**3. Head rotation**

Because of the large size of the 'tubular' eyes and the fact that they are locked into place by a sclerotic ring of bone, ocular mobility in owls is virtually non-existent [6]. To compensate for this lack of eye movement, and a fairly narrow field of view, owls have evolved with the ability to laterally swivel the head smoothly and quickly through 270 degrees and vertically 90 degrees. Owls have 14 cervical vertebrae, but so do many other species of bird. Indeed, 14 is about the average number of cervical vertebrae in birds in general (birds can have between 10 and 26 vertical vertebrae depending on

### *Designed for Darkness: The Unique Physiology and Anatomy of Owls DOI: http://dx.doi.org/10.5772/intechopen.102397*

species) [7]. All birds have to have the ability to turn their heads through 180 degrees and more for preening. The secret to the owl's ability to swivel its head smoothly and quickly through 270 degrees in the manner that it does is down to two areas of adaptation. The first adaptation is to the neck itself. Owls have only one occipital articulation with the cervical vertebrae, while the neck is permanently compressed into a loose 'S' shape [8]. As with a spring coil, this gives the neck great flexibility. It has also been discovered that there are varying degrees of axial rotation within the individual intervertebral joints [9] and that the combination of yawing and rolling in sections of the cervical spine maximises head rotation [10]. The second adaptation is in the reinforcement of the walls of the oesophagus, trachea, and arteries to withstand the enormous torque involved as the head is turned through so many degrees. Also, it has recently been discovered that in the owl neck, one of the major arteries feeding the brain passes through bony holes in the vertebrae. These hollow cavities are approximately 10 times larger in diameter than the vertebral artery travelling through it. The extra space in the transverse foramina, as the holes surrounding the vertebral arteries, are known, creates a set of cushioning air pockets that allow the artery to move around when twisted. Twelve of the 14 cervical vertebrae in the owl's neck were found to have this adaptation. Blood vessels at the base of the head, just under the jaw bone, can also act as contractile blood reservoirs, allowing owls to pool blood to meet the energy needs of their large brains and eyes, while they rotate their heads. The supporting vascular network, with its many interconnections and adaptations, helps to minimise any interruption in blood flow [11].
