Bright lights in big cities may be shrinking the size of bird eyes
PULLMAN, Wash. — Urban light pollution is notorious for blocking out city dwellers’ views of the stars at night, but new findings reveal that the bright lights of big cities may also be sparking an evolutionary change in birds — giving them smaller eyes!
Scientists at Washington State University have discovered that two common songbirds (the Northern Cardinal and Carolina Wren), who live year-round in the urban core of San Antonio, Texas, had eyes that were roughly five percent smaller than members of the same species living farther from the city in communities with less artificial light.
Meanwhile, the research team found no eye-size differences among two species of migratory birds (the Painted Bunting and White-eyed Vireo), regardless of which area of the city the birds dwelled in for most of the year. Study authors stress these findings hold far more than local implications, adding that conservation efforts need to improve across the country as numerous bird populations continue to see rapid declines.
“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,” says Jennifer Phillips, a WSU wildlife ecologist and senior study author, in a university release.
Together, the United States and Canada have lost an astounding 29 percent of their bird populations (three billion birds) since 1970. Most scientists currently subscribe to the belief that habitat fragmentation has been the main driver of this observed decline in bird populations, but this latest work indicates sensory pollutants such as human-made light may also be influencing birds’ ability to cope with city life and subsequent evolutionary patterns.
In collaboration with post-doctoral fellow Todd Jones and graduate student Alfredo Llamas of Texas A&M University, Phillips analyzed over 500 birds from the central and edge areas of San Antonio. They compared birds’ bodies and eye sizes while recording and analyzing noise and light measurements during the day and night in each area.
Researchers did not note a difference in the body sizes of birds living in different areas. However, there was one exception: the Painted Bunting.
Despite that, an additional analysis eventually revealed that this size difference was mostly due to age. Study authors explain that younger, smaller male buntings, who aren’t as capable of competing for mates as their more colorful elders, tended to be seen in the brighter, noisier central locations, which are largely considered less desirable living locations.