an autodidact meets a dilettante…

‘Rise above yourself and grasp the world’ Archimedes – attribution

the amazing physiology of hummingbirds

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The smallest bird on our planet is the bee hummingbird, of Cuba. The average adult weight ranges between 2 and 2.5 grams, with females being slightly larger than males. There are other tiny hummingbirds, including the bumblebee, from Mexico, and the calliope, of Canada and the US. Basically the adults of all these birds weigh little more than a couple of paper clips. Yet, as Jim Robbins reports in The wonder of birds, these featherlight birds are incredibly robust. Calliopes fly from the northern US down to Mexico every winter, often through powerful head-winds and raindrops as big as their ‘eads. They fly back north in spring, early arrivals, living on insects (their principal source of nutrients) until the flowers start blooming (providing nectar, their principal source of energy). It’s an annual journey of nearly 3000 kms.

adult male bee hummingbird

It takes heart to undertake such a journey, and hummingbirds have plenty. The hummingbird heart is the largest of any known animal relative to its size, and its rate has been measured to reach over 1200 beats per minute (in the blue-throated hummingbird). There are some 350 species of hummingbird, all living in the Americas. 

But it’s not just their long-distance flights that astonish, it’s their everyday manoeuvres. They can fly upside-down, change speed and direction smartly, and hover for long periods, even in strong winds, while collecting sweet nectar in vast quantities – as much as 12 times their body weight daily. Their wing-beat speed, which can reach 100 beats per second, is about ten times that of a pigeon, and they have the largest pectorals for their size of any bird. Birds’ pectorals, which power their flight, are always proportionally massive, taking up some 80% of their weight, but hummingbirds are clearly built for flight more than any other, which allows them to remain in the air more or less constantly. ‘It’s their default setting’, says Bret Tobalske of the University of Montana, who studies the mechanics of flight in birds, bats and insects. Tobalske has studied their flight using ultra high-speed cameras and atomised olive  oil illuminated by lasers, so that the revealed air-flow around their wings can help in understanding the mechanical processes involved. He’s also used wind tunnel experiments to investigate how well the birds can withstand wind forces. In a 20mph headwind, they simply increase their wingbeat rate, and can remain hovering for up to an hour and a half. 

calliope hummingbird

Hummingbirds are very trainable and human-friendly, especially if you reward them with sugar water, their favourite energy hit, though the more food is laid on for them the less they’ll visit and pollinate flowers. Their beaks and long tongues are adapted to extracting nectar. The tongues themselves are an extraordinary adaptation. They’re forked at the tip, and when retracted they coil up inside their tiny heads like a garden hose. For years it was thought that the nectar was drawn out of the flowers by capillary action, like a blotter soaks up ink (showing my age), but Margaret Rubega of the University of Connecticut quickly recognised this was a crock, on first hearing of the hypothesis in the 1980s. Capillary action is a slow process, especially with more viscous liquids, but hummingbirds stick their tongues into flowers at a rate of up to 16 times a second. How their tongue works has been revealed by slow-motion photography, another example of technological advances leading to advances in knowledge – though the ingenuity of Rubega and her colleague Alejandro Rico-Guevara in working out the process played a large part. Ed Yong provides a good account here, and the more detailed original paper is also online. The hummingbird’s tongue appears to be a unique evolutionary invention, a bespoke tongue, so to speak. At its tip, where it forks, it curls up at the edges, creating two tubes. Here’s how it works, from Yong:

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.

As Rubega and Rico-Guevara suggest in their abstract, such a unique fluid-trapping mechanism may well have biomimetic applications. As the researchers have shown, the tongue mechanism works even after the bird has died, showing that it’s in some sense independent of the bird itself, and requires none of the bird’s energy. 

It shouldn’t be too much of a surprise to find that hummingbirds have the highest metabolism of any creature (excluding insects). Apart from their record heart rate, they take around 250 breaths a minute, even resting – which they rarely do. Their oxygen intake (per gram of muscle) during flight is ten times higher than that of the most elite human athletes, and they get almost all of their energy for this hyperactive life through ingested sugars – compared to a maximum of 30% for humans. They can utilise sugars for flight within 35 minutes of consumption, which requires a very rapid oxidation rate. Though it isn’t precisely known how this rapid oxidation occurs, it does explain how they can maintain flight while feeding – they’re essentially refuelling while in flight. This raises questions, though, about long-haul flights, for example across the Gulf of Mexico – a distance of 800 kms. It appears they’re able to store fat as a fuel reserve, like other migratory birds, thus almost doubling their weight before the big journey. 

Hummingbird songs and calls are highly varied, and some are even ultrasonic – at a frequency above that of human hearing. These may be used to disturb the flight patterns of small edible insects. Most interestingly, neurological and genetic expression studies suggest that they are capable of vocal learning, something rare among birds as well as mammals. Research in this area is something I hope to explore more fully in future posts – it involves brain design, development and epigenetic factors. 

blue-throated hummingbird, a larger species – only the male has the blue throat

A few other interesting points in closing. Hummingbirds do rest at night, and when there’s no available food – they can enter a state something like hibernation, when their metabolism slows almost to a full stop. They can lose about 10% of their body weight during these states. It’s also notable that they have surprisingly long life-spans for such hyperactive creatures.  Average life-spans have been difficult to measure, but individuals of different species have been known to live for eleven or twelve years at least. 

My growing interest in birds and other creatures, especially with regard to intelligence, has inevitably led me to the load of videos available online, displaying all sorts of amazing traits, as well as profound human-animal relations. There are too many to recommend, but I would strongly suggest to any reader that they sample some of them. Watching them is somehow uplifting, and inspires a sense of hope. Life is nothing if not ingenious, even if accidentally. 


Written by stewart henderson

November 15, 2018 at 10:09 am

2 Responses

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  1. Easy to read and interesting, and I liked the personal message in the last paragraph


    November 15, 2018 at 11:35 am

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