With forward facing eyes the Bald Eagle has
a wide field of binocular vision.
Vision is the most important sense for birds,
since good eyesight is essential for safe
flight, and this group has a number of adapt-
ations which give visual acuity superior to
that of other vertebrate groups; a pigeon has
been described as "two eyes with wings".
The avian eye resembles that of a reptile, but
has a better-positioned lens, a feature shared
with mammals. Birds have the largest eyes
relative to their size within the animal king-
dom, and movement is consequently limited
within the eye’s bony socket. In addition to
the two eyelids always found in vertebrates,
it is protected by a third transparent movable
membrane. The eye’s internal anatomy is
similar to that of other vertebrates, but has a
structure, the pecten, unique to birds.
Birds, like fish, amphibians and reptiles,
have four types of colour receptors in the
eye. Most mammals have two types of recept-
ors, although primates have three. This gives
birds the ability to perceive not just the vis-
ible range but also the ultraviolet part of the
spectrum, and other adaptations allow for the
detection of polarised light. Birds have pro-
portionally more light receptors in the retina
than mammals, and more nerve connections
between the photoreceptors and the brain.
Some bird groups have specific modifica-
tions to their visual system linked to their
way of life. Birds of prey have a very high
density of receptors and other adaptations
that maximise visual acuity. The placement of
their eyes gives them good binocular vision
enabling accurate judgement of distances.
Nocturnal species have tubular eyes, low
numbers of colour detectors, but a high dens-
ity of rod cells which function well in poor
light. Terns, gull and albatrosses are amongst
the seabirds which have red or yellow oil
drops in the colour receptors to improve dis-
tance vision especially in hazy conditions.
The eye of a bird most closely resembles that
of the reptiles. Unli