For the Birds – Bird Vision

The last column discussed color vision in birds. The ability of birds to see colors and to see the longer ultraviolet radiation we cannot see certainly gives birds a different perception of color than humans. In today’s column, I’ll explore some of the other fascinating characteristics of bird eyes, making comparisons with our own eyes.

Birds rely heavily on their vision. Their excellent eyesight allows them to find food and to detect potential predators from long distances. Females often choose their mates based on the appearance of his plumage.

How keen is a bird’s vision? Their visual acuity is often overstated. For instance, I have read fallacious claims that birds can distinguish the type of a newspaper while soaring from above. In reality, the keenest bird eyes that have been tested are those of the American Kestrel. These small falcons can detect an insect only two millimeters long (less than a tenth of an inch) from a distance of 40 feet. The Hobby, a Eurasian falcon that favors dragonflies for its meals, can detect one of these insects from 200 yards away. Bee-eaters can see a honeybee from a distance of nearly 200 feet. In general, birds can resolve details at a distance of 2.5 to 3 times that of a human.

Relative to our eyes, birds’ eyes are huge. The eyes of a Great Horned Owl are as large as yours. The eyes of an Ostrich are the largest eyes of any land vertebrate, exceeded only by much larger marine mammals. The eyes of a bird contribute up to 15% of the total weight of the head.

Humans and other mammals have round eyes. Thanks to pairs of extrinsic muscles on the back of the eyes, mammals can rotate their eyes to perceive images in different directions without moving the head. The shape of birds eyes is much more variable. Some are round while others are quite flat and some are tubular. Furthermore, birds’ eyes have much more limited movement than seen in mammals. Usually the only movement bird’s eyes can make is toward the bill tip, useful for seeing the food that a bird is manipulating with its beak.

Humans and a number of other mammals have binocular vision. Because our eyes are close set, we can see objects with both eyes. Because the angle of the object from each eye is different, our brains can process the differences in the two images to let us know how far away the object is. In other words, we have keen depth perception.

Birds’ eyes tend to be set on the sides of the head. As a result, a bird has a limited field of vision in which it can see an object with both eyes. Birds therefore have less depth perception than humans. Birds of prey tend to have their eyes set more forward, allowing a broader band of binocular vision. Birds like ducks and songbirds that must watch out for predators usually have their eyes set laterally. It’s very difficult for a predator to advance on such a bird without being seen even if the bird can’t gauge the distance to the predator very well. American Woodcocks are unusual in having their eyes set far back on the sides of the head. Their best binocular vision is behind them! A predator is not likely to sneak up on a woodcock from the rear.

Everyone has seen pigeons bobbing their heads as they walk. A number of birds engage in this behavior. The bird is essentially turning one eye into two to achieve a measure of binocular vision. By moving its head up and down, a single eye can see an object from different angles. The brain does the rest, processing the different images to give improved depth perception.

To focus on an object, the eyes of humans and other mammals change the curvature of the lens in each eye. In birds, both the cornea (the clear outermost layer of the eye) and the lens can change their curvature to allow the image of an object to be focused on the retina. The change in corneal shape is caused by muscles not present in mammal eyes.

Changes in the curvature of the cornea are effective for objects seen on land. However, the refractive index of the cornea is virtually the same as the refractive index of water. A change in corneal curvature does little to help a diving bird focus. Diving birds have well developed muscles around the lens and rely on changes in lens shape to focus underwater.

[Originally published on December 16, 2007]