Study shows how size of birds’ eyes provides information on environment

Where do birds live? How do they behave? How vulnerable are they to changes in their habitat?

To answer those questions, researchers used to look toward factors such as size or migration patterns.

But a study suggests the answer might be as simple as looking at a bird’s eyes.

When Ian Ausprey, a recent doctoral graduate of the Florida Museum of Natural History’s Ordway Lab of Ecosystem Conservation, realized there hadn’t been a definitive study of how birds’ eye size relates to their environment, he analyzed the eyes of nearly 3,000 bird species. He found eye size actually predicts birds’ habitat and behavior. The research was recently published in Proceedings of the Royal Society B.

Ausprey used an unpublished dissertation by Stanley Ritland, who measured the eye sizes of several thousand species of animals in the 1970s, as a data set. When he compared birds’ eye size with their behavioral traits, he found that birds’ eyes contain a lot of information about their species.

Birds that eat insects, for example, have large eyes that let them see prey from far away. Birds that live in tropical latitudes and forests also have large eyes that enable them to catch enough light to eat and mate. Birds with smaller eyes tend to spend more time in the sky, where the glare of the sun can be blinding.

The eyes also offer sobering lessons to researchers. Birds whose eyes have adapted to shady forests are affected by deforestation, which makes the landscape sunnier and can prompt migration or behavior change.

“Bright lights can cause something called disability glare,” Ausprey said in a news release. “When you shine a light on birds, they change the way they forage. They also respond differently to vocalizations of experimental predators.”

Since so many tropical birds have large eyes, they stand to be particularly vulnerable to deforestation. But rainforest destruction is plowing forward at a dizzying pace: In May alone, Reuters reports, an area roughly the size of Los Angeles had been destroyed in the Brazilian Amazon.

Now conservationists have another argument against deforestation: The vulnerability expressed in multiple bird species’ eyes.