Everyone likes to watch hummingbirds – tiny, brightly colored blurs that scurry about, hovering around flowers and pugnaciously defending their ownership of a bird feeder.
But for the scientists who study them, hummingbirds offer much more than an entertaining spectacle. Their small size and blazing metabolism mean they live on a razor’s edge, sometimes having to shut down their bodies almost completely just to conserve enough energy to survive the night – or to migrate thousands of miles, sometimes across the ‘ocean.
Their nectar-rich diet leads to blood sugar levels that could put a person into a coma. And their fast, zooming flight sometimes generates g-forces high enough to knock a fighter pilot unconscious. The more researchers look, the more surprises hide inside these tiny bodies, the smallest in the avian world.
“It is the only bird in the world capable of flying upside down and backwards,” explains Holly Ernestconservation ecologist at the University of Wyoming. “They drink pure sugar and don’t die of diabetes.”
Ernest is one of a small number of researchers studying how hummingbirds cope with the extreme demands of their lifestyle. Here’s some of what scientists have learned about hummingbirds’ unique adaptations.
Get to work
For years, most researchers assumed that hummingbirds spent only about 30 percent of their day in the energy-intensive activities of hopping from flower to flower and swallowing nectar, while resting most of the time. But when the physiological ecologist Anusha Shankar Upon closer inspection, she discovered that they often worked much harder than that.
Shankar, now of the Tata Institute of Fundamental Research in Hyderabad, India, tried to understand how southern Arizona’s broad-billed hummingbirds spend their days. Using a combination of experimental methods, she measured the birds’ metabolic rate during various activities and estimated their total daily energy expenditure. By adding previously published data, Shankar was able to calculate the energy cost per minute for perching, flying and gliding – essentially a bird’s three options for spending its time.
She then deduced how much time the birds had to spend feeding rather than roosting over the course of a day.
“We ended up discovering that it’s extremely variable,” says Shankar. In early summer, when flowers were plentiful, the birds could meet their daily energy needs with just a few hours of food, spending up to 70 percent of the day simply roosting, she found. But when the flowers became rarer after the arrival of the summer monsoon rains, the birds of one site perched only 20% of the time and used the rest of the day for feeding.
“That’s 13 hours a day!” Shankar said. “I can’t spend 13 hours a day running. I don’t know how they do it.
Seriously, relax
Hummingbirds have a trick to help them deplete their energy reserves: When a bird is in danger of running out of energy, it can become numb at night, lowering its body temperature almost to that of the surrounding air, sometimes just a few degrees above. freeze. When in torpor, the bird appears almost comatose, unable to respond quickly to stimuli and breathing only intermittently. The strategy can save up to 95% of hourly time metabolic costs during the cold nights, Shankar calculated. This can be essential after days when a bird has fed less than usual, such as after a storm. This also helps birds save energy to accumulate fat before migration.
Shankar is currently studying the parts of their physiology that hummingbirds favor during torpor, looking to see which genetic products they can’t live without. “If you are a hummingbird operating at 10% of your normal metabolism, what is that 10% that keeps you alive? » she asks.
One set of genes that birds seem to leave intact are those responsible for their internal clock. “It is important for them to do things at the right time when they are in torpor,” Shankar says. For example, to be ready for the day, birds begin to emerge from their torpor about an hour before sunrise, well before visible light signals.
Managing sugar
To fuel their extremely high metabolic rate, hummingbirds consume about 80% of their body weight in nectar each day. That’s the equivalent of a 150-pound person drinking nearly a hundred 20-ounce Coca-Colas a day — and the nectar is often much sweeter than soda.
The human gut is unable to absorb sugar as quickly, which is part of the reason why consuming too much soda or Halloween candy upsets the stomach, according to Ken Welchcomparative physiologist at the University of Toronto at Scarborough. Hummingbirds cope with the onslaught by having a leaky gut, which allows sugars to enter the bloodstream between intestinal cells instead of passing only through them. This quickly removes sugar from the intestine, before it can cause problems. This rapid transport, and likely other adaptations as well, allows hummingbirds to achieve blood sugar levels up to six times higher than those seen in humans, Welch says.
Such an amount of sugar in the blood leads to serious physiological problems in humans. This causes more sugar molecules to clump onto the body’s proteins, a process known as glycation; In the long term, excessive glycation leads to many complications of diabetes, such as nerve damage. It’s still unclear how hummingbirds avoid glycation problems, Welch says, but clues are starting to emerge. One study, for example, found that bird proteins contain fewer amino acids that are more prone to glycation than mammalian proteins, and those that remain are often buried deep within the protein, where they are less exposed to circulating sugars.
Other, as yet unknown, strategies for dealing with high blood sugar may one day provide practical benefits for managing diabetes in people. “There could be a gold mine in the hummingbird genome,” says Welch.
Take a Metabolic Leap
By the end of its nightly fast, a hummingbird has almost exhausted its sugar stores, which poses an opposite metabolic challenge. “How does it wake up and fly?” Welch asks. “There’s only fat to burn.”
Hummingbirds have evolved to be remarkably agile at switching their metabolism from burning sugar to burning fat, he found. “It requires a huge 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 could have that kind of control over our fuel consumption, we would be delighted. »
Save water – or not
Sugar isn’t the only challenge posed by a nectar-rich diet. After all, nectar is mostly water – and birds that drink that much fluid have to get rid of most of it, without losing electrolytes. As a result, hummingbird kidneys are highly adapted to scavenge electrolytes before they are excreted. “They almost pee with distilled water,” says Carlos Martinez del Rioecophysiologist now retired from the University of Wyoming.
But this poses another problem: If a hummingbird continued to produce diluted urine overnight, it would die of dehydration before morning. To avoid this, hummingbirds close their kidneys every night. “They go into what, in a human being, would be considered acute kidney failure,” says Martinez del Rio. “Hummingbirds have to do that, otherwise they would pee themselves.”
Fly high – gradually
A hummingbird’s metabolic demands are already quite strong at sea level. But many species live at high altitudes, where the thin air contains less oxygen and provides less resistance to hovering thrust. Consider the giant hummingbird, the world’s largest, which can live in the Andes at altitudes above 14,000 feet, higher than many helicopters can fly. To cope with these conditions, birds developed blood richer in hemoglobin, explains Jessie Williamsonornithologist at Cornell University.
But some birds face an even tougher challenge, as Williamson discovered. The giant hummingbirds are large enough that researchers can attach satellite tracking tags to them, as well as smaller geolocators. Williamson and his colleagues therefore decided to equip the birds with trackers. After thousands of hours spent trying to capture birds with a net, researchers successfully attached trackers to 57 birds using custom-made harnesses made from elastic jewelry cords.
Although they recovered tracking data from only eight birds, even this small sample had a big surprise: Some birds lived in the high Andes year-round, while others – which turned out to be one species distinct, hitherto unrecognized – migrate to the Andes every year from breeding grounds along the Chilean coast. That means they face not only the obvious challenges of a long migration – a round trip of around 5,000 miles – but also the need to adapt to thinner air during their journey.
Their secret? Do it gradually. “It’s a lot like 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 lighter and cheaper, researchers like Williamson also hope to track smaller hummingbird species. This, combined with other technological advances in research, could offer many new surprises about the biology of these small and amazing birds.
This item originally appeared in Knowable Magazinea non-profit publication dedicated to making scientific knowledge accessible to all.
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