Wednesday, April 25, 2018

Avian Vision

How do large birds of prey, like eagles and osprey, see fish in the water? -Jamie C.

With their eyes of course!  

People generally have a egocentric view of their place in the world.  Due to that view most of us believe we are a special breed of organism that reigns atop the heap but, with the exception of our bountiful brains and dexterous digits, there are few things that make us stand out from the crowd.  Elephants are bigger and gorillas are stronger, giraffes are taller and cheetahs faster, turtles live longer and dogs smell better (I should write ‘have a better sense of smell’ but it ruins the flow.  Have you ever smelled wet dog!?).  While we do have notable vision relative to many animals, nearly half of our massive brains is dedicated to vision, it pales in comparison with many birds.  The mechanisms of vision are well understood and while the major components function similarly amongst all sighted vertebrates it doesn’t preclude unique variations which can dramatically alter visual acuity.  Birds have evolved many morphological and behavioral adaptations which make them arguably the animals with the best vision in the animal kingdom.  

The adaptations that make avian eyes so remarkable are more impressive when you understand how they are are different from the human eye.  Go back in time with me for a moment to high school biology class so we can quickly review how the eye works.  It functions much like a camera.  Light enters the eye and passes through the pupil which is the opening in the iris.  The iris is a muscle which can contract or relax, changing the size of the pupil and varying the amount of light entering the eye, just like the camera’s aperture.  Next the light passes through the eye’s lens which changes shape depending on the distance of the object it is trying to focus on.  One difference between the eye and the camera is that the aperture is in the middle of the lens instead of in front of it.  The image is focused on the back of the eye which contains the light detecting retina whose counterpart in the camera would be the film (for those of you old enough to remember film).  

Diagram depicting the major structures on the human eye

There is a small depression in the retina called the fovea which is more densely packed with color detecting cone cells than other parts of the retina.  This is where most of our sharply focused highly detailed vision occurs. Raptors often have a second fovea which makes for a larger field of sharp focus.  Many birds, but not raptors, are tetrachromatic which is a fancy way of saying that they have four types of color detecting cone cells in their retina, comparably humans only have three types of cones.  This extra cone detects wavelengths of light that are much shorter than wavelengths we can see allowing these birds to see ultraviolet light.  Other parts of the retina, used in peripheral vision, contain a greater number of rod cells, which are more sensitive to light than cone cells, but lack the ability to differentiate color.  Owls have mostly rod cells and very few cone cells limiting their ability to see colors allowing their eyes to function like a pair of night vision goggles. 
Wavelengths of light detectable by avian vision compared to human vision

**CAUTION, EXPERIMENT AHEAD**  You can test your rods and cones for yourself on a clear evening.  Look up at the stars and stare at a specific spot, a recognizable constellation is a good place to start.  If you pay close attention you will notice you can see more stars in your peripheral vision than you can in your central vision.  Your rod cells can detect the faint light of distant stars that your cone cells cannot.  If you shift your vision over slightly so the constellation is in your peripheral you may notice more stars surrounding the constellation than before.  Hooray for science!  Now back to our regularly scheduled anatomy lesson.  

The retina is a triumph of evolution but is not without its flaws.  Like all tissues it must be supplied with oxygen and other nutrients which are provided through blood vessels.  These blood vessels crisscross the surface of the retina and obscure many of the photoreceptors resulting in a decrease of resolution.  Avian eyes have evolved a structure called the pecten which resides in the vitreous humour (inner eye fluid/goo) of the eye and bathes the tissues in nutrients allowing for fewer blood vessels across the surface of the retina resulting in higher visual resolution.  Birds also have far more photoreceptor cells than other animals.  We have a photoreceptor density of approximately 200,000/mm² while the common buzzard (Buteo buteo) has a density of over 1,000,000/mm².  In layman’s terms if our vision were the equivalent of a standard definition tube television (I’m really showing my age today) bird vision is a 4k Ultra High Definition theater projector.    
One of the coolest looking structures in all of biology is the sclerotic ring which is a series of small bone plates encircling the eyes of birds, reptiles, and dinosaurs.  

Red Tail Hawk skull showing the sclerotic ring Collection and photo credit: DeLoy Roberts

This ring not only makes these skulls look menacing but it helps provide protection and support for the large eyes most birds have and creates additional anchors for muscles that help with focusing and blinking.  Raptors have lenses which can see extremely far away but use unique Crampton’s muscles that apply pressure to reshape the cornea and aid in closeup vision.  These muscles allow them to have a much deeper field of sharp focus than we do.  Since vision is such an important sense for birds they usually have eyes that are very large compared to their body and in some birds they make up a majority of their head.

great-potoo-editmarty-fieldman One of these photos is a Great Potoo (Nyctibius grandis) exemplifying the proportionally large eyes of birds and the other is Marty Feldman but I’m not sure which is which.

Birds have added protection for their precious eyes in the form of an thin translucent nictitating membrane.  This membrane is a third eyelid that can be pulled back to protect the eye when attacking prey or moisten the eye when blinking without the brief loss of vision opaque eyelids cause.  Osprey use them like swimming goggles to lessen the impact on their eyes when diving after fish.  This conveniently brings us back to our original question:  How do birds of prey see fish in the water? 

 They use all of these amazing adaptive advantages to hunt for distant prey but birds hunting for fish have one other behavior that is important when catching prey that is underwater and it is to attack from a steeper angle than they would when hunting prey on land.  This is vitally important because it cuts down on the refraction caused by the water and means the fish they are closer to where they appear to be.  They are also able to do this because the water is a bit more forgiving than the ground and impacting the ground at such a steep angle would be uncomfortable at best and likely injurious. 

Light refracting as it passes through mediums of two different densities

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