The Duality of Hummingbirds

Dr. Robert Colwell has been measuring hummingbird feet for about 40 years. “This was just a private passion of mine for a few decades,” said Colwell, an evolutionary ecologist at the University of Connecticut and the University of Colorado Museum of Natural History.

Feet aren’t usually a hummingbird’s selling point. Most people gravitate towards their bejeweled plumage or their needle-like beaks. But feet, Colwell said, are just as essential to their nectar-eating lifestyle.

Hummingbirds feed on floral nectar; in return, they shuttle pollen and help plants reproduce. This relationship, which is over 30 million years old, has shaped both parties—especially their beaks and flowers. Some bird-plant pairings have evolved seemingly perfect fits, like the sword-billed hummingbird (Ensifera ensifera), whose bill sheathes neatly into the passionflower (Passiflora mixta). In other cases, hummingbirds have weaponized their bills with hooks, daggers, and serrations to defend flowers against intruders. Colwell, however, felt that beaks didn’t tell the whole story.

Early in his career, he noticed that the fiery-throated hummingbird (Panterpe insignis), despite its short bill, could rob nectar from long flowers. This species, he noted, has huge feet for a hummingbird. “They cling upside down and sideways and climb around with their feet… Once they’re perched, they go boink, like a woodpecker, and stab a hole,” he added, miming with his hands. “While their bill is in there, they can just steal, take all the nectar.” 

Over the next several decades, Colwell and his colleagues would measure over 1,500 hummingbirds to test if other clingers—species that hang sideways and upside down during feeding—also have large feet. Recently, he recruited the help of biomechanists, evolutionary biologists, and statisticians to tease apart the trends. And in a new study published last month, they finally reported that the short-billed, large-footed body plan helps birds grasp using less force. It’s widespread amongst clingers, evolving some two dozen times—“a very unusually large example of evolutionary convergence,” said Colwell. 

Hummingbirds, then, offer a compelling contradiction. They can feed with finesse but also with brute force. As you look closer at the clade, contrasts and contradictions, usually tied to the quest for nectar, continue to arise. Their hovering flight appears static, yet it requires loads of energy. Their zippy movement is fairy-like (Walt Disney noted that Tinker Bell “flits about with the speed of a hummingbird”), yet their battles are fierce jousts. New discoveries continue to surprise, but the plant-pollinator relationships that spawned these biological feats are quickly unraveling due to climate change. With so much still to learn, scientists worry we might not even know what we’re losing. 

Hummingbirds are like race cars, said Dr. Alejandro Rico-Guevara, an evolutionary biologist and ecophysiologist at the University of Washington. They’re the only birds that can hover, and although it looks effortless, it’s actually draining. 

Hummingbirds beat their wings 10 to 80 times each second using powerful pectoral muscles. Their hearts beat up to 1,300 times per minute. Relative to their body size, they have by some estimates the highest metabolic rate among vertebrates.  “Their tank is being depleted constantly because the engine is running all the time,” Rico-Guevara said. That’s where flowers step in. 

Each day, a hummingbird can exceed 1,000 floral visits—each one a tiny pit stop—and consume three times its weight in nectar. As it moves, it shuttles pollen between plants. Compared to insects (or worse yet, the wind), birds are efficient pollinators. They can travel far, traverse unfavorable weather, and because they visit fewer types of plants than insects, they’re less likely to deliver pollen to the wrong species. “It’s like the postal service,” Rico-Guevara said.

About 7,000 plant species across the Americas have specialized on hummingbird pollinators, ranging from sea level to mountaintops and from Alaska to Tierra del Fuego. Called ornithophilous plants, they typically have long or curved flowers, which they extend in positions only accessible to hovering visitors. Some carefully place pollen on the bird’s head where it arrives on the reproductive doorsteps of its future mate. In return, these flowers provide a generous portion of nectar each visit and an exclusive food resource. 

Pairing up seems like a win-win: The more specialized the relationship, the more direct the pollen delivery, and the more monopolized the nectar. In extremely specialized pairings, like the buff-tailed sicklebill (Eutoxeres condamint) and Andean bellflowers (Centropogon granulosus), these flowers and bills fit like a lock and key. 

But robbers prefer a window to a key. 

To rob a flower, hummingbirds either create holes or exploit existing ones near the nectaries. For the pillaging strategy to be profitable, it must be more efficient than orthodox feeding during flight. That’s where large feet step in. Rico-Guevara, who was a coauthor with Colwell on the new study, said that a long hallux toe—the one that faces backwards—reduces the amount of force required to hang on while the bird punctures the flower. 

If the flower is long, the bird gains access to an otherwise unattainable resource. Even if the flower is short and accessible through flight, hovering is expensive, and perched feeding, made even more efficient with large feet, saves energy. “Hummingbirds are very famous for hovering flight, but they actually will take any opportunity to stop hovering,” said Rico-Guevara. “[Perched feeding] is pure energy gain.”

To Dr. Ethan Temeles, a biologist at Amherst College who was not involved in the new study, the repeated evolution of large-footed clingers makes sense. Typically, a hummingbird will do “the most cost effective thing,” he said. “We tend to look at these plant pollinator relationships as mutualisms, but you have to realize that both the animals and the plants are trying to do what’s best for them.” 

Darwin saw this clear as day, he said. “At the end of On The Origin of Species, [Darwin] says he can think of no organism that does something for the benefit of another… the plants are actually in many respects taking advantage of the pollinators.” And in robbery, the pollinators are taking advantage of the plants. 

In floral feeding, hummingbirds aren’t as serene as they might seem. They are no pacifists with each other, either. And, you guessed it, their fights are in the name of nectar. 

Dr. Jay Falk, an evolutionary biologist at the University of Colorado Boulder, has spent countless hours watching hummingbird fights in the lowland rainforests of Gamboa, Panama. “Up in Gamboa, you almost certainly will see aggressive interactions immediately,” he said. 

Most scuffles are simple chases. “You’ll see one bird feeding at your feeder or a flower, and then all of a sudden, swoosh, this other hummingbird will come by,” Falk said. “I’d bet a hummingbird is chased or is the one chasing another many, many, many times a day.” Less commonly, the combatants will grapple with each other in the air, and in rare, highly escalated bouts, they fight to the ground and jab with their bills. For the most part, Falk said, these disputes are over food rather than mates. 

Falk studies white-necked jacobins (Florisuga mellivora). In this species, males are the more aggressive sex, and they don a blue-green cape of feathers over their white underparts. The females, meanwhile, are less aggressive, more petite, and come in two distinct types: About 70% are a drab green, and the other 30% are a bright spitting image of the males. “If you see them just out in the wild, you wouldn’t be able to tell the difference between these females and a male,” said Falk. 

“The males are constantly initiating fights,” Falk added. Hummingbirds of all species see the disguised females, and despite their more waifish stature, think “I’m going to avoid messing with anything that looks like that,” he added.

It’s not just white-necked jacobins fighting, either. Combat is a hummingbird hallmark. Some species in South America have taken extreme measures, arming their bills with weapons. Male long-billed hermits (Phaethornis longirostris), for example, have dagger-like bill tips. Males of other species have bills with hooks or even with saw-like serrations that bend towards their throats. None of these weaponized bills help efficient feeding, Rico-Guevara said. “But, if you can prevent others from accessing the nectar, then you have these resources all to yourself.” 

Hummingbird feeding, with its diverse tactics, embodies a core evolutionary principle: Evolution doesn’t perfect. Different ecological contexts allow for different strategies, and although fairy-like feeding on perfectly matched flowers seems like the pinnacle of coevolution, it’s neither the best way nor the only way. 

The duality of hummingbirds is nothing new; in fact, it’s a staple of Mesoamerican culture. The Aztec god of the sun and war, Huītzilōpōchtli, is often depicted as a hummingbird. When Aztec warriors died in battle, they are thought to be reincarnated as hummingbirds. Meanwhile, in Mayan stories, the gods created hummingbirds by carving an arrow out of jade. To those in touch with nature, this polarity—as beautiful jewels and deadly weapons—is clear. Scientists might just be catching up with traditional knowledge. But with 10% of hummingbird species globally threatened and 60% in decline, they may not have long. 

Temeles called in from his field site in Dominica, a 290-square-mile island in the Caribbean’s Lesser Antilles. Hurricane Maria, a category 5 storm, throttled the island in September 2017. “This place looked like it had been bombed,” he said. 

The hummingbirds weren’t spared. About 80% died—for the most part, not because the storm killed them, but because it killed the plants on which they feed. Temeles worries that as climate change makes extreme storms commonplace, island hummingbirds and mainland ones with small ranges will go extinct.

Heating habitats threaten the plant-pollinator relationship, too. It’s pushing species towards higher latitudes and altitudes, advancing flowering time earlier in the year, and altering when and where birds migrate. If these forces pull flowers and birds out of sync, both in time and in space, then they could tear apart vital species relationships. Perhaps more existentially, Temeles said, climate change could put hummingbirds on an escalator to extinction, en route to a habitat that simply doesn’t exist: “There’s a limit to how far up a mountain you can go before there is no more.”