I was recently asked how birds find a new backyard birdfeeder. This simple question has a complex answer. So, this week we are continuing our look (pun intended) at bird vision. Because the answer to the question is vision.
Indeed birds and other mammals see the world very differently than we humans. Some of this is due to the structures within a birds eye which allow them to see better, further, focus faster and see in a wider range of light conditions. Believe it or not, the position or location of a bird’s eye in its head is also a large factor in how well a bird sees.
We humans have eyes positioned close together in the front of our heads. This gives us nearly equal field of view with both eyes allowing stereoscopic vision known as binocular vision. Many birds have eyes positioned on the sides of their heads leaving them with unequal field of view from both eyes. This is called monocular vision.
At first you may think that having monocular vision is inferior to binocular vision and for the human species that may be true but if you are a small bird looking for tiny insects deep within cracks or crevices or watching out for aerial predators such as hawks and falcons it would be far superior to have eyes located on the sides of your head.
Monocualar vision has many advantages because it results in a wide field of view, sometimes as much as 340 degrees which can allow a bird to see both in front and behind at the same time. This would be like having the proverbial “eyes in the back of your head”. Very handy when fast flying predators are in the neighborhood and they are looking for YOU!
Some birds do have binocular vision. Owls for instance have large eyes close together in the front of their head. Both eyes see the same thing at the same time giving them binocular vision. Consequently they only have a field of view of up to 70 degrees (all in front of them). These birds don’t need to see behind them because generally they don’t have to worry about being someone’s lunch because they are the predator not the prey.
In addition, birds such as snipe and woodcock not only have their eyes on the sides of their head, their eyes are placed near the top of their head giving them excellent vision above them without having to moving their head to see above. Remaining still is an import aspect since these birds relay on camouflage feathers to remain hidden from a predators view. If they had to move their head around to watch for predators the movement of their head would give away their location to a sharp-eyed hawk or falcon. Their eye placement also helps them watch for predators while probing in the mud with their long bills during feeding.
Binocular vision is important not only for owls but also for many of the fast flying predators such as hawks and eagles. Binocular vision gives the bird good depth perception, thus determining distances and spatial relationship easily. This is very important when it comes to capturing prey while traveling at high rates of speed. For example a Cooper’s Hawk often ambushes small birds at a feeder by quickly darting out of the shadows of the woods. The hawk is traveling so fast that in an instance it needs to determine what bird it can catch without running into the birdfeeder, your house or a tree branch.
Although monocular vision doesn’t allow for instant depth perception it doesn’t mean birds with monocular vision can’t judge distances very well. The head-bobbing and weaving movements of the Rock Pigeon or even a barnyard chicken allow the bird to get a series of different views or snapshots to help determine spatial relationships of objects on the ground. When the head moves rapidly forward, objects closer to the bird appear to move across the visual field faster than those at a distance giving them clues to the objects three-dimensional position.
If you want to see monocular vision working you only need to watch an American Robin hunting for worms in your lawn. The bird rapidly runs forward, stops and cocks it head to one side or the other. Many people mistakenly believe the bird is listening for worms when in fact it is looking with one eye. The robin often cocks its head back and forth to gain a prospective on its food before quickly grabbing the worm. Until Next time…