Any animal climbing a mountain experiences a double whammy: the air becomes thinner while cooling, which is especially problematic for creatures that strive to warm up when less oxygen is available. For small animals with the most intense lives, like hovering hummingbirds, the challenges of migrating to higher levels to escape climate change may be too great, but no one knew if these incredible fliers could have more gasoline in the tank to make them fly. at higher altitudes.
Like Anna’s hummingbirds (calypte anna) are comfortable up to altitudes of around 2,800 m (~9,200 ft), Austin Spence of the University of Connecticut, USA, and Morgan Tingley of the University of California, Los Angeles, USA , were curious how hummingbirds native to near sea level and those that live at the higher end of the range would fare when carried high above their natural habitat at an altitude of 3 800 m (12,500 ft). They published their discovery in the Journal of Experimental Biology that the birds have difficulty gliding and experience a 37% drop in metabolic rate at this height – in addition to becoming torpid for most of the night to conserve energy – making it unlikely that they will be able to hover move to higher altitudes.
To learn how agile aeronauts performed at high altitudes, Spence first lured the animals into net traps, from sites 10 m (33 ft) above sea level (Sacramento, CA) to 2,400 m (7,900 ft) (Mammoth Lakes, CA). Then he and Hannah LeWinter (Humboldt State University, USA) transported them to an aviary in western California at 1,215 m (4,000 ft). After the birds spent a few days in their new home, the scientists set up a small funnel the birds could insert their heads into as they hovered sipping tasty syrup, and measured the birds’ O2 consumption (metabolic rate).
Spence and LeWinter also measured the hummingbird’s CO2 production (another measure of metabolic rate) overnight, as the tiny creatures let their metabolism crash when they went torpid – a form of mini hibernation – to conserve energy while they slept. Then the duo moved the birds to a nearby research station near the summit of Mount Barcroft, California (3,800m/12,500ft) where the air is thinner (~39% less oxygen) and colder. (~5°C), and after ~4 days at the new altitude, Spence and LeWinter remeasured the birds’ metabolic rates while soaring and how often and how deep the birds went into torpor while sleeping .
Even though the hovering hummingbirds should have worked harder to stay aloft 1000m above their natural range, the birds actually suffered a 37% drop in their metabolic rate. And when the team compared the energy used by birds native to sea level and the upper end of their range, they all struggled similarly at the top of the mountain. “Taken together, these results suggest that low atmospheric pressure and oxygen availability may reduce the hovering performance of hummingbirds when exposed to the acute challenge of high altitude conditions,” Spence says.
In addition to struggling to soar, the birds resorted to lowering their metabolic rate and becoming torpid for longer periods at night, spending more than 87.5% of the cold night at high altitudes in torpor. “This means that even if they come from a warm or cool place, they use torpor when it’s really cold, which is cool,” says Spence. And when the team checked the size of the animals’ lungs, to find out if birds native to high altitudes had larger lungs to compensate for their low oxygen intake, they didn’t. But birds had bigger hearts to circulate oxygen through the body.
What does this mean for the future of hummingbirds as climate change forces them to find more comfortable conditions? “Our results suggest that low oxygen availability and low atmospheric pressure can be difficult challenges for hummingbirds to overcome,” Spence says, meaning birds will likely have to move north in search of climates. Fresher.
Reference: “Anna’s Hummingbird (calypte anna) Physiological Response to New High Altitude Thermal and Hypoxic Conditions” by Austin R. Spence, Hannah LeWinter, and Morgan W. Tingley, May 26, 2022, Journal of Experimental Biology.
Funding: National Science Foundation, Society for Integrative and Comparative Biology (SICB), White Mountain Research Center