Bright City Lights Linked to Smaller Eyes in Urban Birds: Study

A new study suggests that the bright lights of big cities could be causing an evolutionary adaptation for smaller eyes in certain bird species.

Urban-residing Northern Cardinal (Cardinalis cardinalis) male birds typically have smaller eyes than those of the same species living in less brightly lit neighboring areas. … [+] (Credit: Andrej Chudý / CC BY-NC-SA 2.0)

Photo by Andrej Chudý under a Creative Commons license

A recent study has found that two common species of songbirds that live in urban areas have eyes that are about 5% smaller compared to members of the same species living in less illuminated nearby regions. However, there was no difference in eye size observed in two species of migratory birds, regardless of their breeding location within the city.

“This study shows that the birds that reside in urban areas may adapt over time, while migratory birds are not adapting, likely due to the different light and noise pressures they face in their wintering grounds,” said wildlife biologist Jennifer Phillips, an Assistant Professor at the School of the Environment at Washington State University, in a press release.

Professor Phillips, who has dedicated her career to ornithology, animal behavior, community ecology, and conservation, has always been intrigued by how nature persists despite increasing human encroachment.

The Carolina Wren (Thryothorus ludovicianus) is a common resident in the eastern half of the United … [+] States, extreme south of Ontario, Canada, and extreme northeast of Mexico. (Credit: ADJ82 / CC BY-SA 4.0)

Photo by ADJ82 under a Creative Commons license

A few years ago, a study revealed that over 3 billion birds had disappeared from the skies of North America since 1970 (more here). Initially, it was believed that fragmented habitats were the primary cause of this drastic decline in wild bird populations. However, this new research suggests that sensory pollutants like artificial nighttime lights may also play a role in how well wild birds adapt to urban environments.

“It may make it more difficult for them to adjust to city life during the breeding season,” Professor Phillips explained.

Dr. Todd Jones with a study subject, a male Northern Cardinal. Dr. Jones, a postdoctoral fellow, focuses on fledgling behavior and the effects of artificial light at night. (Image courtesy of Jennifer Phillips)

Image courtesy of Jennifer Phillips

For this study, Professor Phillips collaborated with postdoctoral fellow Dr. Todd Jones, now with the Smithsonian Migratory Bird Center, and graduate student Alfredo Llamas from Texas A&M University, San Antonio. Together, they examined over 500 birds from both central and edge areas of San Antonio to measure the physical characteristics of two resident species — Carolina Wrens (Thryothorus ludovicianus) and Northern Cardinals (Cardinalis cardinalis) — as well as two migratory species — Painted Buntings (Passerina ciris) and White-eyed Vireos (Vireo griseus) — that were breeding in either urban core or edge habitats in San Antonio, Texas.

In addition to measuring and comparing the body and eye sizes of the birds, the researchers analyzed noise and light levels during the day and night in each area.

Professor Phillips, Dr. Jones, and their team discovered no differences in body size between the study birds breeding in urban or edge areas of San Antonio, except for one species: the Painted Bunting. This migratory bird species from North America winters in Central America and breeds in the southeastern United States. Further analysis of the natural history of Painted Buntings showed that the difference in size was due to age: younger and smaller male buntings were less successful in competing for mates or territories with their larger and more colorful older counterparts, often resulting in them being confined to more illuminated, noisier, and less desirable central locations.

An adult male Painted Bunting (Passerina ciris), a bird known for its brilliantly colored plumage. These birds migrate south for the winter and breed in parts of the southeastern United States. (Credit: Don Faulkner / CC BY-SA 2.0)

Photo by Don Faulkner under a Creative Commons license

This study is significant because it is the first to investigate the specific effects of human-created urban lighting on wild birds and how it relates to eye size in both migratory and resident urban species.

According to the lead author of the study, Dr. Jones, having smaller eyes may help urban birds cope with the brighter and more persistent city lights compared to those living on the edges of urban areas. Birds with larger eyes may experience visual impairment due to the glare of city lights or have difficulty sleeping, putting them at a disadvantage in urban environments.

“Humans may be unintentionally impacting birds in ways we do not yet fully understand,” Dr. Jones emphasized. “We have yet to determine whether these adaptations will have positive or negative consequences for the birds in the future, considering that urban environments are here to stay.”

Although the underlying mechanisms of this relationship are still unclear, it appears that being a year-round resident in areas with high light pollution is essential to this pattern.

“It is also crucial to comprehend how to manage such environments for birds that may not be well-adapted to urban life.”

This study is part of a broader research effort that investigates the impacts of both light and noise pollution on various bird species. Professor Phillips and her collaborators are conducting a range of controlled experiments to examine how light and noise affect the stress levels, sleep hormones, song structure, aggression levels, and physiological changes in urban birds caused by altered environments.

“We aim to understand whether these molecular and behavioral patterns influence fitness or not,” Professor Phillips explained. “Essentially, we want to understand the benefits and costs for these animals living in a sensory polluted world.”

Source:

Todd M. Jones, Alfredo P. Llamas, and Jennifer N. Phillips (2023). Phenotypic signatures of urbanization? Resident, but not migratory, songbird eye size varies with urban-associated light pollution levels, Global Change Biology | doi:10.1111/gcb.16935

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