Animals adapt to city environments in many ways - but scientists have found that birds in one city are evolving smaller eyes to cope with light pollution.
The finding could have important implications for the battle to save bird species - with the US and Canada having lost 29% of their bird populations - or three billion birds - since 1970.
The research found that two common songbirds from the city centre in San Antonio, Texas, have eyes about 5% smaller than other members of the same species from the outskirts.
Interestingly, migratory birds show no difference in eye size.
Jennifer Phillips, a Washington State University wildlife ecologist and senior author of the paper, 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.”
Scientists believe that habitat fragmentation is the primary driver of the decline in birds.
The new study suggests that sensory pollutants like human-caused light may also play a role in the birds' ability to cope with city life.
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The researchers studied more than 500 birds from different areas of San Antonio.
They compared body and eye sizes of the birds and analysed noise and light measurements during the day and night of each area.
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.
Why smaller eyes?
The smaller eye size may enable birds to deal with the brighter and more constant light in city environments, said post-doctoral fellow Todd Jones, who worked on the study.
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 realise," said Jones.
"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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The researchers warn that human habits may have unintended consequences on bird life that people d"on’t even realise".
The new research could offer insight into how to adapt urban environments for birds to live more comfortably.