Any animal climbing a mountain has a double whammy of difficulties: the air becomes thinner while it also becomes colder, which is especially difficult for species striving to remain warm when less oxygen is available. The obstacles of shifting to higher levels to avoid climate change may be too much for tiny animals with high-octane lives, such as hovering hummingbirds, but no one knew whether these exceptional aviators may have enough petrol in the tank to keep them flying at higher altitudes.
Austin Spence of the University of Connecticut and Morgan Tingley of the University of California, Los Angeles, we’re curious to see how hummingbirds that originated near sea level and those that live at the loftier end of the range would cope when transported well above their natural habitat to an altitude of 3800 meters. They report their findings in the Journal of Experimental Biology: the birds struggle to hover and experience a 37 percent drop in metabolic rate at that altitude, as well as becoming torpid for the majority of the night to conserve energy, making it unlikely that they will be able to relocate to higher altitudes.
Spence and Hannah LeWinter enticed the animals into net traps from 10 meters above sea level up to 2400 meters to see how they fared at high altitude, then relocated them to an aviary in western California at 1215 meters. The scientists built up a little funnel into which the birds could enter their heads as they hovered while sipping delectable syrup after the birds had spent a few days in their new habitat, and assessed the birds’ O2 intake.
Spence and LeWinter also monitored the hummingbirds’ CO2 production overnight, as the small critters allowed their metabolism to slow down when they went torpid (a type of micro hibernation) to save energy while sleeping. The birds were then relocated to a nearby research station near Mount Barcroft, CA (3800 meters), where the air is thinner (39 percent less oxygen) and colder (5°C), and after four days at the new altitude, Spence and LeWinter remeasured the birds’ metabolic rates as they hovered, as well as how often and deeply the birds went into torpor as they slumbered.
Despite the fact that the hovering hummingbirds should have been working harder to stay aloft in the thin air 1000 meters above their usual range, their metabolic rate dropped by 37%. When the researchers analyzed the amount of energy consumed by birds that began around sea level with those that originated at the higher end of their range, they found that they all suffered similarly on the mountain top. “Overall, our findings imply that when hummingbirds are subjected to the acute challenge of high-elevation settings, low air pressure and oxygen availability may decrease hovering performance,” adds Spence.
In addition to straining to hover, the birds reduced their metabolic rate and went into torpor for longer periods of time at night, sleeping for more than 87.5 percent of the frigid high-altitude night. “It indicates that they employ torpor when it’s really cold, regardless of whether they’re from a warm or chilly environment,” Spence explains. When the researchers measured the size of the animals’ lungs to see if the birds from higher elevations had larger lungs to compensate for their low oxygen supply, they discovered that they didn’t. However, the birds’ hearts were enlarged, allowing more oxygen to be circulated throughout the body. What does this suggest for the future of the hummingbird as climate change compels them to seek for more hospitable environments? “Our findings show that low oxygen availability and low air pressure may be significant obstacles for hummingbirds to overcome,” adds Spence, implying that the birds may most likely migrate north in search of colder climates.
