Hummingbirds thrive on an extreme lifestyle. Here’s how.

This article was originally published on Interesting magazine.

Everyone loves watching hummingbirds—tiny, colorful blurs that scurry around, hover on flowers, and fiercely defend their ownership of a bird feeder.

But to the scientists who study them, hummingbirds offer much more than just an entertaining spectacle. Because of their small size and rapid metabolism, they live on a knife’s edge, sometimes having to shut down their bodies almost entirely just to conserve enough energy to survive the night – or to migrate thousands of miles, sometimes across open sea.

Their nectar-rich diet leads to blood sugar levels that would put a human in a coma. And their fast, furious flight sometimes generates G-forces so high that a fighter pilot loses consciousness. The more researchers search, the more surprises lurk in these tiny bodies, the smallest in the bird world.

“They are the only bird in the world that can fly upside down and backwards,” says Holly Ernest, a conservation ecologist at the University of Wyoming. “You drink pure sugar and don’t die of diabetes.”

Ernest is one of a few researchers studying how hummingbirds cope with the extreme demands of their lifestyle. Here’s what scientists have learned about hummingbirds’ unique adaptations.

Enter the work

For years, most researchers assumed that hummingbirds spent only about 30 percent of their day in the energy-intensive activity of flying from flower to flower and gobbling up nectar, resting most of the rest of the time. But when physiological ecologist Anusha Shankar looked closely, she found that they often work much harder.

Shankar, now at the Tata Institute of Fundamental Research in Hyderabad, India, was trying to figure out how broad-billed hummingbirds spend their days in southern Arizona. Using a mix of experimental methods, she measured the birds’ metabolic rates during different activities and estimated their total daily energy expenditure. By adding previously published data, Shankar was able to calculate the energy cost per minute of perching, flying and hovering – essentially the three ways a bird spends time.

She then determined how much time the birds must have spent eating rather than sitting over the course of a day.

“In the end, we found that it was very variable,” says Shankar. At the beginning of summer, when flowers are plentiful, birds could meet their daily energy needs with just a few hours of feeding and spend up to 70 percent of the day just perching, she found. But when flowers became rarer after the onset of the summer monsoon rains, the birds only sat at one location 20 percent of the time and used the rest of the day to feed.

“That’s 13 hours a day!” says Shankar. “There is no way I can walk 13 hours a day. I don’t know how they do it.”

Really chill

Hummingbirds have a trick that helps them use up their energy reserves: When a bird is in danger of running out of energy, it may become lethargic at night and its body temperature will drop to almost the temperature of the surrounding air – sometimes just a few Degrees above freezing. During torpor, the bird appears almost comatose, cannot react quickly to stimuli and only breathes intermittently. Shankar has calculated that the strategy can save up to 95 percent of hourly metabolic costs on cold nights. This can occur after days when a bird has eaten less than usual, e.g. B. after a thunderstorm, can be of crucial importance. It also helps birds conserve energy to store fat before migrating.

Shankar is now studying which parts of their physiology hummingbirds prioritize during torpor by figuring out which gene products they can’t live without. “If you’re a hummingbird operating at 10 percent of your normal metabolism, what 10 percent keeps you alive?” she asks.

A set of genes that birds seem to leave untouched are those responsible for their internal clocks. “It is important for them to do things at the right time when they are in a state of torpor,” says Shankar. For example, to be ready for the day, birds begin to emerge from their torpor about an hour before sunrise, long before any visible light signals are visible.

Take care of the sugar

To boost their extremely high metabolism, hummingbirds suck up around 80 percent of their body weight in nectar every day. That’s the equivalent of a 150-pound person drinking nearly a hundred 20-ounce Cokes a day—and nectar is often much sweeter than a soda.

The human gut isn’t able to absorb sugar as quickly, which is one reason consuming too much soda or Halloween candy upsets the stomach, says Ken Welch, a comparative physiologist at the University of Toronto in Scarborough . Hummingbirds cope with the onslaught by having leaky guts, allowing sugar to pass into the bloodstream between the gut cells, not just through them. This means the sugar is quickly removed from the intestines before problems can occur. This rapid transport and likely other adaptations allow hummingbirds to reach blood sugar levels up to six times higher than humans, Welch says.

So much sugar in the blood causes serious physiological problems in humans. It causes more sugar molecules to bind to body proteins, a process known as glycation. In the long term, excessive glycation causes many complications of diabetes, such as nerve damage. It’s still unclear how hummingbirds escape the problems of glycation, says Welch, but there are early clues. For example, one study found that bird proteins contain fewer of the amino acids most prone to glycation than mammalian proteins, and that the remaining amino acids are often hidden deep within the protein, where they are less exposed to circulating sugars.

Other, as-yet-unknown strategies for managing high blood sugar may one day have practical benefits for treating diabetes in humans. “There could be a gold mine in the hummingbird genome,” says Welch.

Do a metabolism flip

By the end of its nightly fast, a hummingbird has nearly depleted its sugar stores – presenting an opposite metabolic challenge. “How does it wake up and fly?” Welch asks. “There is nothing but fat available to burn.”

He found that hummingbirds have evolved to be remarkably adept at switching their metabolism from burning sugar to burning fat. “This requires an enormous change in the biochemical pathways involved,” says Welch – and it happens in just a few minutes, much faster than other organisms can manage. “If we had that kind of control over our fuel consumption, we would be happy.”

Save water – or not

Sugar isn’t the only challenge that comes with a nectar-rich diet. After all, nectar is mostly water—and birds that drink so much fluid need to get rid of most of it without losing electrolytes. Therefore, hummingbird kidneys are highly adapted to recover electrolytes before excretion. “They almost pee with distilled water,” says Carlos Martinez del Rio, an ecophysiologist now retired from the University of Wyoming.

But that brings with it another problem: If a hummingbird continued to produce diluted urine overnight, it would die of dehydration before morning. To avoid this, hummingbirds shut down their kidneys every night. “They lead to what would be considered acute kidney failure in a human,” says Martinez del Rio. “Hummingbirds have to do this or they would piss themselves to death.”

Fly high – gradually

The metabolic demands of a hummingbird are high enough at sea level. However, many species live at high altitudes, where thin air contains less oxygen and offers less resistance when floating. Consider the giant hummingbird, the largest in the world, which can live in the Andes at altitudes over 14,000 feet – higher than many helicopters can fly. To cope with these conditions, the birds evolved more hemoglobin-rich blood, says Jessie Williamson, an ornithologist at Cornell University.

But some birds face an even greater challenge, Williamson discovered. Giant hummingbirds are large enough for researchers to attach both satellite tracking tags and smaller geolocators. So Williamson and her colleagues decided to equip the birds with trackers. After spending thousands of hours catching birds with nets, the researchers managed to attach trackers to 57 birds using custom-made straps made from elastic decorative cords.

Although they collected tracking data from only eight birds, there was a big surprise even in this tiny sample: Some of the birds lived in the high Andes year-round, while others – it turned out to be a separate one, so far unrecognized species – Andes migrated to the high Andes every year from breeding areas along the coast of Chile. That means they not only face the obvious challenges of a long migration — a round trip of about 5,000 miles — but also the need to adapt to thinner air as they travel.

Your secret? Do it gradually. “It’s very similar to the way human climbers climb something like Mount Everest, with bursts of climbing and breaks to acclimatize,” says Williamson. “The journey takes months.”

As tracking technology becomes easier and cheaper, researchers like Williamson hope to track smaller species of hummingbirds. This, along with other advances in research technology, could hold many new surprises about the biology of these tiny, amazing birds.

This article originally appeared in Knowable Magazine, an independent journalism outlet of Annual Reviews. Sign up for the newsletter.