CU Boulder researcher analyzes 50 years of data to show the relationship between certain birds’ unorthodox behavior and their traits

Hummingbirds are iconic, easily recognized by their plumage, needlelike beaks and unique way of flying. With several hundred species in the family, different species of hummingbirds are distinct from one another in ways that are sometimes less noticeable. 

“It is the great diversity of forms in this family of birds which renders the study of them so very interesting,” John Gould, a 19th-century English ornithologist and collector of hummingbirds, wrote. “If these little objects were magnified to the size of eagles, their structural differences would stand out in very bold relief.” 

This belief led Robert Colwell, museum curator adjoint of entomology and zoology for the University of Colorado Museum of Natural History—as well as co-researchers Gregor Yanega, Alejandro Rico-Guevara, Thiago Rangel, Karolina Fučíková and Diego Sustaita—to collect and evaluate a large amount of data on hummingbirds’ physical features for a research paper published in The American Naturalist. 

The researchers found that large feet—an uncommon trait for hummingbirds, whose feet are usually small to the point of seemingly disappearing when tucked away—correlated with short bills in hummingbirds that engage in a particular, unorthodox feeding behavior. 

Legitimate and illegitimate feeding 

Hummingbirds are pollinators, and the flowers they feed from deposit pollen onto different parts of their bodies so that their flight from flower to flower functions to fertilize the plants.

“The flowers they visit produce nectar for the sole purpose of attracting hummingbirds,” Colwell explains, adding that different species of plants deposit pollen on different species of hummingbirds or different parts of a single species’ body. This specificity is necessary because “if the bird delivers the wrong pollen, then it just clogs up the plant’s female organ, the stigma, without fertilizing the flower,” he says. 

While some plants have adapted to get pollen onto different parts of hummingbirds, the focus of this research is on species-based pollen delimitation. The main way that plants attract only certain hummingbird species is to develop corollas (the whorl of petals that protects the flower’s reproductive organs) with lengths or curvatures that not all hummingbirds’ bills can fit into. 

“The plants sort of partition the hummingbirds based on bill length, bill curvature and flowering season,” Colwell explains. “It gets more complicated the more species are involved. In a tropical lowland community, there could be 50 or 60 hummingbird-pollinated species of plants.” 

This evolutionary strategy is successful only when hummingbirds feed “legitimately”—that is, through the mouth of the corolla. A hummingbird with a short beak cannot reach the nectar of a flower with a long corolla; however, such a bird may access that nectar “illegitimately” by inserting its beak through natural opening near the base of a flower, poking a hole in the base using its beak, or using a hole made by another hummingbird. This method is called illegitimate because, according to Colwell, it “does nothing to pollinate the plant and imposes an energetic cost on both the plant and legitimate visitors by depleting nectar.” 

Why feed illegitimately? 

Considering the consequences of feeding illegitimately for both the flowers that a nectar thief relies on and other birds, why does this behavior exist? There are a couple of reasons, Colwell says. For one thing, it gives short-billed hummingbirds access to nectar that they otherwise could not reach.

The other reason is that, while most plants force legitimately feeding hummingbirds to hover, according to Colwell, this is not necessary for illegitimate feeders, who can instead cling to a nearby surface while stealing the nectar. Birds that cling to plants to feed, instead of hovering (called clingers), are therefore able to conserve energy in a way that non-clingers cannot. 

Hovering is the most expensive means of vertebrate locomotion, Colwell says. Consequently, hummingbirds are “on a very tight schedule” in terms of energy, “and if the birds have no nectar and insufficient insects to capture for a couple of days, they could die.”

For these reasons, saving energy by perching to feed can be the difference between life and death for a hummingbird. This is especially true in the case of the coquettes, a high-elevation Andean group that developed perching behavior early in the evolution of hummingbirds, Colwell says. Their habitat makes it even more expensive to hover: the air is thinner, making it harder to fly and breathe, and it’s colder, making the maintenance of healthy body temperatures more difficult. 

“So, there’s strong natural selection to avoid hovering, if possible,” he explains. “There are some species that actually walk on the ground and feed on flowers that are near the ground.” 

Although clinging and stealing nectar saves energy, all species of hummingbird feed legitimately while hovering at least sometimes, Colwell says. This is because if illegitimate feeding was ubiquitous, “the flowers would go extinct because they wouldn’t be getting pollinated. So, it’s kind of a game theory thing, where there are cheaters, but you can’t have all cheaters because then the game won’t go on.” 

Morphological manifestations of clinging 

As Colwell recounts, the study began with an observation that he made about the morphological differences between clingers and non-clingers: “It was an accidental discovery I made 50 years ago in Costa Rica. I was studying a high-elevation site with four species. The ones that are important to this are a very long-billed hummingbird with a large body and a smaller bird with a shorter bill.” 

The expedition was using mist nets to humanely capture birds for measurement, and he noticed that the smaller bird with the short bill had feet that were bigger than those of the larger bird. The little bird perched on and pierced flowers to steal nectar that the larger bird would consume legitimately.

“I got the idea,” Colwell says, “that maybe this is general; maybe there’s a negative correlation between bill size and foot size. That’s how it all started. Sometimes scientific discoveries are accidental in that way, or intuitive, and then you have to go on and look at it statistically.” 

Specifically, Colwell and his research colleagues hypothesized that clingers would have relatively longer toes and claws, as well as shorter tarsi (the bones connecting to bird’s digits to their lower legs) to make it less energetically costly to cling while feeding.

“The claw is very important in grasping the flower, or the stem, or the leaf, or whatever it’s perching on,” Colwell explains. “Biomechanically, it’s a crucial part of the gripping force.” 

To determine if this hypothesis were supported by statistics, the researchers collected measurements of hummingbird feet (including the tarsus, hallux or hind toe, hallux claw and middle toe claw) and bills over many years. Ultimately, they pooled three datasets consisting of 1,154 museum specimens and 404 field captures, with 220 of about 340 recognized species of hummingbird represented. 

Within these data, they found that clingers showed a negative correlation between bill and hallux claw size when body weight was accounted for, with no other strong correlations detected. This confirmed part of the hypothesis: among clingers with small bills, the foot span is increased by a longer claw on the hallux. However, clingers did not have smaller tarsi. 

According to Colwell, a role for tarsi was anticipated based on its presence in biomechanical studies of clinging behavior in other birds, such as woodpeckers. “We expected that to happen, and it didn’t,” he says. “It just means that hummingbirds do it their own way.” 

Losing big feet

In addition to determining the correlation between bill and hallux claw size in clingers, the researchers used phylogenetic inference, a method of finding the evolutionary “family tree” of related species, to estimate the number of independent origins of clinging behavior in hummingbirds. “We were surprised at how many different, independent times perching to feed with larger feet arose in the hummingbird phylogeny,” Colwell says, adding that it was over two dozen times. 

Despite this, clinging to feed doesn’t seem to be a good long-term strategy, as it doesn’t lead to much speciation (i.e., further evolutionary development) except in the coquette clade, Colwell explains. This may be in part because the additional weight of larger feet would be strongly selected against in most cases, he says. Consequently, a branch of hummingbirds with large feet will tend to lose that trait once it is no longer useful. 

For example, Colwell recounts, “there are some species that walk on the ground and feed on flowers that are near the ground, so they have big feet. Late in that branch of the evolutionary tree, some of that group diversified the tropical lowlands, where they lost their big feet and now have longer bills. It beautifully confirms the overall pattern.” 

Colwell adds that what makes the study significant is its focus on an often-overlooked feature of hummingbirds.

“When you see hummingbirds, you don’t think about their feet, you think about their wings, their color, their dives, their voice, their behavior,” he says. “Their feet have been ignored for 150 years, since John Gould, who was a very good observer, marveled at them. Nobody paid any attention to it until we got interested in it 50 years ago.” 

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