Nature up close: How birds' eyes are different from other vertebrates'

Ospreys cannot move their eyes, so they move their heads instead.

Verne Lehmberg

By "Sunday Morning" contributing videographer Judy Lehmberg. 

Years ago my husband decided he wanted to learn to fly a glider. He already had his private pilot's license and thought gliding quietly through the sky sounded like a wonderful thing to do. We went to a small local airport where an 80-year-old man taught glider lessons. They went up. While the old man fell asleep in the back of the glider, my husband had a wonderful time.

While he flew, I sat in a small waiting room with a couple of dilapidated couches and waited. There were several private pilots talking about flying small, single-engine airplanes. The conversation turned to aerobatic planes and one of the guys said, "A Bellanca can fly straight down at 160 mph and pull out without ripping its wings off." Another pilot started bragging about a different brand of plane. Finally the biologist in me couldn't stand it anymore, and I said, "A peregrine falcon can dive 200 mph and not get its wings ripped off when it pulls out of a dive."

One of the pilots looked over at me and said, "Who makes that one?"

My answer: "Evolution."

To be fair evolution has had much longer to work on making birds better flyers than humans have had to perfect airplanes. During millions of years of evolution, birds have developed many adaptations for flight, from lightweight bones to feathers. Several of those adaptations have occurred in the avian eye.

Birds have an eye bone (the sclerotic bone) which reduces their ability to move their eye, especially in raptors. It varies in shape, from a simple ring in pigeons to an elaborate tube-shaped bone surrounding the eye in owls. 

pigeons-eye-bone-sclerotic-ring-verne-lehmberg-620.jpg
A pigeon's eye bone, or sclerotic ring. In a live bird it would be centrally located in the eye socket but has been glued in place here. Verne Lehmberg

The cornea in peregrine falcons is firmly attached to the sclerotic ring that keeps the bird's eye from changing shape in a dive. Without it, their eye shape would change, their vision would be distorted, and they would not know exactly where their prey – or the ground – was during a dive, a mistake they cannot afford to make.

Although bird eyes are structurally similar to ours, based on the number of rods and cones, their visual acuity is far superior. An eagle has five times the number of rods and cones in each of their eyes as we do. Their eyes are also much larger in relationship to the size of their head, especially in eagles and other raptors such as hawks and owls. In fact, their eyes are so large some birds must move their head, rather than their eyes, because their eye sockets don't have enough room for muscles to move their eyes.

Think about the last time you saw a bird following you with its eyes and you realize most of them don't – they must move their head to follow you, or any object.  Some birds, especially raptors, move their heads to accurately locate their prey because they can't move their eyes.  Others hold their head very still to find the exact location of their prey while their body moves. 

The way birds observe their world can be seen in the accompanying video. 

The head movements of birds

The first bird is a perching osprey looking for a fish lunch. It sees a fish and needs to know exactly where it is to catch it. The second osprey sees a threat (another osprey). The third one, a juvenile, is watching its dad bring a fish to the nest. None of the ospreys can move their eyes, so they move their heads instead.

The fourth bird is a pied kingfisher doing what kingfishers do: looking for a fish. It must hold its head very steady so that it knows exactly where the small fish is if it is going to successfully catch it in the water below.

The last two birds are a lilac-breasted roller and a purple roller. Both species are experts at catching flying insects. Both need to be able to determine the exact trajectory of their prey, and can be seen compensating for the movements of their bodies by keeping their heads in the same position. 

Judy Lehmberg is a former college biology teacher who now shoots nature videos.

       
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