Anna's hummingbirds, a species native to the west coast of North America, float around California's lowlands before migrating to higher altitudes for the summer. A warming climate may make it harder for the tiny critters to migrate to higher ground, and the vibrant bird might not be able to adapt, reports Inverse's Tara Yarlagadda. Those are the findings of a study published this week in the Journal of Experimental Biology.
Animals and plants have previously shown their ability to adapt to warming climates by moving towards the poles and to higher elevations, where temperatures are cooler. "Think of an AC. If I'm too hot, I can turn it down. If an animal or plant is too hot, they can't turn the AC down," Austin Spence, conservation biologist and study lead author at the University of Connecticut, tells Inverse.
However, when animals trek to higher ground to escape the heat, they may face not just colder temperatures but also thinner, less oxygen-rich air, reports David Nield for Science Alert. Humans have already displaced Anne's hummingbirds (Calypte anna) because of changing habitats. "Anna's hummingbirds are a great study system because they've already changed their life history because of humans," Spence says to Inverse. The hummingbirds have already started a progression to higher terrain due to warmer climates. Their high-energy lifestyles make them a great model for study because the hummingbirds can cope with various weather conditions, per Science Alert. The birds are now found at elevations between 10 to 2,800 feet.
To study Anna's hummingbirds’ adaptability, researchers relocated 26 birds from across all average elevation ranges to a location 12,467 feet above sea level, and recorded their oxygen levels while they ate or slept, Inverse reports. Using a modified feeder that forced hummingbirds to put their face inside a breathing mask to access food, scientists measured the bird's oxygen consumption while they ate from syrup-filled funnels, per Science Alert. To measure metabolism while they slept, the birds were placed in a metabolic chamber that could measure the carbon dioxide the birds produced.
The team found that the tiny birds responded to the colder shift in temperature by increasing torpor, a state that allows the hummingbirds to save energy by slowing down their metabolism. This included lowering their heart rate, per Inverse. Researchers also found that the birds displayed a lower metabolic rate, the birds' rate of energy consumption, when hovering. Reduced oxygen and the lower air pressure at the high elevation impacted the birds’ ability to fly by reducing their energy efficiency.
At higher elevations, the hummingbirds entered torpor, a mini-hibernation state, where their metabolism slowed significantly for 87.5 percent of the night, reported Science Alert. This longer torpor occurred no matter where the birds originally lived. "It means that even if they're from a warm or cool spot, they use torpor when it's super-cold, which is cool," Spence said to Science Alert.
Overall, the team found that while the birds could adapt to the cold, it was harder for the hummingbirds to adapt to lower oxygen levels. Spence tells Inverse that the lower levels of oxygen at higher altitudes may slow the bird's ability to migrate to higher ground. Despite this, it might be possible that Anna's hummingbirds could acclimate to environments with lower oxygen levels over a more extended research period. Anna's hummingbirds living in higher elevations were already found to have larger hearts than those in lower areas, and it implies that their bodies eventually adapted to areas with lower oxygen levels.