior. What surprised us was just how much flappers operate in costly conditions where their fuel economy is really low, he adds, expressing hope that the framework will drive further investigations of the different trade-offs the two groups face, and of potential energy-saving mechanisms still missing.

The laboratory rules tell us that condors and swans must have the same energy bill because they have the same body mass, says Shepard. Yet seeing how condors soar and swans flap, we know that clearly there must be a difference. 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, she adds.

Beyond rewriting a textbook ranking, 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. If atmospheric conditions change, so too will the cost of flight, and therefore how animals decide to migrate and establish communities, says Safi. In future work, the authors aim to build on their findings by measuring animals’ energy use directly, such as through heart rate monitoring, which will capture differences in individual physiology and provide an even more realistic measure of the energetic cost of flight. For conservation and management of aerial species, the study provides a key: the atmosphere is not a backdrop for flight but a resource, and protecting soaring migrants may mean protecting the winds and rising air on which their fuel economy depends.

Subject of Research: Cost of transport in wild flying animals

Article Title: Wild birds rewrite rules for travel

Article References: Wild birds rewrite rules for travel. Original publication (Bibliographic details unavailable.)

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Keywords: flight, soaring, Andean condor, cost of transport, atmospheric flows, GPS tracking, accelerometry, migration, vultures, Science, Wild, birds