Birds with big eyes avoid bright human-altered landscapes

A new study from the Florida Museum of Natural History suggests that light pollution in human-altered landscapes such as crop fields serves as a barrier to birds with big eyes. The findings indicate that eye size is an important factor that has been largely overlooked when assessing how vulnerable various  birds species are to changes in their habitat.

The researchers established that bird eye size, habitat, and foraging techniques are strongly connected. Birds with larger eyes were found to stay in the shade of the forest, while birds that were abundant in the canopy and in agricultural settings had smaller eyes. 

“Many bird species literally disappear from highly disturbed, anthropogenic habitats such as agricultural landscapes,” said study lead author Ian Ausprey. “That’s probably due to many reasons, but this paper suggests light could be part of that.”

Ausprey said while some of the findings may seem like “a no-brainer,” the study is the first to document the relationships between light, eye size, and how birds navigate their world.

Light is particularly important for birds to detect food. Big eyes, which are commonly found in birds of prey such as owls and raptors, contain more photoreceptors. This allows the birds to identify images at longer distances and in darker settings.

On the other hand, large eyes can be detrimental when exposed to the glare of bright environments. Previous studies have shown that when birds become overwhelmed by too much light, it can alter their feeding behavior and reduce their awareness of threats.

In collaboration with study co-author Felicity Newell of the University of Florida, Ausprey surveyed birds in the forests of northern Peru for four years. Within these forests, gaps in the canopy expose patches of forest to startling brightness. “You can go from being very dark to very bright within inches,” said Ausprey.

The researchers explained that a difference of just 1,000 feet in elevation can lead to dramatic changes in which birds are present. The study region in Peru is also home to small farms with livestock pastures and vegetable fields that are interspersed with the forest.

Newel noted that the broad range of ambient light, from the deep, dark forest interior to wide open country, made an ideal model system for measuring birds’ use of light.

The team measured eye size relative to body size in 240 species that make up the cloud forest bird community. The analysis revealed that the insect-eating birds with the biggest eyes were far-sighted species, while the eye size in near-sighted species increased the closer to the ground they lived.

The researchers attached tiny light-sensing trackers to 71 birds representing 15 species. The sensors recorded the intensity of light that was encountered by the birds over several days. This experiment provided the experts with a new look at the various light “micro-environments.” 

Among the birds tracked, the rusty-tinged antpitta inhabited the darkest environment. This bird, which is exclusive to Peru, spends most of its life walking along the forest floor. Meanwhile, the blue-capped tanager was found to reside in the brightest environments.

With predominantly small-eyed bird species present in agricultural settings, the researchers derived that birds adapted to the dark forest understory would struggle to adjust to the flood of light in a field. These patterns are likely to be found on a global scale, and the trend may also apply to urban areas, which are “basically extreme forms of agricultural landscapes in some ways,” said Ausprey.

In fact, the rufous-collared sparrow, Zonotrichia capensis, the bird most commonly found in agricultural fields, is also the most abundant species in Latin American cities, said Newell.

The study is published in the journal Ecology.

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By Chrissy Sexton, Earth.com Staff Writer