Bright light in big cities is linked to smaller eyes in birds

flying birds

Large areas of the planet are now exposed to anthropogenic disturbance sources due to urbanization, which accelerates environmental change and creates novel ecosystems. Given that populations found in urban and more natural systems may exhibit phenotypic divergence, responses of creatures to urbanization serve as essential models for evolutionary ecologists.

However, the generality of phenotypic change remains unclear. Scientists from Washington State University examined whether morphological phenotypes in two residential species, the Northern Cardinal and Carolina Wren, and two migratory species (Painted Bunting [Passerina ciris] and White-eyed Vireo [Vireo griseus]) differed between urban core and edge habitats in San Antonio, Texas, USA.

Collared Sunbird
Collared Sunbird

In particular, scientists examined whether urbanization, associated sensory pollution (light and noise), and brightness (open, bright areas caused by anthropogenic land use) influenced measures of the avian body (mass and frame size) and lateral eye size. Their study indicated that the bright lights of big cities could be causing an evolutionary adaptation for smaller eyes in some birds.

Two common songbirds, the Northern Cardinal and Carolina Wren, were discovered to have eyes roughly 5% smaller than those of the same species in the less bright outer suburbs of San Antonio, Texas, where they spend the entire year. No matter which area of the city the Painted Bunting and White-eyed Vireo spent most of the year in, scientists observed no variation in the size of the eyes of these two migratory bird species.

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

A past study has suggested that the U.S. and Canada have lost 29% of their bird populations, or 3 billion birds, since 1970. The latest study raises the possibility that sensory pollution, like light produced by humans, may also affect birds’ capacity to adapt to city life, even though scientists currently assume that habitat fragmentation is the main reason for reducing bird populations.

While other studies have examined how urban light affects the timing of birds’ “dawn song” and circadian rhythms, this is the first known study to show a connection to eye size.

Scientists studied more than 500 birds from central and edge areas of San Antonio. They studied the bird species’ body and eye sizes and examined data on each area’s noise and light levels during the day and at night.

Except for one species, the Painted Bunting, there was no change in the body proportions of birds in various locations. Further investigation revealed that age played a significant role in this size disparity. Younger, smaller male buntings were more frequently observed in the brighter, noisier central areas, which are less desirable since they cannot fight for mates as effectively as their more colorful elders.

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

“Humans may have 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.”

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