As Earth’s climate warms, some animals may seek respite from the heat in colder, northern climates or higher altitudes. For some species, these cooler locations may offer greener pastures, so to speak, than their current homes as average annual temperatures continue to rise.
For the tiny Anna’s hummingbird, which calls the west coast of North America from California to Vancouver, British Columbia its home, that may not be an option. According to a study published Thursday in the Journal of Experimental Biology, moving to cooler and higher altitudes would only do two things: They would have trouble levitating when their metabolic rate dropped, and they would sleep most of the day.
“The further uphill you go, the colder it gets and there is less oxygen available. You can think of it like Everest; People have to go to base camp and bring supplemental oxygen and get used to it up there,” Austin Spence, one of the paper’s authors and a postdoctoral fellow in the University of California Davis’ Department of Wildlife, Fish, and Conservation Biology, told Ars.
To study this, Spence and a team of researchers went into the wild in the summer of 2018 and captured 26 Anna’s hummingbirds by luring them into net traps with hummingbird feeders. “One reason we worked with hummingbirds is because they’re easy to catch,” Spence said. “We also knew from previous research that hummingbirds move uphill.”
These birds originated from locations between 10 meters above sea level in Sacramento and 2,400 meters above sea level (Mammoth Lakes, California). They were then transported to an aviary in western California at 4,000 feet (1,215 meters) above sea level and the birds were allowed time to acclimatize to their cages to allow birds from different altitudes to acclimate to them. From there, the birds were transported to a research station near Mount Barcroft, California, which is 12,000 feet (3,800 meters) above sea level.
“If they keep moving uphill with climate change, that’s where they’re going to live. So we wanted to see how that would affect her,” Spence said.
In the wild, the birds are used to up to 2,800 meters above sea level. Near the summit of Mount Barcroft, the air has 39 percent less oxygen and is around 5°C colder than you’re used to. After four days, the team used a field metabolism system, which Spence describes as a box that sucks in and reads the ambient air and the air the animals expel, to calculate how much energy the animals are expending.
A tough fight
This measurement allowed them to get a feel for the birds’ metabolic rates. Overall, the birds suffered a 37 percent drop in their metabolic rate and, the study found, had difficulty hovering. In addition, the birds spent 87.5 percent of their nights in a state of paralysis. That inertia helped them conserve energy, Spence said. “They could conserve energy, but they didn’t respond well to the oxygen…they just didn’t fly that well,” he said.
As the climate changes, it can be difficult for Anna’s hummingbird to migrate to, and survive at, higher and cooler altitudes.
However, research has only looked at acute exposure lasting a few days. The team wants to study how the birds would cope with the higher altitude over longer periods of time, Spence said. “If people want to climb Mount Everest, they have to stay at base camp for a while because staying at base camp helps them get used to the new conditions,” he said.
Experimental Biology, 2022. DOI: 10.1242/jeb.244313 (About DOIs)
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