The bright lights of big cities could be causing an evolutionary adaptation for smaller eyes in some birds, a new study indicates.

Researchers found that two common songbirds, the Northern Cardinal and Carolina Wren, that live year-round in the urban core of San Antonio, Texas, had eyes about 5% smaller than members of the same species from the less bright outskirts. Researchers found no eye-size difference for two species of migratory birds, the Painted Bunting and White-eyed Vireo, no matter which part of the city they lived in for most of the year.

The findings, published in Global Change Biology, have implications for conservation efforts amid the rapid decline of bird populations across the U.S.

“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,” said Jennifer Phillips, a Washington State University wildlife ecologist and senior author.

 

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