All birds have very mobile heads, partly to aid preening, feeding and flight, and partly to compensate for the fact that their eyes are fixed in their sockets (making it difficult for birds to glance around them).
They have 11–25 neck vertebrae compared with just 7 in most mammals.
Owls take this flexibility to extremes: they can rotate their heads up to 270 degrees and twist them nearly upside-down.
To prevent such contortions bursting blood vessels or cutting off the blood supply to the brain, the arteries in an owl’s neck – which sit inside tubes within the hollow bones – are cushioned by air sacs.
Other arteries at the base of the skull can dilate to store blood, so if the flow is interrupted lower down, there is a reserve to supply the brain.
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