Urbanization now exposes large portions of the Earth to sources of anthropogenic disturbance, driving rapid environmental change and producing novel environments. In new research, ornithologists examined whether urbanization-associated sensory pollution (light and noise) and brightness (open, bright areas cause by anthropogenic land use) influenced measures of avian body and eye size in two residential species — the Carolina wren (Thryothorus ludovicianus) and the northern cardinal (Cardinalis cardinalis) — and two migratory species — the painted bunting (Passerina ciris) and the white-eyed vireo (Vireo griseus) — in San Antonio, Texas, the United States. They found that the two residential species had eyes about 5% smaller than members of the same species from the less bright outskirts. They also found no eye-size difference for two species of migratory birds, no matter which part of the city they lived in for most of the year.
The painted bunting (Passerina ciris) in Freeport, Texas, the United States. Image credit: Dan Pancamo / CC BY-SA 2.0.
The United States and Canada have lost 29% of their bird populations or 3 billion birds since 1970, according to previous research.
Scientists believe that habitat fragmentation is the primary driver of the decline in birds, but the current study suggests that sensory pollutants like human-caused light may also play a role in the birds’ ability to cope with city life.
“Our 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,” said Washington State University wildlife ecologist Jennifer Phillips.
“It may make it more difficult for them to adjust to city life during the breeding season.”
In the study, Dr. Phillips and colleagues studied more than 500 birds from central and edge areas of San Antonio, Texas.
They compared body and eye sizes of the birds and analyzed noise and light measurements during the day and night of each area.
There was no difference among the body sizes of birds in different areas, except for one species: the painted bunting.
Upon further analysis, the researchers found that this size difference was mostly due to age.
Younger, smaller male buntings, who cannot compete as well for mates as their more colorful elders, were more often found in the brighter, noisier central locations, which are likely less desirable.
While other studies have looked at 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.
“The smaller eye size may enable birds to deal with the brighter and more constant light in city environments,” said Dr. Todd Jones, a postdoctoral researcher at the Smithsonian’s Migratory Bird Center.
“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 some unintended consequences on birds that we don’t realize,” he said.
“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.”
The study is published in the journal Global Change Biology.
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Todd M. Jones et al. Phenotypic signatures of urbanization? Resident, but not migratory, songbird eye size varies with urban-associated light pollution levels. Global Change Biology, published online September 20, 2023; doi: 10.1111/gcb.16935
