An Andean condor spends about 0.1% of its flight time flapping its wings, according to a study published Oct. 15 in Science. The rest of the time, the 24-pound (11-kilogram) bird rides rising air and wind, covering each kilometer on roughly a fifth of the energy a trumpeter swan of the same weight burns.

That contrast is the headline of a much larger finding. Drawing on 17 years of GPS and motion-sensor tracking from 850 birds and bats across 38 species, plus four years of radar data on smaller flyers, the researchers concluded that how animals use the atmosphere matters as much as body size in setting the cost of flight.

It also upends a classic textbook ranking. Biology courses have long taught that swimming is the cheapest way for an animal to travel, followed by flying and then running. In the wild, the study found, soaring flight can be cheaper than swimming.

A Condor and a Swan Built Alike, Flying Very Differently

The comparison starts with two birds that weigh about the same. Laboratory rules based on body size would predict similar energy bills for both, but the condor flaps for roughly 0.1% of its flight time while the swan flaps for about 99%, according to the Max Planck Institute of Animal Behavior, which led the work with Swansea University.

The result was a cost of transport for the condor about 79% lower than the swan's. Seven vulture species in the dataset flew a combined 4.3 million kilometers (about 2.7 million miles) over 128,000 days, flapping only 0.1% to 7% of the time.

The new figure sharpens earlier work. A 2020 study of Andean condors in Patagonia, reported at the time by CBC News, found the birds flapped about 1% of their flight time. The new study's larger dataset puts the condor's flapping share even lower.

Senior author Emily Shepard of Swansea University summed up the advantage in a university release, saying animals with access to natural air currents have "free fuel from the sky."

Wind and Rising Air Set the Price of a Trip

The team, which included 68 researchers who contributed data, matched each tracked flight segment with the wind and rising air the animal encountered. Accelerometers told them when an animal was flapping, which is costly, and when it was soaring, which is cheap.

The study covered animals from about 12 grams (less than half an ounce) to more than 11 kilograms. Among soaring birds, larger species flapped less and spent less energy. Among flapping birds, wind strongly shaped the bill, with tailwinds lowering costs and headwinds raising them.

The American Association for the Advancement of Science, which publishes Science, said in its own summary that soaring cut flight costs in some cases to levels comparable to swimming or even resting. That framing is the AAAS's description; the Max Planck and Swansea releases compare soaring with swimming and running, not resting.

There is a catch for the biggest soarers. Large soaring birds were more restricted in when and where they could fly, since they depend on usable updrafts. Flapping species could cope with more variable weather but paid for that flexibility in energy.

Measured in the Wild, Not in a Wind Tunnel

The study's strength is also its main limitation. Earlier estimates of flight cost came largely from swim tanks, flight tunnels and treadmills. This work measured animals in real conditions, but it estimated energy use from movement and airflow data rather than measuring metabolism directly with heart-rate monitors, which the authors have said they plan to add in future work.

The species mix also matters. The study reported results for 38 tracked species and radar-based size classes, and the public releases do not list every species, so readers should not assume a given North American bird was included.

In a related perspective article, Lund University biologist Anders Hedenström encouraged further development of tracking technology and detailed wind-tunnel measurements of different flight modes, according to the AAAS release. That reflects a broader view that field and lab approaches need to be combined.

Shifting Winds Could Reshape Migration

The practical message reaches beyond condors. If the atmosphere sets the price of flight, then changes in wind patterns and rising air could change where and when birds and bats migrate. Senior author Kamran Safi of the Max Planck Institute said in the institute's release that changing atmospheric conditions would change flight costs and, with them, migration decisions.

That matters for wildlife planning. Soaring birds such as vultures, eagles and storks rely on the same updrafts along ridges and over open land that wind energy developers often favor. Better maps of where flight is cheap could help wildlife agencies and planners understand where birds concentrate, although the study itself does not address turbine siting.

For birdwatchers, the finding explains a familiar sight. Large soaring birds often wait until midmorning, when the sun has warmed the ground enough to produce thermals, before taking off. On cool, still or rainy days, they may stay perched because flapping is simply too expensive.

Readers who want to see the behavior can watch hawks and vultures circling over open fields or ridgelines on sunny fall afternoons, a peak season for raptor migration across much of the United States. Local hawk-watch sites and Audubon chapters often post daily migration counts.

The researchers say the next step is linking these field estimates to direct physiological measurements. Nature World News will follow additional results as the team expands its work.

The bottom line is that the sky is not space for flying animals. For an Andean condor, it is an energy source so rich that the bird barely needs to flap, and how that resource shifts in a changing climate may shape where soaring birds can thrive.

What Readers Want to Know

How often do Andean condors flap their wings?

About 0.1% of their flight time, according to the new Science study. The rest of the time they soar on rising air and wind.

How much energy does a condor save compared with a swan?

The condor covers each kilometer on about a fifth of the energy a trumpeter swan of the same weight uses, a cost about 79% lower.

Is flying really cheaper than swimming?

For soaring birds in the wild, the study found it can be. Older lab-based rankings put swimming first because they ignored wind and rising air.

How was the study done?

Researchers combined 17 years of GPS and accelerometer data from 850 birds and bats across 38 species with four years of radar data on smaller flyers.

Could climate change affect bird migration through this effect?

The authors say changing atmospheric conditions would change flight costs and could affect how animals migrate, but the study does not forecast specific changes.

When is the best time to see soaring birds?

Sunny midmorning to afternoon hours, when thermals form, are typically best, especially during fall raptor migration.