Turkey vultures fly faster to defy thin air

How large turkey vultures remain aloft in thin air

The Company of Biologists

Mountain hikes are invigorating. Crisp air and clear views can refresh the soul, but thin air presents an additional challenge for high-altitude birds. ‘All else being equal, bird wings produce less lift in low density air’, says Jonathan Rader from the University of North Carolina (UNC) at Chapel Hill, USA, making it more difficult to remain aloft. Yet this doesn’t seem to put them off. Bar-headed geese, cranes and bar-tailed godwits have recorded altitude records of 6000 m and more. So how do they manage to take to the air when thin air offers little lift? One possibility was that birds at high altitude simply fly faster, to compensate for the lower air density, but it wasn’t clear whether birds that naturally inhabit a wide range of altitudes, from sea level to the loftiest summits, might fine-tune their flight speed to compensate for thin air. ‘Turkey vultures are common through North America and inhabit an elevation range of more than 3000 m’, says Rader, so he and Ty Hedrick (UNC-Chapel Hill) decided to find out whether turkey vultures (Cathartes aura) residing at different elevations fly at different speeds depending on their altitude. They publish their discovery in Journal of Experimental Biology that turkey vultures fly faster at altitude to compensate for the lack of lift caused by flying in thin air.

First the duo needed to select locations over several thousand meters’ altitude, so they started filming the vultures flying at the local Orange County refuse site (80 m above sea level); ‘Vultures on a landfill… who would have guessed?’, chuckles Rader. Then they relocated to Rader’s home state of Wyoming, visiting Alcova (1600 m) before ending up at the University of Wyoming campus in Laramie (2200 m). At each location, the duo set up three synchronized cameras with a clear view to a tree that was home to a roosting colony of turkey vultures, ready to film the vultures’ flights in 3D as they flew home at the end of the day. ‘Wyoming is a famously windy place and prone to afternoon thunderstorms’, Rader explains, recalling being chased off the roof of the University of Wyoming Biological Sciences Building by storms and the wind blurring movies of the flying birds as it rattled the cameras.

Back in North Carolina, Rader reconstructed 2458 bird flights from the movies, calculating their flight speed before converting to airspeed, which ranged from 8.7 to 13.24m/s. He also calculated the air density at each location, based on local air pressure readings, recording a 27% change from 0.89kg/m3 at Laramie to 1.227 kg/m3 at Chapel Hill. After plotting the air densities at the time of flight against the birds’ airspeeds on a graph, Rader and Hedrick could see that the birds flying at 2200m in Laramie were generally flying ~1m/s faster than the birds in Chapel Hill. Turkey vultures fly faster at higher altitudes to remain aloft. But how do they achieve these higher airspeeds?

Rader returned to the flight movies, looking for the tell-tale up-and-down motion that would indicate when they were flapping. However, when he compared how much each bird was flapping with the different air densities, the high-altitude vultures were flapping no more than the birds nearer to sea level, so they weren’t changing their wingbeats to counteract the effects of low air density. Instead, it is likely that the 2200 m high birds were flying faster simply because there is less drag in thin air to slow them down, allowing the Laramie vultures to fly faster than the Chapel Hill birds to compensate for generating less lift in lower air density.

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REFERENCE: Rader, J. A. and Hedrick, T. L. (2024). Turkey vultures tune their airspeed to changing air density. J. Exp. Biol. 227, jeb246828. doi:10.1242/jeb.246828

DOI: 10.1242/jeb.246828

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Journal

Journal of Experimental Biology

DOI

10.1242/jeb.246828

Method of Research

Experimental study

Subject of Research

Animals

Article Title

Turkey vultures tune their airspeed to changing air density

Article Publication Date

1-Aug-2024

Flamingos don’t preen more than other waterbirds

University of Exeter

Caribbean flamingo preening. — image:
Caribbean flamingo preening
view more
Credit: Dr Paul Rose

Despite their famously fancy feathers, flamingos don’t spent more time preening than other waterbirds, new research shows.

Scientists watched five of the world’s six flamingo species to see how they spend their time.

While preening time varied, overall flamingos were roughly average compared to existing studies of other waterbirds.

The researchers also examined time spent standing on one leg – finding that captive flamingos most often do this indoors and in water.

The study was carried out by the University of Exeter and WWT Slimbridge Wetland Centre.

“It is widely believed – even among zookeepers – that flamingos spend more time than other birds preening their feathers,” said Dr Paul Rose, from Exeter’s Centre for Research in Animal Behaviour.

“But this hadn’t been tested until now – and we were surprised to find they are roughly ‘middle of the road’ among waterbirds.

“The waterbirds that spend the most time preening are actually Pelecaniformes such as pelicans, gannets and cormorants – which makes sense, as these birds spend the most time in water, and therefore need to maintain feathers for waterproofing.

“Even so, we expected flamingos to be top of the preening charts, as lush pink feathers are so important in their courtship displays.”

The study also found that captive waterbirds spend more time preening than wild birds.

Like many birds, flamingos stand on one leg to save energy and to allow half of their brain to sleep. By doing so in water, flamingos reduce heat loss (only one foot loses heat to the water, rather than two).

Rachael Kinnaird, who worked on the study as part of an MSc in Animal Behaviour at Exeter, said: “Our study shows the value of observing animals to understand their behaviour.

“In this case, a widely held belief about flamingos preening more than other waterbirds didn’t hold true.”

Commenting on the wider importance of the study, Dr Rose said: “By understanding why flamingos behave in certain ways, we can predict how climate and habitat change might affect them.

“At present, greater flamingos are seeing their range expand due to climate change. Meanwhile, mountain species such as Chilean and Andean flamingos are really suffering.

“Studying captive flamingos can help us understand what their wild counterparts need.

“We hold so many species in zoos around the world – so the opportunities to study them, to learn how and why they have evolved, are huge.”

The paper, published in the journal Applied Animal Behaviour Science, is entitled: “What influences feather care and unipedal resting in flamingos? Adding evidence to clarify behavioural anecdotes.”

Chilean flamingo preening

hadean flamingo standing on one leg

Credit

Dr Paul Rose