In a revelation with profound implications for bird populations and conservation, a recent study suggests that the brilliant luminance of urban environments may be driving an evolutionary adaptation towards smaller eyes in certain bird species. Published in Global Change Biology, the findings highlight the potential impact of human-caused light pollution on birds and raise concerns amid the rapid decline of avian populations across the United States.
Two common songbirds, the Northern Cardinal and Carolina Wren, residing year-round in the brightly lit urban core of San Antonio, Texas, exhibited eyes approximately 5 percent smaller compared to their counterparts from the less illuminated outskirts. Surprisingly, no such variation in eye size was observed among two migratory bird species, the Painted Bunting and the White-eyed Vireo, regardless of their year-round habitat.
Jennifer Phillips, a wildlife ecologist at Washington State University and senior author of the study, noted, “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.”
Previous research has already revealed that the United States and Canada have witnessed a staggering 29 percent decline in bird populations since 1970, amounting to a loss of 3 billion birds. While habitat fragmentation was considered the primary driver of this decline, the current study suggests that sensory pollutants like human-induced light may also play a role in birds’ ability to cope with city life.
To investigate this phenomenon, Phillips collaborated with post-doctoral fellow Todd Jones and graduate student Alfredo Llamas from Texas A&M University, San Antonio, studying over 500 birds from central and peripheral areas of San Antonio. The researchers compared the body and eye sizes of the birds and analyzed noise and light levels during both day and night in each location.
Among the species examined, the study identified a size difference in the Painted Bunting, which was primarily attributed to age. Younger, smaller male buntings, less competitive in seeking mates compared to their more colorful elder counterparts, were more commonly found in the brighter and noisier central urban areas.
While previous studies have explored the effects of urban light on birds’ circadian rhythms and “dawn song,” this study is the first to establish a link to changes in eye size. The smaller eye size is believed to help birds cope with the constant and intense urban light, potentially enabling them to see more effectively in city environments. In contrast, birds with larger eyes may experience glare from city lights or disrupted sleep, rendering them less adapted to urban life.
Todd Jones, the study’s first author and a post-doctoral fellow at the Smithsonian’s Migratory Bird Center, commented, “Humans may have some unintended consequences on birds that we don’t realize. 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.”
Jennifer Phillips is leading a team to investigate the effects of both light and noise pollution across multiple bird species, supported by a recent $2.1 million grant from the National Science Foundation. Their research aims to determine how sensory pollution impacts 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,” Phillips explained.
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