For most birds, eyes are essential to life in flight. They perform instant aerobatics in dense branches and pinpoint predators or prey in the distance. However, when studying how birds adapt to our rapidly changing world, ornithologists have largely ignored the size of their eyes, and tended toward features such as wing length and beak shape. However, the lost “treasure house” of bird eyeball measurements now provides a new perspective.
In 1982, University of Chicago graduate student Stanley Ritland used preserved museum specimens to carefully measure the eyeballs of nearly 2,800 bird species—one-third of all terrestrial birds. He has never published his data, but Ian Ausprey, a graduate student at the University of Florida and the Florida Museum of Natural History, has just revisited it. Ausprey’s analysis, published inside Proceedings of the Royal Society B, Supporting previous work in Peru, showing that small-eyed birds can better adapt to changing habitats.
“We were able to prove that there is a strong correlation between eye size, the type of habitat the birds use, their foraging behavior, and where in the world they live,” Ausprey said. Ritland’s measurement results show that there is an inverse relationship between the size of the eye and the scope. Birds with smaller eyes tend to migrate and travel through many habitats; large-eyed species have a narrower distribution range, concentrated near the equator, and are often shrouded in dense forest canopies. The study hypothesized that small-eyed birds can seamlessly handle different light levels when traveling, while large-eyed birds struggle with glare outside the dim woodland.
Ausprey has seen this in the mountainous cloud forests of Peru.He said that in these biodiversity hotspots, “the size of the eyes and how [birds] Deal with agricultural disturbances. “Big-eyed birds tend to disappear from brightly lit agricultural and deforestation landscapes; small-eyed birds adapt. This new study extends Ausprey’s observations of Peru to more birds in other places, including parrots, woodpeckers, and Finches.
Alison Schultz, an ornithologist at the Natural History Museum of Los Angeles County, who was not involved in the study, praised it for emphasizing the importance of exposure of birds to light. Her own work has discovered the connection between bird color and ambient light. She said that she is looking forward to future research to explore how light pollution and deforestation can further shape bird eyes. “If we really see the eyes evolving to better match the new light environment, I would be very curious,” Schultz added.
Ausprey said the study emphasizes the importance of protecting habitats throughout the entire range of light, especially dense rainforest patches, to protect birds of all sizes from habitat loss.