Hummingbirds Are Where Intuition Goes to Die
A long-held belief about how hummingbirds drink is wrong—and the truth is really weird.
When Margaret Rubega first read about how hummingbirds drink, she thought to herself: That can’t possibly be right.
Hummingbirds drink nectar using tongues that are so long that, when retracted, they coil up inside the birds’ heads, around their skulls and eyes. At its tip, the tongue divides in two and its outer edges curve inward, creating two tubes running side by side. The tubes don’t close up, so the birds can’t suck on them as if they were straws. Instead, scientists believed that the tubes are narrow enough to passively draw liquid into themselves. That process is called capillary action. It’s why water soaks into a paper towel, why tears emerge from your eyes, and ink runs into the nibs of fountain pens.
This explanation, first proposed in 1833, was treated as fact for more than a century. But it made no sense to Rubega when she heard about it as a graduate student in the 1980s. Capillary action is a slow process, she realized, but a drinking hummingbird can flick its tongue into a flower up to 18 times a second. Capillary action also is aided by gravity, so birds should find it easier to drink from downward-pointing flowers—and they don’t. And capillary action is even slower for thicker liquids, so hummingbirds should avoid supersweet nectar that’s too syrupy—and they don’t.
“I was in this very odd position,” says Rubega. “I was only a graduate student and all these really well-known people had done all this math. How could they be wrong?”
Even while she turned her attention to other birds, the hummingbird dilemma continued to gnaw at her. And decades later, as a professor at the University of Connecticut, she hired a student named Alejandro Rico-Guevara who would help her solve the mystery.
Born in Colombia, Rico-Guevara remembers spotting a hermit hummingbird on a fateful field trip in the Amazon. In the jungle, most animals are heard rather than seen, but the hermit flew right up and hovered in front of his face. “It was just there for a split second but it was clear that it had a completely different personality than other birds in the forest.” He fell in love, and started studying the birds. And when he read the capillary-action papers, he felt the same pang of disbelief that Rubega did. “We decided to go after it,” says Rubega. “Is it capillary action? And if not, what’s going on? We just wanted to know.”
Rico-Guevara handcrafted artificial flowers with flat glass sides, so he could film the birds’ flickering tongues with high-speed cameras. It took months to build the fake blooms, to perfect the lighting, and to train the birds to visit these strange objects. But eventually, he got what he wanted: perfectly focused footage of a hummingbird tongue, dipping into nectar. At 1,200 frames per second, “you can’t see what’s happening until you check frame by frame,” he says. But at that moment, “I knew that on my movie card was the answer. It was this amazing feeling. I had something that could potentially change what we knew, between my fingers.”
Here’s what they saw when they checked the footage.
As the bird sticks its tongue out, it uses its beak to compress the two tubes at the tip, squeezing them flat. They momentarily stay compressed because the residual nectar inside them glues them in place. But when the tongue hits nectar, the liquid around it overwhelms whatever’s already inside. The tubes spring back to their original shape and nectar rushes into them.
The two tubes also separate from each other, giving the tongue a forked, snakelike appearance. And they unfurl, exposing a row of flaps along their long edges. It’s as if the entire tongue blooms open, like the very flowers from which it drinks.
When the bird retracts its tongue, all of these changes reverse. The tubes roll back up as their flaps curl inward, trapping nectar in the process. And because the flaps at the very tip are shorter than those further back, they curl into a shape that’s similar to an ice-cream cone; this seals the nectar in. The tongue is what Rubega calls a nectar trap. It opens up as it immerses, and closes on its way out, physically grabbing a mouthful in the process.
“This has been going on literally under our noses for the entire history of our association with hummingbirds and there it was,” says Rubega. “We were the first to see it.”
This same technique is also how the hummingbird swallows. Every time it extends its tongue, it presses down with its beak, squeezing the trapped nectar out. And since there’s limited space inside the beak, and the tongue is moving forward, there’s nowhere for that liberated nectar to go but backward. In this way, the tongue acts like a piston pump. As it pulls in, it brings nectar into the beak. As it shoots out, it pushes that same nectar toward the throat. The tongue even has flaps at its base, which fold out of the way as it moves forward but expand as it moves backwards, sweeping the nectar even further back.
The thing that really astonishes Rico-Guevara about all of this is that it is passive. The bird isn’t forcing its tongue open—that happens automatically when the tip enters liquid, because of the changing surface tension around it. Rico-Guevera proved that by sticking the tongue of a dead hummingbird into nectar—sure enough, it bloomed on its own. Likewise, the tongue closes automatically. It releases nectar automatically. It pushes that nectar backward automatically. The bird flicks its tongue in and out, and all else follows.
In hindsight, the surprising reality of the hummingbird tongue should have been entirely unsurprising. Almost everything about these animals is counterintuitive. Hummingbirds are the bane of easy answers. They’re where intuition goes to die.
Consider their origins. Today, hummingbirds are only found in the Americas, but fossils suggest that they originated in Eurasia, splitting off from their closest relatives—the scythe-winged swifts—around 42 million years ago. These ancestral hummingbirds likely flew over the land bridge that connected Russia and North America at the time. They fared well in the north, but they only thrived when they got to South America. In just 22 million years, those southern pioneers had diversified into hundreds of species, at least 338 of which are still alive today. And around 40 percent of those live in the Andes.
As evolutionary biologist Jim McGuire once told me, “the Andes are kind of the worst place to be a hummingbird.” Tall mountains mean thin air, which makes it harder to hover, and to get enough oxygen to fuel a gas-guzzling metabolism. And yet, the birds flourished. Their success shows no sign of stopping, either. By comparing the rates at which new species have emerged and old species go extinct, McGuire estimated that the number of hummingbird species will probably double in the next few million years.
As they evolved, they developed one of the most unusual flying styles of any bird—one that’s closer to insects. The wings of medium-sized species beat around 80 times a second, but probably not in the way you think. When I ask people to mimic a hummingbird’s wingbeats, they typically stick their hands out to the side and flap them up and down as fast as they can. That’s not how it works. Try this, instead. Press your elbows into your sides. Keep your forearms parallel to the ground and swing them in and out. Now, rotate your wrists in figure eights as you do it. Congratulations, you look ridiculous, but you’re also doing a decent impression of hummingbird flight.
That unusual wingbeat allows them to hover, but it also allows for more acrobatic maneuvers. Hummingbirds use that aerial agility to supplement their nectar diet with insects, which they snatch from the air. While many birds can do that, they typically have short beaks and wide gapes. Hummingbirds, by contrast, have long flower-probing bills and narrow gapes. “It’s like flying around with a pair of chopsticks on your face, trying to catch a moving rice grain,” says Rubega.
But once again, she has shown that there’s more to these birds than meets the eye. Another of her students, Gregor Yanega, found that as the birds open their mouths, they can actively bend the lower half of their beaks, giving it a pronounced kink and getting it out of the way. Then, the hummingbirds essentially ram insects with their open mouths.
High-speed cameras again revealed their trick. “The moment Gregor first saw a bird fly into frame and open its beak, he stopped, and said: Hey, can you look at this?” says Rubega. She walked in and he played the footage. She asked him to play it again, and he did. Just one more time, she said. He played it again.
“That is wild, and you should know that nobody has ever seen that before you,” she told him.