- Written by Nadia O'Carroll
Ironically the characteristics that make owls so appealing to people, such as the wide eyes, dished face, and fluffy feathers are actually attributes of an extremely sophisticated night hunter.
Eyes: the eyes of an owl are adapted to night vision. Their eyes are large (up to 5% of body weight). Their eyes’ forward facing position give them binocular vision (when an object is seen at the same time by both eyes) this gives them a field of view of about 110 degrees of which 70 degrees is binocular and 20 degrees left monocular and 20 degrees right monocular. Owls do not have eyeballs as such, their eyes are more like tubes held in place by bony structures called sclerotic rings. Their eyes do not have sclera (the whites in human eyes) so that the pupil of an owl can dilate to the very edge of the eye for maximum light exposure. Because of this eye structure, an owl cannot move its eyes, it. can only look straight ahead. To overcome this restriction, it has an extremely flexible neck, with 14-neck vertebra, which allow it to turn its head through a range of 270 degrees. In flight the owl to keeps its head level while pivoting its body and wings up to 270 degrees. Owl retinas, like those of humans, contain rod and cone cells. Cone cells are sensitive to colour, and rod cells are sensitive to light and movement. Owls have a high ratio of rod cells which give them fuzzy, low colour vision that is effective in low light and highly sensitive to movement. To protect their eyes owls have 3 eyelids – an upper and lower eyelid, as well as a nictitating membrane which moves diagonally across the eye to clean and protect its surface.
Ears: An owl’s range of auditory sounds is similar to that of humans, but its hearing is much more acute at certain frequencies, so that the slightest rustle in the undergrowth can be heard. To be an effective hunter the owl has to first hear, then locate its prey. Owl ears are vertical slits set on either side of their heads, in many species they are asymmetrical ie the slits are not level with each other. The facial disc of owls acts as a radar dish – the owl uses its facial muscles to alter the shape of the disc, this can amplify and funnel sounds to the ears. When a sound is heard, the owl is able to analyse the minute time difference between when the sound is perceived by the left ear, and when it is perceived by the right ear (owls can detect a left/right time difference of 0.00003 seconds ie 30 millionth of a second.
They can also analyse the minute up/down difference in sound volume and pitch between the asymmetrical right and left ears. The owl raises a central ridge in its facial disc to prevent the sounds mixing. The minute left/right and up/down time differences fix the horizontal and vertical position of the sound; its intersection is the source of the sound and the location of the potential prey. The owl’s brain has an instant mental image of the sound, space and direction and this is constantly corrected as the owl flies.
Silence: The owl also is a stealthy night hunter and its soft downy feathers and its flexibility are designed to cause minimum air disturbance so it can hunt silently.
These days we seem to see owls as bumbling professorial characters in cartoons and children’s stories. A closer look at this species shows that earlier cultures had a more accurate measure of owls, when they feared or revered them as creatures of mystery and beauty.