Hiking in the mountains is an exhilarating experience. The crisp air and breathtaking views can truly rejuvenate the soul. However, the thin air at higher altitudes presents an additional challenge for birds. According to Jonathan Rader from the University of North Carolina (UNC) at Chapel Hill, USA, bird wings produce less lift in low-density air, making it more difficult for them to stay airborne.
Despite this obstacle, birds such as bar-headed geese, cranes, and bar-tailed godwits have been recorded reaching altitudes of 6000 m or more. The question remains: how do they manage to fly in thin air, which offers little lift? One possible explanation was that birds at high altitudes simply fly faster to compensate for the lower air density. However, it wasn’t clear if birds that naturally inhabit a wide range of altitudes, from sea level to the highest peaks, might adjust their flight speed to adapt to thin air.
“Turkey vultures are common through North America and inhabit an elevation range of more than 3000 m,” says Rader.
So Rader and Ty Hedrick (UNC-Chapel Hill) set out to investigate whether turkey vultures (Cathartes aura) flying at different elevations exhibit varying flight speeds in relation to their altitude. Their findings reveal that turkey vultures fly faster at higher altitudes to counteract the reduced lift experienced when flying in thinner air.
To conduct their study, the pair needed to identify locations at several thousand meters’ altitude. They began by capturing footage of vultures flying at the local Orange County refuse site (80 m above sea level). Rader jokingly remarks, “Vultures on a landfill… who would have guessed?”
They then moved on to Alcova, Wyoming (1600 m), before wrapping up their research at the University of Wyoming campus in Laramie (2200 m). At each site, the team positioned three synchronized cameras to capture the 3D flights of the turkey vultures returning to their roosting tree.
“Wyoming is a famously windy place and prone to afternoon thunderstorms,” Rader explains, recalling being forced off the University of Wyoming Biological Sciences Building’s roof by the wind that blurred the footage of the flying birds.
In North Carolina, Rader meticulously analyzed 2458 bird flights from movies to determine their flight speed. He then converted the flight speed to airspeed, which ranged from 8.7 to 13.24m/s. Additionally, Rader calculated the air density at each location by using local air pressure readings, noting a 27% change from 0.89kg/m3 at Laramie to 1.227 kg/m3 at Chapel Hill.
By plotting the air densities against the birds’ airspeeds, Rader and Hedrick observed that birds flying at 2200m in Laramie were generally flying approximately 1m/s faster than the birds in Chapel Hill. This phenomenon indicates that turkey vultures fly faster at higher altitudes to stay airborne. The question remains: how do they achieve these higher airspeeds?
Rader revisited the flight footage, observing the subtle movements indicating the flapping of the bird’s wings. He noted that the high-altitude vultures were not adjusting their wingbeats to compensate for the low air density, but instead, it seemed that they were flying faster due to less drag in the thin air, allowing them to compensate for generating less lift.
Journal reference:
- Jonathan A. Rader and Tyson L. Hedrick. Turkey vultures tune their airspeed to changing air density. Journal of Experimental Biology, 2024; DOI: 10.1242/jeb.246828