I’m gonna have to go with hummingbirds. You may well have seen some firsthand, and while they are generally considered novel creatures, I don’t think many are aware of just how bizarre they really are.
First and foremost, there’s their metabolism. Hummingbirds have the fastest metabolic rate of any animal on Earth, except for some insects. Their hearts beat at a positively mind-blowing rate of 1,260 BPM during flight, and even at rest they have to take 250 breaths per minute. For comparison, the average person’s max heart rate and resting breathing rate are 180 and 12–20, respectively.
Having a breakneck metabolism brings with it many, many challenges. To prevent themselves from overheating, many hummingbird have featherless patches under their shoulders, on their wings, and around their eyes, where excess heat can dissipate efficiently.
That said, if a human were to have the same metabolic rate, despite being wholly hairless, they would surely perish. This is, of course, because we are much, much, much bigger than hummingbirds. In fact, hummingbirds are the world’s smallest birds by far.
In Cuba, you can find the aptly named bee hummingbird, whose size cannot be understated. The males can measure as little as 5 centimetres in length, and weigh a paltry 2 grams. Despite their tininess, bee hummingbirds are quite beautiful.
You would think these minuscule birds wouldn’t eat much food, but - bearing in mind their metabolism - that is far from the case. Every day, a typical hummingbird will suck nectar from roughly 2,000 flowers, in addition to eating plenty of small insects, consuming triple their bodyweight in food total. They have to eat once every 10–15 minutes.
They also take in a huge amount of water, up to five times their bodyweight daily. That would be equivalent to me downing more than 1,600 glasses in a single day, i.e. a few glasses in every minute I’m awake. Because of this massive influx of hydration, their kidneys have evolved to precisely filter the levels of water and electrolytes in the body.
You might also be wondering how hummingbirds cope with lean times, given that they need so much food just to survive. If there is a shortage of nutrition, they take extreme measures - that is, they enter a hibernation-like state of torpor in order to save energy. The heartbeat slows down to 1/25th of its normal pace, and their body temperature more than halves.
Rather adorably, hummingbirds hang upside down while they are torpid!
The hummingbird metabolism means that the mitochondria in their cells must produce energy as quickly and efficiently as possible. So, instead of performing glycolysis - breaking glucose down into pyruvic acid and using the resulting free energy to form molecules like ATP - like normal organisms, they have the amazing ability to instantly convert glucose into energy, by simply oxidizing it.
What’s even more interesting is that they can do the same thing to another sugar, fructose. As well as being high in glucose, flower nectar contains plenty of fructose, so by being able to immediately metabolize it they get a lot more fuel out of their food.
Speaking of nectar, hummingbirds ingest this sweet liquid using one of the most fascinating tongues in the animal world. This tongue is so long that it coils around the skull when retracted, and has two twin tubes. When the tongue is ejected, the tubes split apart, and a row of flaps unfurls on each side.
Droplets catch on these flaps, which snap shut as the tongue is drawn back into the mouth, trapping the nectar. This repeats in incredibly rapid succession, as the hummingbird flicks their tongue in and out of the flower with a pumping action.
Another cool adaptation one species has for nectarivory is an absurdly long beak. The sword-billed hummingbird of the Andes has the proportionally longest bill of any bird in the world; at more than 15 cm, it even exceeds the body length.
This allows the species to access flowers which have very deep corollas (petal tubes), which are out of reach to all of their would-be competitors.
While we’re on the topic of odd and novel species of hummingbird, I should also mention a couple of types which have beautiful plumage. Specifically, the long-tailed sylph:
And the white-booted racket tail:
The former has extraordinarily long tail feathers, and the latter has a very interesting sexual display feature. Male racket-tails get their name from the two little oval-shaped flags which trail behind them, used for attracting females. Both sexes also have lovely fluffy pom poms on their legs - hence “white-booted”.
Lastly, hummingbird behaviour is almost as fascinating as their diet and metabolism. Take flight, for example Hummingbirds are perhaps the most skilled aviators in nature (insects aside); their aerial acrobatics are simply jaw-dropping. They can fly in any direction, including backwards and sideways, and can hover on the spot.
To perform these feats, they beat their wings at an unimaginably fast pace of 80 times per second!
Focusing one’s eyes with so much rapid aerial movement is also highly challenging; in order to refine their vision, the optic tract of the hummingbird brain is greatly enlarged. In addition to that, they are tetrachromatic; they have four types of colour receptor cells, in comparison to our three. This enables them to perceive five extra colours which are “invisible” to us humans.
Not only do they have enhanced vision - they appear to have enhanced intelligence, too. Relative to their body size, hummingbirds have the biggest brain of any bird - they account for nearly 5% of the mass, whereas our brains are around 2% of our total body weight.
Of course, we have to keep in mind that this is partially because they’re so small - brain size does not scale directly proportionally to body weight. However, hummingbirds show behavioural signs of intelligence as well: they are one of only three bird groups which have vocal learning abilities, i.e. listening to, imitating and reproducing the sound of others.