Heaviest Soaring Bird

In a new study, a team of researchers from the United Kingdom, Germany and Argentina deployed high-tech flight-recorders on the world’s heaviest extant soaring bird, the Andean condor (Vultur gryphus), to assess the extent to which this species can operate without resorting to powered flight. Their data revealed the lowest levels of flapping flight recorded for any free-ranging bird, with condors remarkably spending 99% of all flight time in soaring/gliding flight.

A young female Andean condor (Vultur gryphus) in Argentina. Image credit: Hugo Pédel / CC BY-SA 3.0.

“Flight costs are predicted to vary with environmental conditions, and this should ultimately determine the movement capacity and distributions of large soaring birds,” said lead author Dr. Hannah Williams of Swansea University and the Max Planck Institute of Animal Behaviour and her colleagues.

“Despite this, little is known about how flight effort varies with environmental parameters.”

“In the study, we assessed whether, and if so, how, flight effort varies with environmental conditions in the world’s heaviest extant soaring bird, the Andean condor.”

The study involved eight immature Andean condors (9.5 to 13.9 kg), each equipped with an archival ‘daily diary’ unit, a GPS unit and a miniature VHF.

“Given that flapping was likely to be relatively infrequent, we developed custom logging and tag-release systems to obtain uninterrupted, high-frequency data on the flight behavior of these birds,” the scientists explained.

“These continuous data enable us to identify each and every wingbeat, from which we assess the precise conditions that elicit flapping.”

During the study, they discovered that more than 75% of the condors’ flapping was associated with take-off.

However, once in the sky condors can sustain soaring for long periods in a wide range of wind and thermal conditions.

One bird remained airborne for over 5 hour without using flapping flight, covering over 170 km.

“Watching birds from kites to eagles fly, you might wonder if they ever flap,”

“This question is important, because by the time birds are as big as condors, theory tells us they are dependent on soaring to get around.”

“Our results revealed the amount the birds flapped didn’t change substantially with the weather.”

“This suggests that decisions about when and where to land are crucial, as not only do condors need to be able to take off again, but unnecessary landings will add significantly to their overall flight costs.”

“As all the birds we studied were immature, it demonstrated that low investment in flight is possible even in the early years of a condor’s life,” said Swansea University’s Professor Emily Shepard, co-author of the study.

Closer examination showed the challenges the birds faced as they moved between weak thermals.

The condors were seen to flap more as they reached the end of the glides between thermals when they were likely to be closer to the ground.

“This is a critical time as birds need to find rising air to avoid an unplanned landing. These risks are higher when moving between thermal updrafts,” said senior author Dr. Sergio Lambertucci, a researcher at the Universidad Nacional del Comahue-CONICET.

“Thermals can behave like lava lamps, with bubbles of air rising intermittently from the ground when the air is warm enough. Birds may therefore arrive in the right place for a thermal, but at the wrong time.”

“This is a nice example of where the behavior of the birds can provide insight into the behavior of the air.”

The study was published this week in the Proceedings of the National Academy of Sciences.

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H. J. Williams et al. Physical limits of flight performance in the heaviest soaring bird. PNAS, published online July 13, 2020; doi: 10.1073/pnas.1907360117