Birds' eyes see more, better than ours do

We know what birds look like to us, but what do we look like to them?

Birds’ eyes and human eyes have evolved separately for 300 million years, so it is to be expected they differ. Birds’ eyes are much bigger than those of comparably sized mammals; for example, at five times bigger than a human eye, the ostrich has the biggest eye of any land animal.

These huge eyes allow for better resolution, as the incoming image gets projected by the lens onto a larger retina across a longer eyeball. Each retinal receptor cell has to resolve a smaller portion of the image. But the cost of these huge eyes is that birds’ brains are pushed farther back into the skull, as their eyes take up the entire frontal portion and meet in the middle.

The avian retina has increased surface area for visual receptors due to deep folding of the focal area. These extra visual receptors (called fovea) allow birds to resolve small objects at least five times farther away than humans, with a large bird of prey being able to spot a small mammal such as a rabbit from a distance of over a mile.

Owls and many other birds feed and migrate at night, and must gather as much light as possible. One trick they share with many nocturnal animals is to reflect the light that passes through their eye back into the eyeball with a special reflective layer. When you see eyeshine from a deer or a moth, you are seeing this second pass of light through the eye. Humans and other primates, mostly active during the day, lack this layer (called a tapetum lucidum).

But with such a well-appointed retina, avian vision might be compromised by requiring extra blood vessels, which cause blind spots as they pass in front of the retina.

Not to worry, birds have evolved a unique structure to solve the problem: A condensed cluster of blood vessels (called a pecten oculi) sits in the middle of the eyeball and reduces the number of blind spots, while nourishing the huge eyeball.

Not only are avian eyes bigger and better than ours, but they receive color differently. Birds have four visual pigments, instead of the three (red, green and blue) of humans. They break the color spectrum up differently and can see way out into the ultraviolet. Thus, a bluebird may not look blue to another bluebird, and feather patterns that are invisible to us are apparent to the birds.

As one example from our own research, the blue-and-white Belted Kingfisher is well-known to be one of the only bird species in which females have more colors than males.

Females have a third color, the brown “belt” across their belly. However, when we measured the ultraviolet reflectance of the belly feathers in males, we could detect a bright, flashy pattern of ultraviolet absent in females. Thus, to a kingfisher, males are still the more ornate gender, with females lacking ultraviolet patterns and covering up their belly under dull brown feathers.

Cristol teaches in the Biology Department at the College of William and Mary and can be contacted at dacris@wm.edu. To discover local birding opportunities visit williamsburgbirdclub.org.

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