Some male hummingbirds wield their bills as weapons | Science News for Students

Some male hummingbirds wield their bills as weapons

Some species may trade feeding efficiency for fighting prowess
Feb 19, 2019 — 6:45 am EST
two male hummingbirds fighting at a bird feeder

Hummingbirds fight over a feeder. The shapes of some male bills appear to have evolved to equip them for such combat.

CarolinaBirdman/iStockphoto

A hummingbird’s long, curved bill (or beak) is perfectly designed to sip the nectar deep inside trumpet-shaped flowers. In fact, the types of flowers a species will visit are closely tied to the shape of the birds’ beaks. Long, narrow flowers, for instance, are visited by hummers with equally long bills. Flower shape equals bill shape. But there’s more to that equation, suggests a new study. And it involves a fair amount of combat.

For decades, scientists had argued that the shape of hummingbird bills must depend on the flowers these birds tap for food.

Some hummingbirds can beat their wings up to 80 times per second. This lets them zip from flower to flower and hover while eating. But all that movement requires a lot of calories. Hummingbirds sip plenty of sugary nectar to fuel that activity. Bills that fit perfectly inside flowers help birds reach more nectar and drink it down faster. Their long tongues lap up the sweet reward located at the base of the bloom.

Flowers pollinated by those birds get more pollen moved from flower to flower, because these birds tend to visit the same types of flowers again and again. So the close tie between bill shape and flower shape seemed like an open-and-shut case of co-evolution. (That’s when the traits of two different species that interact in some way change together over time.)

a close-up photo of saw-like teeth on a male hummingbird bill
Some males’ bills have saw-like “teeth” and hooked tips that they use to bite other birds.
Kristiina Hurme

Except for one thing: Males of some tropical species don’t show the same bill adaptation to fit flowers that the females have. Instead, their bills are stronger and straighter with pointy tips. Some even have sawlike structures along the sides. In short, they kind of look like weapons. They are not slicing open flowers. So what’s up with their beaks?

Maybe males and females simply feed from different types of flowers, scientists proposed. That might explain their different bills. But Alejandro Rico-Guevara was not convinced. He’s an evolutionary biologist at the University of California, Berkeley. And he has a passion for hummingbirds.

There’s another difference between the sexes, he notes: Males fight one another. Each defends a territory, and all of the flowers and females within it. He thinks that competition between males — and the combat that results — led to the weapon-like features on the guys' bills.

Taking it slow

Studying hummingbirds isn’t easy. They’re fast fliers, clocking in at speeds up to 55 kilometers per hour (34 miles per hour). They can change direction in an instant. But Rico-Guevara knew that if males had weaponized bills, it would come at a cost. Bills designed to fight would not be as well adapted to eating. So he first had to learn how hummingbirds drink nectar to test his hypothesis.

To do that, he teamed up with researchers at UC Berkeley and the University of Connecticut in Storrs. Using high-speed cameras, they filmed hummingbirds feeding and fighting. They placed some cameras underneath hummingbird feeders. This let the scientists record how the birds used their bills and tongues while drinking. The researchers used the same high-speed equipment to record males fighting.

a close-up photo of the sharp bill of a hummingbird
The pointed tip of this male’s beak is perfect for stabbing competitors, but maybe not so good for sipping nectar.
Kristiina Hurme

Slowing down the videos, the team saw that hummingbirds lap up nectar with their tongues. This was a new discovery. Before this, scientists thought nectar moved up the tongue almost like liquid sucked up a straw. Instead, they found that the tongue unfurls as it enters liquid, like a palm frond opening. This creates grooves, allowing the nectar to flow in. When the bird pulls its tongue back in, its beak squeezes the nectar out of those grooves and into its mouth. Then the bird can swallow its sweet reward.

Females, the team found, had curved bills that were perfectly designed to max out the amount of nectar picked up in each sip. But the straighter beaks of some males didn’t seem to get as much out of each drink.

Slow-motion video of males fighting showed that those straight bills might have an advantage in combat, though. These birds stab, bite and pull feathers from males invading in their territory. Straighter bills are less likely to bend or be damaged than curved ones. It’s like poking someone with a straight finger, rather than one that’s bent, explains Rico-Guevara. The pointy tips make it easier to jab through a protective layer of feathers and pierce the skin. And the birds use the sawlike “teeth” along the edges of some bills to bite and pluck feathers.

“We were really surprised by these results,” says Rico-Guevara. This was the first time anyone had seen what happens when male hummingbirds fight. No one knew they wielded their bills as weapons. But that behavior helps explain some of the strange structures found on the males’ bills.

It also highlights the trade-offs these birds face, he says. His team is still studying the videos of males feeding. But if they really get less nectar per sip, it would suggest they can either be good at getting food, or good at defending flowers from others (keeping the food to themselves) — but not both.

His team’s findings were published January 2 in Interactive Organismal Biology.

Rico-Guevara has many more questions. For example, why don’t males in all species that fight have weapon-like bills? Why don’t females have these features? And how could such structures evolve over time? He has experiments planned to test these and other questions in the future.

This study shows there’s still a lot to learn, even about birds that people thought they understood well, says Erin McCullough. The behavioral ecologist at Syracuse University in New York was not involved with this study. Its findings also highlight how an animal’s shape and body structures almost always reflect trade-offs, she notes. “Different species prioritize different tasks,” such as feeding or fighting, she says. And that affects how they look.

Hummingbird bills are perfect for sipping — unless they are modified to fight off intruders.
UC Berkeley/YouTube

Power Words

(more about Power Words)

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

behavioral ecologist     A scientist who studies how animal behavior relates to where animals live.

biology     The study of living things. The scientists who study them are known as biologists.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

calorie     The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.

diet     The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

equation     In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary biologist     Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).

evolve     (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, such an evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.

frond      The large leaf (or leaf-like) part of a fern, palm or other plant whose edges have many parts to it, sometimes known as leaflets.

hover     To above the ground, maintaining an intentional position.

hypothesis     (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

liquid     A material that flows freely but keeps a constant volume, like water or oil.

nectar     A sugary fluid secreted by plants, especially by flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.

palm     A type of evergreen tree that sprouts a crown of large fan-shaped leaves. Most of the roughly 2,600 different species of palms are tropical or semitropical.

pollen     Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.

reward     (In animal behavior) A stimulus, such as a tasty food pellet, that is offered to an animal or person to get them to change their behavior or to learn a task.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

Citation

Journal:​ ​​A.​ ​Rico-Guevara et​ ​al.​ Shifting paradigms in the mechanics of nectar extraction and hummingbird bill morphology.​ Interactive Organismal Biology.​ ​​Vol.​ ​1,​ ​Published online January 2, 2019. doi:​ ​10.1093/iob/oby006.