Science explains the odd irony behind the fact that the cutest birds – the ones gazing out at you with those big eyes – are also the ones in the most imminent danger from chopping down all the rain forests. It’s because they’re the ones that live in the shadiest places and hunt the smallest prey:
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.
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.
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.
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.