Eye Size Of Birds May Predict Sensitivity To Environmental Change

WASHINGTON — The size of the birds’ eyes can reveal broad patterns of their biology and behavior, including where they live, eat, and how they hunt. This, further, provides a potential roadmap for future efforts of conservation, states a new study.

With a limited range of taste and smell, birds primarily rely on vision to navigate, find food and avoid predators. Since Charles Darwin’s classic work on finches, very little has been known about how eye size in birds influences their behavior compared to other traits such as beak shape and body size.

Birds are claimed to have some of the most prominent eyes relative to the bodies of all vertebrate land animals, second only to frogs.

“I was shocked to find out while doing literature searches that there was no definitive publication on how eye size in birds relates to their environment,” said Ian Ausprey, a recent doctoral graduate of the Florida Museum of Natural History‘s Ordway Lab of Ecosystem Conservation.

Previous studies on bird eyes have been limited in scope, typically including only a few dozen species of birds in specific regions. This gap in scientific knowledge was all the more glaring, given that a graduate student measured the eyes of more than 4,000 species of birds in museum collections in the late 1970s, creating the largest dataset of its kind.

Ausprey relied on this resource to analyze eye size for 2,777 species— about one-third of the world’s bird diversity— revealing that this single trait more powerfully predicts where birds live and how they behave than better-studied characteristics such as size, anatomy, and movement.

Audrey had the idea for the study while conducting fieldwork with colleagues in the Andean forests of Peru. For five years, the researchers measured the eyes of Peruvian birds. They attached small light sensors to more than a dozen species of tanagers, finches, wrens, and woodpeckers to determine how these birds were coping with increased amounts of forest fragmentation due to agriculture.

Their results were troubling. They revealed that birds with large eyes avoided agricultural fields, keeping to diminishing forest habitats. But the researchers could also use eye size to predict where these birds mated and laid eggs and what they were eating, valuable information for future conservation efforts.

Audrey wanted to know whether this pattern held for all birds, not just those in Peru. But with over 10,000 species spread out across all seven continents, answering a question as broad as how eye size influences bird behavior would have taken years.

Fortunately, the data Ausprey needed had already been collected in the form of a dissertation, a nearly 2,000-page tome completed by Stanley Ritland during his time as a doctoral student at the University of Chicago.

“He spent his time traveling around museums, extracting eyes out of specimens preserved in alcohol and then measuring them,” said Asprey. “He did it for several thousand species of birds, as well as mammals and reptiles.”

Ritland left academia upon graduating, however, and never published his data in a scientific journal. Researchers have used small portions of the massive dataset, initially relegated to the University of Chicago library’s stacks to answer small-scale questions. Still, comprehensive analyses have so far been lacking.

Although the data was available, the time-consuming task of digitizing it remained. Ausprey hired two undergraduate students, Savannah Montgomery and Kristie Perez. They spent five months transcribing Ritland’s measurements into spreadsheets, so they could be analyzed and shared more broadly with the scientific community.

Because eye size tends to increase with body size, Ausprey standardized all the measurements for each species by mass and intentionally omitted birds that operate at optical extremes, such as far-sighted raptors and nocturnal owls. Scientists already know these species have enormous eyes.

Instead, he focused on land-dwelling birds that hunt for food close to the ground and are most active during daylight hours.

Stark patterns began to take shape as eye size was compared with a host of behavioral traits.

Birds with more giant eyes live closer to the equator, where the planet’s belt of rainforests creates dark understory habitats. Regardless of latitude, birds that hunt or forage most relative to the forest floor have large eyes to take in as much light as possible. In contrast, those that spend more time in the sky had correspondingly smaller eyes to reduce glare.

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

Scientists worry that such behavioral changes may negatively affect avian understory specialists, many of whom have already been displaced by deforestation.

“Understory tropical birds may be susceptible to fragmentation because they are adapted to dark forested environments and are unable to cope with rapid changes in brightness associated with forest edges and human-modified habitats,” Asprey said.

Eye size is also strongly correlated with diet. More enormous eyes not only absorb more light, but they can also confer increased focal length and resolution, the equivalent of upgrading your camera with a longer lens.

Birds that eat insects have more enormous eyes, better suited for spotting prey at long distances, regardless of whether they lived in the forest understory or open habitats. Birds with the most petite eyes relative to body size were often nectar feeders, suggesting they may rely on color more than shape when looking for food.

Asprey also analyzed how eyes have changed throughout the birds’ evolution, finding that once eyes became larger in a particular group, they stayed that way. This meant that closely related groups, such as the hummingbird and swift families, could have eyes of vastly different sizes.

Within a family, however, size did not change much among species. Flycatchers, for example, spend a lot of time sallying out and catching prey, which requires long-distance, binocular vision, Ausprey said.

“And it turns out; flycatchers tend to have larger eyes, as you’d expect. All the finches and tanagers and such that eat fruits and seeds tend to have tiny eyes.”

To Audrey, the data collected by Ritland decades ago offer an unparalleled glimpse into bird diversity and behavior, which may help conserve species for the future.

“Nearly half a century has passed, and yet the same datasets are relevant,” Asprey said.

Ritland relied entirely on alcohol-preserved museum collections, meaning the identical specimens he measured are still accessible to scientists stitching together patterns in the natural world.

Some of the birds he encountered during his museum visits were already of considerable antiquity by the time he began taking his measurements, including two birds collected during Captain Cook’s first voyage around the world.

“Museum collections are invaluable, indispensable, and essentially irreplaceable,” Ritland said in an email.

Asprey, who knew firsthand the difficulty of collecting eye-size data in the Andean cloud forests of Peru, gained a newfound respect for natural history collections while using Ritland’s work. 

(With inputs from ANI)

Edited by Saptak Datta and Ritaban Misra