How do owls rotate their heads?

Owls don't need eyes in the back of their heads to see what's behind them — they can just swivel their heads all the way around. 

In fact, many owl species, such as the barred owl, can rotate their heads 270 degrees in each direction, which means they can look to the left by rotating all the way to the right, or vice versa.

Unlike people and other animals who can simply move their eyes to follow an object or use peripheral vision to scan a room, owls must turn their heads for the same effect. 

These birds have fixed eye sockets, which means their eyeballs can’t rotate, forcing them to stretch their necks - a seemingly supernatural feat.

Owls are more flexible than humans because a bird’s head is only connected by one socket pivot. Whereas, people have two, which limits our ability to twist. 

Owls also have multiple vertebrae, the small bones that make up the neck and spine, helping them achieve a wide range of motion.

Yet, even with these skeletal advantages, a bird’s body shouldn’t be able to withstand such extreme levels of movement. In people, a spinning head would cause all kinds of internal bleeding and breakage!

But two US-based scientists have found that it's really the way the animal manages the flow of oxygenated blood to its brain that underpins the impressive feat. 

They found that the big carotid arteries, instead of being on the side of the neck as in humans, are carried close to the centre of rotation just in front of the spine. As a consequence, these arteries experience much less twisting and stretching. The potential for damage is therefore greatly reduced.

Although, this arrangement is not specific to owls. It is actually seen in other birds as well. What does appear unique to owls, however, is the way the vertebral arteries - the vessels that travel through channels within the neck bones - are given extra space.

In humans, the bony cavities are just big enough to carry the vertebral arteries. But the canal in owls is about 10 times bigger and is filled with an air sac.

Birds have air sacs to make them lighter, and somehow they manage to put some of this inside that bony canal, and cushion the vessel."

In addition, between the carotid and vertebral arteries, owls have a lot of smaller connecting vessels that permit the blood to find alternative pathways should one of the main flow routes close down during rotation.

But perhaps most significant of all is the discovery that owls have wide segments in their carotids just under the skull base. The researchers found that these could dilate and fill with a reservoir of blood. They believe that this is probably a way to pool blood and get some continuity of flow even if there is disruption below at the next level.

The finding is just another example of how the birds are perfectly adapted to suit their environment, enabling them to see despite having relatively fixed eyes. 

How remarkable!