City birds are developing smaller eyes due to light pollution

Researchers have discovered that two common birds are developing smaller eyes – most likely as a response to urban light pollution. The species are the Northern Cardinal and Carolina Wren, both of which inhabit urban areas around the country. 

Remarkably, these birds were found to have eyes that were about 5% smaller than their counterparts living on the outskirts of the city. This contrasts with two migratory species, the Painted Bunting and White-eyed Vireo, which showed no variance in eye size based on their urban or suburban habitats.

Declining bird populations

The implications of these findings are profound, especially in the context of the worrying decline of bird populations across the U.S. 

According to past research, the U.S. and Canada have witnessed a devastating reduction of 29% – or 3 billion birds – since 1970. 

Although habitat fragmentation has been identified as the primary culprit, this new research suggests that human-generated sensory pollutants, particularly light, could also be influencing birds’ ability to thrive in urban settings.

Studying birds’ eyes

“This study shows that residential birds may adapt over time to urban areas, but migratory birds are not adapting, probably because where they spend the winter–they are less likely to have the same human-caused light and noise pressures. It may make it more difficult for them to adjust to city life during the breeding season,” explained study senior author Jennifer Phillips, a wildlife ecologist at Washington State University. 

For the investigation, Phillips collaborated with Todd Jones and Alfredo Llamas from Texas A&M University, San Antonio. The trio analyzed over 500 birds, comparing body and eye sizes. They also evaluated the noise and light measurements of each habitat. 

What the researchers discovered 

Interestingly, no notable differences were detected in the body sizes of birds from various locations, except for the Painted Bunting. This discrepancy, the researchers concluded, was primarily age-driven. 

Younger male buntings, struggling in the mating competition, were more frequently found in brighter, louder city centers, presumably less sought-after habitats.

This research is the first to link urban light pollution to physiological adaptations in birds, specifically eye size. 

Birds with smaller eye size

The smaller eye size may enable birds to deal with the brighter and more constant light in city environments, said Jones, the study’s first author who is now a post-doctoral fellow at the Smithsonian’s Migratory Bird Center. 

According to Jones, birds with bigger eyes can be somewhat blinded by the glare of city lights or be unable to sleep well, putting them at a disadvantage in urban areas.

Unintended consequences 

“Humans may have some unintended consequences on birds that we don’t realize,” said Jones. “We don’t know if these adaptations could have good or bad consequences for the birds down the road, considering that urban environments aren’t going away anytime soon.”

“It is also important to understand how to manage such environments for the birds that maybe aren’t urban adapted.”

Future research 

With the support of a $2.1 million grant from the National Science Foundation, Phillips is leading a team to investigate the effect of both light and noise pollution across multiple bird species.

The team will investigate how light and noise affects the birds’ stress levels, sleep hormones, song structure, aggression levels, and overall fitness.

“We want to know whether patterns at molecular and behavioral scales affect fitness or not. Essentially, we’re trying to understand what are the benefits and costs to these animals living in a sensory polluted world,” said Phillips.

The study is published in the journal Global Change Biology.

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