Biology Reference
In-Depth Information
CHAPTER
10
Of Eagle Eyes and Ultrasound: How Birds Perceive the World
Vision that extends into the ultraviolet. Hearing that extends to frequencies well
above what the keenest teenager can perceive. Most birds have senses of touch, taste,
and smell that may be quite similar to our own, or poorer, but they can also per-
ceive the earth's magnetism and polarized light. How well can they really see and
hear? Those are just some of the questions people ask about how birds experience the
world.
A Bird's Eye View
Q How good are “eagle eyes”?
A
Birds in general have extraordinarily good eyesight. Imagine how acute a hawk's vision
mustbeifitcansoarhundredsoffeetintheair,scanningthegroundfortheslightestmove-
ment to home in on a mouse.
Bird eyes are bell-shaped, with a large retina. A 3-pound Great Horned Owl's retina is
larger than an adult human's. Bird eyes move very little, and so the only parts that are not
covered by skin and feathers are the iris and pupil. But beneath the surface, their eyes are
huge. Eagle eyes are as large as or larger than human eyes, and in some species the com-
bined weight of the eyes is heavier than the brain!
Rather than being shaped like mammalian “eyeballs,” avian eyes are concave in back,
giving them a relatively larger retina than humans have. Bird eyes also have a distinctive
feature: a large projection from the rear surface of the eye near the optic nerve, called the
pecten oculi.
Scientists don't yet understand why birds have this, but it probably nourishes
the retina.
We know that birds' visual acuity is far better than ours, allowing them to perceive ob-
jects much smaller and farther away than we can. The cones — the cells in the retina re-
sponsible for visual acuity — are packed about five times more densely in areas of a rap-
tor's retina than in ours. There are about a million cone cells per square millimeter!
In birds, each eye sends information to just one half of the brain, allowing birds to pro-
cess information from separate visual fields independently, but an area of the brain called
the “Wulst” processes the information from both eyes to provide stereoscopic vision, the
means by which birds have depth perception, which is critical when an eagle or heron is
homing in on a fish or when a flycatcher is snapping up a moth.
