Owls cannot turn their heads all the way around.
They can rotate them about 270 degrees in either direction, which is still an extraordinary amount.
The reason has a lot to do with their eyes.
Owl eyes do not swivel much
An owl’s eyes are large and shaped more like tubes than human eyeballs.
They sit tightly in the skull and have very limited movement inside their sockets.
That gives owls excellent visual performance, but it creates a problem.
If an owl wants to look far to one side, the eyes cannot simply roll over there.
The whole head has to move.
Their necks contain more vertebrae than ours
Humans have seven cervical vertebrae in the neck.
Owls have more.
That gives the neck additional flexibility and allows rotation to be spread across more joints.
The neck is doing work that human eyes usually handle.
What about blood flow?
Twisting a neck that far could create serious circulation problems in an animal built like a human.
Owls have specialized blood-vessel arrangements that help maintain blood flow as the head turns.
Their anatomy helps reduce the risk of cutting off blood supply to the brain during extreme rotation.
Do owls ever rotate the head a full 360 degrees?
No.
The famous full-circle owl head is a myth.
A 270-degree turn is enough to make it look almost supernatural because the bird can face far behind itself without moving the body.
Why not just move the whole body?
Sometimes they do.
But being able to scan a wide area without shifting the torso can be useful for a predator watching and listening carefully.
It can also help an owl remain relatively still while observing prey.
Do other birds have similar neck flexibility?
Many birds have more neck vertebrae than humans and can move their heads impressively.
Owls are especially famous because of the combination of large fixed eyes and extreme rotational range.
So the dramatic head turn is not a creepy party trick.
It is the solution to having very useful eyes that do not swivel much.
The owl sacrificed eye-rolling and made up for it with neck engineering.
A related animal question is Why do birds fly in a V formation?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: Why do birds fly in a V formation?, Why can birds sit on power lines?, and Why do flamingos stand on one leg?