What do Birds See?

A 'Look' at Bird Sight and Seeing

Bald Eagle Showing Forward Vision [Internet Photo]

Recent research has revealed a lot of interesting features about bird eyesight.  This post will present some of them.  The Bald Eagle shown above has excellent eyesight.  It can clearly see prey from hundreds of feet in the air prior to making its catch.  A relative of the Bald Eagle, the Australian Wedge-tailed Eagle has the greatest visual acuity of any known animal.  We have good reason to feel envious of avian visual abilities because they can see so much better than we can.  That gives us good reason to study their eyesight.

I will first present what birds see followed by some of the anatomical and physiological reasons.  We see objects with what we think is good visual acuity.  Objects to us are clear, bright, crisp and in vivid colour.  Well, take it as a fact now that we are desperately disadvantaged compared to birds.  Birds can see with greater clarity, more shades and contrasts and they can even see ultraviolet (UV) light.

Black-crowned Night-Heron [Internet Photo]

Humans see light with the red/green/blue spectrum.  We are trichromats.  Birds are tetrachromats!  They see with four colours; red/green/blue/UV.  By being able to see UV, birds have a whole realm of colour and form available to them that we do not.  UV has no colour but birds can see many different 'colours' (for want of a term) in the UV range.  Note the photo below.  On the left is how we see the bird with no UV.  On the right is how birds see the bird with UV.  When you translate these differences over the plumage of all birds, vegetation and other parts of the bird universe, you can see that birds live in a vastly difference system than we do.  

How Humans See (left); How Birds See (right) [unknown source]

In a study done in 2007 scientists analyzed the colours of 166 North American songbird species that (to us) did not appear to have any physical differences between the two genders.  What they found reveals in one way how different the bird environment is compared to us.  They found that even though the two genders looked alike to us, their colours differed greatly when viewed in the UV spectrum.  So, we are the ones lacking here!

So, birds see with high density resolution; they see well from great distances; and they see with great colour variation and acuity.  Just imagine a colour you have never seen and multiply it by 11!  Wow!  Plus, they can see different colours in the infrared spectrum which we can see only as red.  I am feeling severely disadvantaged now!

Brown Pelican [Internet Photo]

Birds are the most colourful animals on the planet.  That in itself shows that they have good perception of colour.  It is also why we find them so appealing.  Let's consider one of the most highly coloured birds on Earth, the Andean Cock-of-the-Rock.  The male of this species is intensely scarlet red with a black tail and outer wing feathers.  The inner wing feathers are silver.  It gets its name because it nests on rocky cliff edges.  The males perform in groups in leks.  These leks are in small sunny spots in the heavy South American forest.  The strange thing is that when the males step out of the sunshine, they appear black and become difficult to see.  So how do these birds change from such brilliant plumage to a well-camouflaged plumage?  It appears that as the light is filtered through the heavy green foliage of the forest it neutralizes the brilliant red to a dark colour.  There are many other species with a similar plumage trait.  They need sunshine in order to display for their mates (and us if we are lucky enough to be watching).  They have to choose their display sites with care.  Some birds-of-paradise in New Guinea and manakins in South America actually create their own sunny spots for their leks by clearing away the foliage.  

Peregrine Falcon [Internet Photo]

So how have birds developed anatomically and physiologically to have such good eyesight?  A lot of it is out of necessity.  Birds with better eyesight would naturally have survival and reproductive advantages.  Raptors, for example, need exceedingly good visual acuity to spot prey for afar and to focus at extreme speeds.  The Peregrine Falcon seen above is the fastest bird on Earth on its prey dives.  Seabirds can see their mates flying in with food for the nestlings from great distances.  

Great Horned Owl [Internet Photo]

Anatomically birds have large eyes.  The globes are very big compared to mammals.  Large eyes give them large retinas and larger fields of view both of which improve sight.  In fact, their eyes are bigger than they appear because a large part of the globe is concealed in bone and soft tissue.  Excellent vision is necessary for avoiding collisions and capturing fast-moving prey.  Nocturnal species such as owls have larger eyes than birds that are active during the day.  Larger eyes gather more light making it so they can see better in the dark.

Birds cannot (for the most part) move their eyes.  We are all familiar with the owl swivelling its head nearly 360º in order to see around it.  They can actually see 270º from a single perch.  Some birds have their eyes facing forward, some on the sides of the head and some on the side of the head but placed so that the bird can see forward as well.  See the photo of the Bald Eagle at the beginning of this post.  All these different positions are suited to the visual needs of each species.  

Birds, being tetrachromats, have four single-cone types in their eyes.  Not only do they have more cone types than we do, but they have more of them as well.  Most other mammals other than humans and primates have two cone types.

Another interesting anatomical feature of avian sight is that many bird species use their right and left eyes differently.  Each eye is controlled from a separate hemisphere of the brain; left controls right and right controls the left eye.  Many studies have shown that birds use the right or the left eye for different activities.  Peregrine Falcons, for example, zero in on their prey in an arc using their right eye.  Male Black-winged Stilts use their left eye to direct courtship displays to the female.  It has been shown the strong sidedness produces better performance and efficiency.  

As if better eyesight and seeing much better through UV sightedness isn't enough, it has been shown that birds can sleep with one eye open (and thus alert through that open eye).  So far it has been shown that songbirds, ducks, falcons and gulls can sleep with one eye open.  This makes them alert to predation.  It enable them to sleep more efficiently.  And, they can even fly while sleeping!  That is correct.  They are asleep but the one open eye keeps them on track while migrating!  That idea seems ridiculous but it has been shown to be true.  No wonder they can fly from Mary's Point non-stop to Surinam!  

The last physiological trait I mention here is their ability to use their senses in combination.  This really helps in migration.  We have known for a long time that birds can navigate by the stars and sun.  It has been found recently that they also can read the Earth's magnetic fields.  They have magnetite receptors in the beak which help with navigation.  It is also possible that they have a chemical structure in the eye which also aids with navigation.  The eye structure provides the direction of the magnetic field and the beak receptors detect the strength of the magnetic field.  The birds can then integrate stimuli from both receptors to find their way home without land or celestial features.  More research is being done in this area.  

This post has provided a lot of information about bird eyesight and behaviour.  We are just beginning to understand the abilities of birds by studying their amazing vision.  We have a lot to learn yet!

Reference:  Audubon.org; 'Bird's-Eye View'; nwf.org; Google