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Bird’s Eye – Why Birds are Superheroes
By Jennifer Neale ©
They are superheroes. Don’t try to argue with me. I can see you opening your mouth ever so slightly in protest, raising your finger, scrunching your left eye. Those little things in your backyard, the little twerps and their little tweets, the ones who prey on helpless bugs? Hardly crime fighters. They won’t even chase the squirrels from the birdfeeder, let alone save the world from impending natural disaster or a disgruntled molecularly-altered nuclear-accident-victimized villain. They may not do that (not yet, anyway). However, everyone knows that one becomes a superhero when one has a superpower. Regardless of the scale of your do-goodery, the ability to do good is what does it.
Birds are superheroes and their super-power is super-vision. There’s no arguing with it. You’ll have to believe me.
Migratory songbirds can see magnetic fields. Seriously. They have specialized molecules in their eyes that act in conjunction with nerve connections to the brain, giving migratory birds a compass when they’re thousands of kilometers from their summer home.
The molecules are called cryptochromes (how superhero is that?), and they react to the magnetic fields, sending signals to the brain, allowing the birds to “see” the magnetic fields.
Many migratory birds can also see polarized light. The Savannah Sparrow has been found to use polarized light at sunrise and sunset to recalibrate their internal compass. While I’m trying to retrace my steps to where I might have left my keys fifteen minutes ago, the Savannah Sparrow is discerning bent light and magnetic fields to find a tree halfway across the continent.
Many birds can also see ultraviolet light. Birds that are already painted with brilliant colours often have whole other patterns of ultraviolet in their feathers that we can’t see—but their prospective mates can, and they love it. Male Blue Grosbeaks with a greater amount of UV plumage have been shown to have larger territories, and to be generally socially superior. It’s a fourth colour receptor in their eyes, instead of the three that we have, which allows them to see ultraviolet light. Birds of prey are more commonly brown hues and don’t have these ultraviolet accents, since they can’t see the colour as well. Pigeons are pentachromatic, meaning that they have five different colour receptors, and may be able to distinguish up to 10 billion different colours.
Some (human) hunters, in a super villain tactic, have started using decoys painted with ultraviolet so that they look more realistic—hopefully our little bird friends have the superability to spot the difference!
Built-in goggles. All birds have three membranes that can cover the eye—a top lid, bottom, and a nictitating membrane, which goes sideways across the eye. They can act as windshield wipers, cleaning everything off, or, for sea birds such as the cormorant, as transparent goggle that protects their eyes when underwater. Just before cormorants reach the surface of the water, they close the nictitating membrane to protect their eyes as they dive for fish. They can then paddle at will until they find their dinner or run out of breath.
Many birds have oil droplets in their eyes that help to filter colours. In sea birds, the droplets are yellow and red, which allows them to discern colours and shapes with awesome effectiveness in their blue world.
Birds of prey. These guys are awesome. Movements that appear as a blur to us are a clear image to a hawk or kestrel. Smaller birds tend to rely on monocular vision, seeing things with one eye at a time, which is why they orient themselves sideways (so yes, while that pigeon is facing away from you, it sees you very clearly, and it will be gone by the time you start your chase). Birds of prey, though, use their phenomenal binocular vision to spot a mouse from high up in the air, and may be able to distinguish objects at distances 2 to 3 times further than humans. They have a much larger number of rods and cones than humans, and two fovea (dense areas of photoreceptors and nerve cells) instead of the mere one that we have.
Diurnal birds of prey use these skills to catch their victims during daylight hours. Nocturnal birds of prey have all rods since they don’t need to see colour in their nighttime hunt. Owl eyes are about 2.2 times larger than a diurnal bird of the same size, and they rely almost entirely on binocular vision, which is why their eyes are on the front of their head. They also have a reflective layer on their retina called tapetum lucidum, which reflects light to increase brightness in their field of vision when they’re catching mice by night.
You’re on my side now, yeah? Birds have the largest eyes relative to body size in the animal kingdom (the largest total eye size belongs to the colossal squid at 30cm diameter or more, but that’s hardly a fair comparison since they can also be over 10m long). Where we have 200,000 photoreceptors per mm2, the common buzzard has 1,000,000. They can travel across whole chunks of the world, ending up in the same small field or tree, without so much as a map. They can see colours that we can’t imagine, they can pick up a tiny bug that they spotted from 20m away. That, to me, is enough to qualify them as superheroes.
Also, they can fly.