Feisty feeding for flowers foments fighting: a tale of the hummingbird bill

Feeding, flower shape, and fighting: Shifting paradigms in the mechanics of nectar extraction and hummingbird bill morphology

Alejandro Rico-Guevara, Margaret A. Rubega, Kristiina J. Hurme, & Robert Dudley

  1. What makes hummingbirds such ‘extreme’ animals anyway – why should we study them?

A: Hummingbirds are a study in extremism, especially the extremes of being small: they are some of the smallest endotherms (warm-blooded animals) and have some of the smallest amniote genomes, yet have one of the highest metabolic rates of any vertebrate for their size. They can hover in mid-air like insects and helicopters and absorb pure sugar straight from their blood serum. Biologists study extremes in the animal kingdom to get an idea of what factors of the environment and ecology shape how animals look and behave. Hummingbirds fuel their high-energy lifestyle by consuming nectar, a sugary ‘reward’ produced by plants to attract pollinators. To eat as much nectar as possible, some species of hummingbird must control territories containing certain flowers, and others (and there are over 300 of them) visit only certain kinds of flowers. In turn, some flowers have adapted to being pollinated by specific hummingbird species. This close relationship between a given hummingbird and its chosen flower is called diffuse coevolution. This kind of evolutionary relationship can shape how two species look in tandem… as the flower shape changes, for example, so does the beak the hummingbird uses to feed from the flower.

  1. Why do hummingbird bills look so different if they all sip nectar from flowers?

A: Not all flowers look alike, right? Some flowers are tall and narrow, whereas others are short and squat. Different hummingbirds use different-shaped bills to feed from different flowers. Now in general, hummingbird beaks are long and skinny, made for reaching the bottom of flowers, where all the sugary nectar is. Up until recently, hummingbirds’ tongues were thought to act like capillaries when soaking up nectar – i.e. how water will move up a tiny glass tube, seemingly against the force of gravity. Capillary action doesn’t seem to be the only driving force in the design of hummingbird beaks. So, researchers used very high-speed cameras to film hummingbirds feeding on nectar. They found that hummingbirds have forked tongues, like snakes, that once in contact with nectar unfurl into fringed ends. The rapid expansion of little channels along the tongue propel nectar from the fringes upwards. The tongue is sucked back into the beak, where nectar is wrung out into the mouth.

  1. So hummingbird beaks work like straws?

A: Nope… hummingbird tongues act like little pumps, or ‘elasto-hydrodynamic’ pumps, to be precise. As hummingbird tongues are shot out of the beak into the flower, they remain bundled up – only to expand as they contact the nectar. This sudden expansion generates a space for nectar to fill and does so very quickly. Nectar fills the once-folded grooves along the tongue and then the whole apparatus is slurped back into the beak, and then wrung out like a wet towel when protruded again. Some hummingbird species even have little ‘teeth’, called tomial serrations, that help clean the ends of the tongue and keep the nectar from squirting out the sides of the beak… hummingbirds are fastidious feeders.

  1. WAIT – Hummingbirds have teeth?!

A: Tomial serrations aren’t teeth like we know them – they’re extensions of the beak or bill, so instead of being made of bone (like our skeleton) or enamel and dentine (like our teeth), tomia are made of keratin – the same material in our fingernails, hair, and a rhino’s horn. Weirdly enough, in most hummingbirds these teeth are tiny, point away from the throat and towards the tip of the beak, but in some species, the tomia look like the sort of recurved fangs you’d expect from a snake. Why? Turns out that hummingbirds are scrappy little animals, with personalities and conflicts that seem much, much bigger than their tiny frames would suggest. These serrations help miserly hummingbirds tear feathers off would-be intruders.

See, in order to control enough flowers to feed their fervent lifestyle, hummingbirds need to be constantly on the lookout for other birds trying to pilfer their stash of nectar-producing flowers. These territories vary between species and the sexes, based on how many flowers a bird needs to maintain itself, or how productive those flowers might be. Maybe a couple big flowers are all one species of hummingbird might need, or maybe a species needs a lot of smaller flowers – this directly affects the size of the territory birds need to patrol (and some individuals even horde more flowers than they need).

  1. So what are hummingbird bills good for anyway?

A: Your cat or dog uses its mouth to eat, to groom, and to gnash their teeth at those pets that sniff too ungraciously. Hummingbirds follow suite, using their long beaks to sip nectar from flowers, preen their feathers, as well as bite and poke each other, signaling ‘stay out of my flower garden!’ However, only adult males seem to have beak adaptations for fighting, while females and juveniles don’t. This suggests that evolution is promoting different kinds of beak shapes for members of the opposite sex. In order to control territories as well as compete for mates, male hummingbirds use hooked, dagger-like bills with serrated edges to harass, intimidate, and injure other males and competitors in general, even bees and larger birds! This means that understanding how beak shape relates to feeding, flower shape, and fighting is a more complex story than we could have imagined for these tiny, feisty little birds.

By Dr. Matt Kolmann (@KolmannMA)

Dr. Kolmann is an NSF-funded Post Doctoral Research Fellow at George Washington University studying the biomechanics of skulls, teeth, and muscles in piranhas, pacus, and their relatives. He is interested in how evolution has shaped predators according to the properties of what they eat, using any and all fishes as study systems. He gained a grudging appreciation for hummingbirds, now grown to full-blown admiration, working in the lab of his avian collaborator and committee member… on stingray feeding. https://mattkolmann.jimdo.com/

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