Wednesday, December 17, 2014

Avian Senses Part One - Vision

Eyesight in birds is the most developed of all their senses. 

Some species have the ability to capture an entire scene in one look as opposed to putting it together one section at a time as we humans must.  This is called wide-angle vision. 

Notice the placement of the eyes on the far side of the head on this Northern Mockingbird (photo: Gay Schroer)

The eyes of birds vary in shape and can be flat, round or tubular, unlike human eyes, which are uniformly round. The avian eyes fill the orbit completely and are in most cases placed on the sides of the head (usually seen in birds that can be a prey species).  Because of this, these birds see better to the side.  This is the reason that, upon observation, you see birds rapidly bob or turn their head from side to side when looking at an object.  Bobbing the head allows them to compensate by seeing objects from two different angles.  Depth perception should seemingly be limited due to this monocular vision, but since birds move so much faster than most of earth’s animals, they must have excellent depth perception to land smoothly on any perch, avoid obstacles, and catch the items they eat. 
The Eurasian Eagle Owls uses its forward facing eyes to zero in on prey (photo: Gay Schroer)

Not all birds see in a monocular fashion, though.  Some birds use monocular vision for near nearsightedness and forward binocular viewing for farsightedness.  The forward facing alignment of the eyes is common to most predators, especially the owls.

Conversely, humans have strictly binocular vision.  Because our eyes face forward, we are able to use them together to focus on objects. 

The sclerotic ring is made up of small bones that are called ossicles.  There are two banded muscles called Crampton's and Brucke's, that are attached to the ossicles and are used for focusing. The sclerotic ring is what forces the lens into positions that allow for very fast and precise visual acuity, whether the subject is near or far.  Birds have voluntary, or manual control over the rings, just as we can move our fingers and toes when we think about it.

Diagram of the avian eye (photo: wikipedia files)

 The Crampton's muscle adjusts the curvature of the lens and thus controls the way the light bends as it enters the eye. 
The Brucke's muscle controls the shape of the lens.  This process is similar to using a pair of binoculars.  Think of looking through the lens and the image being blurred.  As you turn the focusing mechanism, the image becomes either clearer or more blurred.

Cones and rods (named from their actual shapes) are types of photoreceptors, located on the retina at the back of the eye, that allow animals to see light and color. Rods are sensitive to light, but they only create coarse gray images.  Cones are sensitive to color and allow for detail.  The large number of cones in the avian eye enables birds to form very sharp images.

The next time you visit the World Bird Sanctuary be sure to pay special attention to the differences between the eyes of the birds that visit our feeders (prey birds) and the raptors (birds of prey…predators) in our exhibits.

More to come on the topic of avian vision next month.

Submitted by Leigh French, World Bird Sanctuary Naturalist

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