Birds
and people are "sight animals." For both, the eyes are the
dominant sense organs, vastly more important than their
inferior sense of smell. The reasons for our sensory
similarity to birds can be found in human evolutionary
history. At one point the ancestors of Homo sapiens were
small, tree-dwelling primates. When leaping from limb to
limb and snatching of insect prey with the hands, sharp,
binocular vision was very handy; those of our forebears that
tried instead to smell the location of a branch on which to
land were unlikely to survive to reproduce. And since in the
breezy treetops odors quickly dissipate, they do not provide
good cues for detecting food, enemies, or mates. Birds,
flying higher and faster than primates leap, naturally also
evolved sight as their major device for orienting to the
world. Most birds have binocular
vision. It is especially well developed in predators that
must precisely estimate ever-changing distances to moving
prey. Their eyes tend to be rotated toward the front of the
head, so that the visual fields of each eye overlap to some
degree. This trend is most pronounced in owls, whose eyes
are almost as completely overlapping in field as ours. Small
birds that are likely to be prey for raptors tend to have
their eyes set on the sides of the head, permitting them to
watch for danger in all directions. At the opposite extreme
from the owls are the woodcocks, mud probers with eyes set
high and back on the head, out of the way of vegetation and
splattering mud and in a position to look out for predators.
In fact, the woodcock has better binocular vision to the
rear than to the front! Shorebirds, waterfowl,
pigeons, and other birds that have minimal binocular vision
seem to depend on differences in apparent motion between
close and distant objects for much of their depth
perception. When a bird's eye is moving, closer objects
appear to move at a faster rate than do distant objects -- a
phenomenon familiar from the way roadside telephone poles
seen from the window of a moving car appear to pass more
rapidly than the distant landscape. Presumably to enhance
this distance-measuring method, shorebirds, and waterfowl
often bob their heads up and down, and pigeons move theirs
back and forth while walking. Even birds with relatively
good binocular vision may use apparent motion to aid them in
estimating distance; perched New Guinea kingfishers often
"post" up and down on their legs before diving after prey.
To see how this works, move your head with one eye closed
and note the relative motion of close and distant
objects. The term "hawk-eyed"
accurately describes many birds. For example, both raptors
that must see prey at great distances and seed eaters that
must pick tiny objects off the ground have eyes designed for
high "visual acuity" -- the capacity to make fine
discriminations. There is, in fact, evidence that hawks can
distinguish their prey at something like two or three times
the distance that a human being can detect the same
creature. Interestingly, even with such visual acuity,
Cooper's Hawks are known to hunt quail by their
calls. One way that birds have
attained such a high degree of acuity is by having
relatively large eyes. A human eye weighs less than I
percent of the weight of the head, whereas a starling's eye
accounts for some 15 percent of its head weight. But more
than size alone appears to account for the astonishing
performance of the eyes of hawks. Evolution has arranged the
structure of their eyes so that each eye functions very much
like a telescope. The eye has a somewhat flattened lens
placed rather far from the retina, giving it a long "focal
length," which produces a large image. A large pupil and
highly curved cornea admit plenty of light to keep the image
on the retina bright. Visual acuity in birds is
also enhanced by the structure of the retina itself, which
has tightly packed receptors and possesses other adaptations
for producing a fine-grained image. Most of those receptors
are the type called "cones." "Rods," the receptors of the
vertebrate retina that are specialized to function in dim
light, are relatively rare. Thus daytime acuity is, in part,
achieved at the expense of night vision -- a small price to
pay for birds that are inactive at night anyway. In those
relatively few species that are nocturnal, such as owls,
rods predominate. Considering the frequent
evolution of gaudy colored plumage, it is not surprising
that birds active in the daytime have color vision
(nocturnal birds are thought to be color blind), and that
color perception is often obvious in bird behavior. One can
watch a hummingbird moving from red flower to red flower;
bowerbirds show color preferences when decorating their
bowers. Just how refined that color vision may be has proven
difficult to determine. However, the diversity of visual
pigments found in birds' eyes, and the presence of an array
of brightly colored oil droplets inside the cones, suggest
that avian color perception may surpass our own. There is
also evidence that some birds' eyes are sensitive to
ultraviolet light. In hummingbirds the adaptive significance
of this is clear, since some flowers from which they drink
nectar have patterns visible in the ultraviolet end of the
light spectrum. Why pigeons have the ability to see
ultraviolet remains a mystery. Equally surprising is the
recently discovered ability of pigeons to detect the plane
of polarized light. This probably serves them well in
homing. SEE: Raptor
Hunting;
The
Color of Birds;
How
Owls Hunt in the Dark;
The
Avian Sense of Smell. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.