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The chestnut-crowned gnateater (Conopophaga castaneiceps) lives in the dark understory of tropical cloud forests and has relatively large eyes.

IAN AUSPREY/FLORIDA MUSEUM OF NATURAL HISTORY

Eye size predicts where birds breed and feed

When Ian Ausprey outfitted dozens of birds with photosensor-containing backpacks, the University of Florida graduate student was hoping to learn how light affected their behavior. The unusual study, which tracked 15 species in Peru’s cloud forest, has now found that eye size can help predict where birds breed and feed—the bigger the eye, the smaller the prey or the darker the environment. The study also suggests birds with big eyes are especially at risk as humans convert forests into farmland.  

The study reveals a “fascinating new area of sensory biology,” says Richard Prum, an evolutionary biologist at Yale University who was not involved in the new work. It also shows the size of a bird’s eye says a lot about its owner, adds Matthew Walsh, an evolutionary ecologist at the University of Texas, Arlington, also not involved with the work.

Light matters—not just for plants, but also for animals. Large eyes have long been associated with the need to see in dim conditions, but very little research has looked in depth at light’s impact on behavior. Recently, scientists have shown that the relative size of frogs’ eyes corresponds to where they live, hunt, and breed. And several studies published in the past 3 years suggest the eyes of killifish and water fleas vary in size depending on the presence of predators. With no predators, even slightly larger eyes offer a potential survival advantage.

To find out how eye size might matter for birds, Ausprey and his adviser, Scott Robinson, an ecologist at the Florida Museum of Natural History, turned to the 240 species they had identified in one of Peru’s many cloud forests. The study area included a range of habitats—dense stands of trees, farms with fencerows, shrubby areas, and open ground. Because light can vary considerably by height—for example, in the tropics, the forest floor can have just 1% of the light at the tops of the trees—they included species living from the ground to the treetops.

Researchers temporarily mounted a light sensor (see arrow) to the back of this chestnut-capped brush finch (Arremon brunneinucha) and other birds to learn how light affected behavior.

IAN AUSPREY/FLORIDA MUSEUM OF NATURAL HISTORY

Over 4 years, the researchers measured eye width in 192 netted bird species, and estimated the size of the remaining species’ eyes’ from photographs. Larger birds tend to have larger eyes, so they used relative eye size for their subsequent analyses. They divided the birds into two groups based on hunting habits: those that typically grab morsels from their perch (and tend to be near-sighted) and those that usually pounce or dive to grab their meals. The team then documented roughly where these birds spend most of their time.

They also put commercially available light detectors on 71 birds from 15 species, attaching them to tiny backpacks with a medical adhesive that lasted several weeks. Once they fell off, Ausprey used a pack’s radio signal to track it down and retrieve the data. The light measurements tracked the light the birds were experiencing. “It’s amazing that they were able to glue little things on the birds,” says Jeffry Dudycha, an evolutionary biologist at the University of South Carolina, Columbia, who was not involved with the work.

After analyzing the data, Ausprey and his colleagues determined that eye size predicted not only where the birds spent their time, but also what they ate, they report this month in Ecology. As one might expect, birds that live deep in forests or needed to chase down insects from afar, like flycatchers, had relatively large eyes. Birds that lived in the brightest environments, such as the blue-capped tanager, have comparatively small eyes.

Moreover, the birds tended to stay where their eyes worked the best, which could explain the rise and fall of some species as humans clear forests for farming and development. Species with smaller eyes are thriving in pastures, fields, and tree plantations, Ausprey and his colleagues found. But larger eyed species that avoid bright light may be threatened as their forest habitat becomes fragmented and they cannot travel as far as they need to successfully reproduce or hunt. “Light can have an impact,” Ausprey says, on which species survive where.

Meanwhile, Prum predicts more researchers will start to look at the impact of light on their favorite organisms. And that pleases Ausprey. “It’s been a real privilege getting to share an entirely new dimension in ecology.”