The size of bird eyes reveals a lot about them


The size of birds’ eyes can reveal broad patterns of their biology and behavior, including where they live, what they eat and how they hunt, according to a new study.

The results provide a potential roadmap for future conservation efforts.

Birds have some of the largest eyes compared to their bodies of any vertebrate land animal, just behind frogs. With a limited range of tastes and smells, birds primarily rely on vision to navigate, find food, and avoid predators.

Yet surprisingly, little is known about how the size of birds’ eyes influences their behavior relative to other traits, such as beak shape and body size, which scientists have meticulously studied since the 1970s. classic works of Charles Darwin on finches.

“I was really shocked to find, while doing bibliographic research, that there was no definitive publication on the relationship between the size of birds’ eyes and their environment,” says Ian Ausprey, a recent PhD from the Florida Museum of Natural History from the Ordway Lab of Ecosystem Conservation at the Florida Museum of Natural History. .

Bird eye preview

Previous studies of bird eyes have been limited in scope, typically including only a few dozen specific species or regions. This gap in scientific knowledge was all the more glaring when 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 in the world. kind.

Ausprey relied on this resource to analyze the eye sizes of 2,777 species, or roughly one-third of global bird diversity, revealing that this single trait more effectively predicts where birds live and how they behave than traits. better studied such as size, anatomy and movement.

Ausprey got the idea for the study while conducting fieldwork with colleagues in the Andean forests of Peru. For five years, researchers measured the eyes of Peruvian birds and attached small light sensors to more than a dozen species of tanagers, finches, wren and woodpeckers to determine how these birds coped with increased fragmentation. of forests due to agriculture.

Their results were disturbing: Big-eyed birds avoided agricultural fields, sticking to shrinking forest habitats. But researchers could also use eye size to predict where these birds mate and lay eggs and what they eat, valuable information for future conservation efforts.

Ausprey wanted to know if this model applied to all birds, not just those in Peru. But with more than 10,000 species spread across seven continents, it would have taken years to answer a question as large as the influence of eye size on bird behavior.

Ritland to the rescue

Fortunately, the data Ausprey needed had already been collected in the form of a thesis, a nearly 2,000-page book written by Stanley Ritland while he was studying for his doctorate at the University of Chicago.

“He spent his time traveling through museums, extracting eyes from specimens preserved in alcohol and then measuring them,” Ausprey explains. “He has done this for several thousand species of birds, as well as mammals and reptiles.”

Ritland, however, left academia after graduating and never published his data in a scientific journal. Researchers have used small portions of the massive data set, initially relegated to stacks at the University of Chicago library, to answer small-scale questions, but comprehensive analyzes have so far been lacking.

Even though the data was available, the tedious task of digitizing it remained. Ausprey hired two undergraduates, Savannah Montgomery and Kristie Perez, who spent five months transcribing Ritland’s measurements into spreadsheets so they could be analyzed and shared more widely with the scientific community.

Because eye size tends to increase with body size, Ausprey standardized all measurements for each species en masse and intentionally omitted birds that operate at optical extremes, such as clairvoyant raptors and nocturnal owls. Scientists already know that these species have unusually large eyes.

Instead, he focused on land birds that hunt for food near the ground and are more active during the day.

Old data, new discoveries

Austere patterns began to take shape as Ausprey compared eye size with a multitude of behavioral traits.

The larger-eyed birds live closer to the equator, where the planet’s rainforest belt creates dark undergrowth habitats. Regardless of the latitude, birds that hunt or feed closest to the forest floor have large eyes to absorb as much light as possible, while those that spend more time in the sky have smaller eyes to reduce light. ‘glare.

“Bright lights can cause what is called glare for people with disabilities,” says Ausprey. “When you light up the birds, they change the way they feed. They also react differently to vocalizations from experimental predators.

Scientists fear that such behavioral changes could negatively affect avian undergrowth specialists, many of whom have already been displaced due to deforestation.

“Tropical understory birds can be particularly susceptible to fragmentation as they are adapted to dark forest environments and are unable to cope with the rapid changes in light associated with forest edges and human-modified habitats,” explains Ausprey.

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

Birds that eat insects have larger eyes, which are better suited for spotting prey at long distances, whether they live in the undergrowth of the forest or in open habitats. Birds with the smallest eyes for body size were often nectar eaters, suggesting that they may rely more on color than shape when foraging.

Ausprey also analyzed how the eyes changed throughout the evolution of birds, finding that once the eyes got bigger 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 very different sizes.

Within a family, however, the size has not changed much between species. Flycatchers, for example, spend a lot of time going out and catching prey, which requires long-range binocular vision, Ausprey says.

“And it turns out that flycatchers tend to have larger eyes, as you would expect. All finches and tanagers and the like that eat fruits and seeds tend to have very small eyes.

For Ausprey, data collected by Ritland decades ago provides unprecedented insight into bird diversity and behavior, which may help conserve species for the future.

“Almost half a century of time has passed, and yet the same data sets are relevant,” says Ausprey.

Ritland has relied entirely on museum collections preserved in alcohol, which means the same specimens he measured are still available to scientists who assemble patterns in the natural world.

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

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

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

“As an environmentalist, it has become extremely obvious that collections are invaluable in providing data on traits that we really cannot easily collect in the field. “

The search appears in the Proceedings of the Royal Society B.

Research funding comes from the Katharine Ordway Chair in Ecosystem Conservation at the Florida Museum of Natural History.

Source: University of Florida

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