Climbing to survive climate change isn’t the best option for hummingbirds

Climate change is forcing many hummingbird species to move to colder habitats in higher elevation regions, but are hummingbirds ready for this change? A study published in the Journal of Experimental Biology highlights the struggles and challenges hummingbirds can face when climbing a slope.

Hummingbirds sipping nectar from flowers. Image credits: James Wainscoat/Unsplash

Temperature plays an important role in deciding the migratory and reproductive behavior of hummingbirds due to their high body metabolic rates. Compared to the human heart, which beats between 60 and 100 times per minute at rest, a hummingbird’s heart beats more than 1,000 times per minute. Thus, to satisfy their body metabolism, birds must constantly take in calories.

But an increase in temperature spoils their main source of calories, which is flower nectar. Additionally, the birds become less social and do not prefer to breed during intense solar activity. However, due to their high metabolic levels and constant caloric needs, the birds can no longer stay in a warm habitat, so they often migrate to high altitude regions. Unfortunately, this type of migration due to rapid global warming induced by climate change forces birds to adapt quickly, which is not always possible.

Can hummingbirds easily move to higher altitudes?

Researchers from the University of Connecticut, Humboldt State University and the University of California studied Anna’s hummingbirds (calypte annalisten)), a common hummingbird species found in North America. During their experiment, they flew the birds from a location 10 meters up to 2,400 meters in the Mammoth Lake area. Then the birds were brought to a bird sanctuary in California located at an average altitude of 1,215 meters.

image credits: Zdeněk Macháček/Unsplash

Oxygen consumption, carbon dioxide production and other parameters related to the birds’ body metabolism were measured during their stay in California. After a few days, the researchers moved the hummingbirds again, this time to a research station at 3,800 meters. Here too, the metabolic levels of the birds were checked.

Researchers have noticed that hovering birds often fall into torpor (a state of partially suspended physical or mental activity that results in conservation of energy) while sleeping during their uphill journey. Additionally, the hummingbirds’ metabolic rates suffered a 37% loss when they flew 1000 meters above their natural habitats. These results highlighted that the birds had difficulty adapting to higher altitude environments.

“So what we tested was an acute test – it was a very quick test once we got to high altitudes. We found the hummingbirds had a hard time right away,” said Austin Spence, one of the study’s authors and a postdoctoral researcher at the University of California.

To overcome energy losses at higher altitudes during extremely cold nights, hummingbirds spent more than 87.5% of their time in torpor and their hearts increased in size to meet the needs of blood circulation. While explaining all these results of their experiment, Spence said:

“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.” He added that “low oxygen availability and low atmospheric pressure can be tough challenges for hummingbirds to overcome, meaning birds will likely have to move north in search of cooler climates.”

The researchers told ZME Science that they only did a rapid test and that their study therefore has several limitations. They plan to conduct long-term acclimation experiments and breeding bird surveys that would reveal more information about the difficulties hummingbirds face due to climate change-induced temperature changes.

Teresa H. Sadler