Flying Can Use Less Energy Than Swimming, and these Birds Have Been Doing It All Along

Scientists thought swimming was the most energy-efficient way for animals to travel.

An Andean condor and a trumpeter swan weigh roughly the same: about 11 kilograms. Both can fly, and both have to overcome gravity to stay in the air. So you might be tempted to think that they use up comparable amounts of energy.

You’d be very wrong.

The swan spends around 99% of its flight time flapping its wings. The condor, just 0.1%. And according to new research, the condor uses 79% less energy to cover the same distance.

While the swan relies primarily on its muscles, the condor has learned to harness an enormous, invisible source of energy: the atmosphere itself. In fact, it’s so efficient, that under the right conditions, its flight can be more energy-efficient than swimming.

The old rules of movement

Swimming is generally the “cheapest” way to travel from an energy standpoint. It makes a lot of sense; water supports an animal’s weight, allowing fish and other swimmers to move without constantly fighting gravity.

Flying comes next. Sure, you can “ride the wind”, but you need to generate lift to stay airborne, which can require considerable energy. Running is typically the most expensive. Land animals have to support their own weight while repeatedly accelerating their limbs and bodies.

Scientists describe this relationship using a quantity called the cost of transport: the energy required to move a given distance, often calculated relative to body mass. Think of it as the animal equivalent of fuel economy.

Now, an international team of 68 scientists challenges the classic thought on the cost of transport. Much of our understanding, they say, comes from experiments conducted in laboratories, where fish swim in tanks, birds fly in wind tunnels, and mammals run on treadmills. Those controlled environments make it easier to measure energy expenditure, but nature doesn’t really work like that.

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“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.

The sky is full of free energy

If you’ve ever watched an eagle or vulture circle around the sky, you’ve probably seen it: they can remain airborne for long periods of time while barely flapping their wings.

These birds are masters of exploiting physics.

When the Sun heats the ground, the ground warms the air above it. In some places, pockets or columns of warmer air rise through the atmosphere. These are called thermals.

A soaring bird can enter one of these rising columns and circle upward, gaining altitude without having to generate all that lift through muscular effort. Then, once they’re high up, it can glide toward its destination, gradually losing altitude until it finds another thermal.

It’s kind of like taking an elevator to the top of a building and then gliding down toward the next one.

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This “free” energy comes from the Sun, which heats the landscape and drives atmospheric motion. But from the bird’s perspective, it’s an external energy subsidy.

So far, nothing new. But that’s where the new study comes in.

Millions of kilometres, almost no flapping

To find out how much energy birds actually spend travelling, the researchers assembled an unusually large dataset.

They combined 17 years of GPS and accelerometer records from approximately 850 individual birds and bats belonging to 38 species. They also analysed four years of radar observations of smaller flying birds, divided into 12 size categories.

Altogether, the analysis covered nearly 200,000 GPS-based flight segments and thousands of additional radar observations. The animals ranged from tiny birds weighing little more than 12 grams to enormous soaring birds weighing over 11 kilograms.

GPS tracking showed where and how fast animals travelled. Accelerometers, which measure movement, helped researchers distinguish between energetically expensive wingbeats and more economical gliding or soaring.

The team then matched the animals’ movements to information about wind and rising air currents, allowing them to estimate the energetic cost of travelling through real atmospheric conditions.

The results were striking.

Across seven species of vultures, the researchers analysed flights representing more than 4.3 million kilometres of travel. That’s enough to circle the Earth more than 100 times. Collectively, the birds accumulated approximately 128,000 days of flight.

Yet depending on the species, they spent only 0.1% to 7% of that time flapping their wings. The Andean condor, the heaviest vulture in the comparison, flapped least of all.

Granted, these birds are exceptional flyers to begin with; but the secret lies in how they use the energy that was already present in their surroundings.

And when the researchers translated their movements into estimated energy costs, some birds proved more economical travellers than swimming animals.

Bigger can actually mean better

The study uncovered another seemingly surprising pattern. In general, larger species tended to spend less time flapping and were more efficient flyers.

That sounds counterintuitive. Surely a heavier bird needs more energy to stay airborne?

Well, in absolute terms, yes, larger animals generally require more energy. But the question here is how much energy they spend moving a given distance relative to their mass.

Large soaring birds have evolved wings and flight behaviours that let them extract considerable benefit from rising air. Under favourable conditions, they can travel enormous distances with relatively little muscular effort.

But there’s a catch: larger birds are more dependent on finding suitable atmospheric conditions.

Smaller birds are more flexible, they don’t need to rely on thermals as much, but they spend more time flapping their wings and use up more energy as a result.

A new way of understanding migration

Perhaps the most surprising finding is that some soaring birds travel more efficiently than swimming animals, overturning the traditional ranking of animal locomotion.

But the findings could also reshape our understanding of migration. A longer route with favourable winds and thermals might require less energy than a shorter journey through difficult conditions.

That’s particularly relevant as climate change alters atmospheric conditions. Changes in wind and rising air currents could affect how much energy birds need to migrate, and potentially where they can travel.

For now, these energy costs are modelled estimates rather than direct metabolic measurements. But the message is clear: the cost of getting somewhere depends not just on the animal, but also on the environment carrying it; and we need more complex studies to truly map that out.

The study was published in Science.