Owls are famous for their vision. With their great ability
to detect and capture small prey even in the darkest nights, they do not have
round eyeballs like we do. Instead, they are elongated and sit on
"sclerotic rings," a bony structure in the skull. Owls have binocular
vision similar to that of humans, since their eyes are oriented towards the
front, which gives them (and us) a great capacity to judge height, weight and
distance. However, while humans have a 180-degree field of vision, with
approximately 140 degrees of binocular vision, owls only have a 110-degree
field of vision with approximately 70 degrees of binocular vision. What they
lack in binocular vision, they make up for with their legendary night vision
and their vision of the future.
Their shape also provides a comparatively larger retina that
is filled with light-sensitive rods, leading to the extraordinary
light-gathering properties of the owl's eye that are enhanced in many species
by a reflective layer behind the retina called tapetum lucidum, which reflects
back onto the rods any light that may have passed through the retina without
hitting it once.
What species of owl has better vision? It seems that tawny
owls have the best developed eyes of all varieties of owls, even of all
vertebrates, probably about 100 times more sensitive in low light levels than
our vision.
What about the color of their iris? It's a fascinating
aspect. The color of the iris is related, in part, to the time of day or night
when an owl hunts. Although not true for all owl species, eye color tends to
indicate what time of day owls prefer to be active. There are always
exceptions, but most of them hunt almost at the same time as other owls of the
same color as their eyes, according to Raptor Rescue, a bird of prey
rehabilitation charity. This link is not fully understood, but there is plenty
of evidence: the species' eye color correlates with periods of daily activity.
Colors tend to be dark brown or black for strictly nocturnal
owls, orange (and almost red) for twilight owls that hunt at sunrise and
sunset, and yellow for daytime owls that hunt during the day.