Yesterday’s spectacular Goshawk video reminded me of one of the coolest things I ever learned in a long-ago zoology class: why birds never need bifocals the way humans do.
Think about this: If you’re running through the woods, you have to focus on your route ahead (distant focus) so that you know where you’re going, as well as watching the ground at your feet (close focus), so that you don’t trip over roots and rocks. You’re constantly switching back and forth between close and distant focus. And if you’re middle-aged like me, that switch from close to far and back again doesn’t happen as quickly as it did in your younger years. Our human eyes age, and the muscles that change the shape of our lenses age, which is why many of us need bifocals after a certain number of years. We either take too long to make the switch between near and far, or we lose the ability altogether.
Now think about that Goshawk. It’s not running through the woods; it’s flying. It’s moving much, much faster than a running human, so it’s having to switch between close focus (quick, dodge that branch!) and far focus (where’s that bird I’m chasing?) in mere fractions of seconds.
Birds solve the problem of high-speed vision changes by having two sets of muscles that change their eye shape. They can change not just the lens, but also the cornea — the actual surface of the eye. We humans can’t do anything about the shape of our corneas, which is why so many of us need glasses even when we’re children. All those eye surgeries to give nearsighted people better vision? They all depend on changing the shape of the cornea. (Permanently.)
Birds don’t need glasses or surgery. They just flex some muscles and pow, the cornea changes shape. In conjunction with the muscles that also change lens shape, birds have instant and very finely-tuned control over their vision. They make us humans look like slow, blinking blind things. Which, by comparison, we are.
Cool! I didn’t know that! And yes, humans are worse off than lots of other organisms in many ways, which we discussed (slightly off-topic…) in the seminars of my Human Ethology course.
I still don’t get it. Why would two sets of musscles not tire out, if we had them?
I didn’t go into all the anatomy on this, because I was trying to keep it simple…but it’s very cool stuff. The human eye has a muscle to pull the lens into one shape, but it has no second muscle to then pull the lens back into the original shape. Instead, the single muscle must relax, and allow the lens to naturally revert to the first shape. For young people, that relaxation/reversion process is almost instant. For older people, the muscle doesn’t relax as far and the lens doesn’t shift as easily.
Birds, on the other hand, have a second muscle to yank the lens back into the original shape. Much more efficient! So even if both sets tired out with age, they would still work far better than the underpowered human arrangement. And that’s not even counting the additional muscles birds have to rearrange their corneal shape.
I get it.Thanks for the explanation. Interesting. Which begs the next question: How can the muscle be attached to yank it back, without being in view?