Fast beat of the city
Blackbirds’ biological rhythms are altered in an urban setting
The researchers captured adult male European blackbirds from the city of Munich and a nearby rural forest. Each bird was equipped with a lightweight radio-transmitter which monitored their daily levels of activity in the wild for 10 days before they were recaptured. They were then kept in light-proofed, sound-insulated chambers and their circadian rhythms were measured under constant conditions, without any environmental information that could serve as a “clock”. In this way, each bird’s own, internal rhythm could be tested. Once the tests were complete the birds were returned to the wild.
Barbara Helm, of the University of Glasgow’s Institute of Biodiversity Animal Health and Comparative Medicine, said: “The daily cycles of activity and rest are based on biological rhythms which have evolved as an adaptation to the rising and setting of the sun. Our tests were designed to benchmark the internal rhythms of the birds under controlled conditions and to determine a link to the birds’ chronotype in the wild. Chronotype is a measure of an individual’s consistent timing relative to environmental factors, ie., its relative “morningness” or “eveningness”.
“We found that the rhythms of urban birds in the wild differ significantly from their forest counterparts. On average, they began their daily activities around 30 minutes before dawn, while forest birds began their day as the sun rose. The city birds ended their days around nine minutes later, meaning they were active for about 40 minutes longer each day. “In constant laboratory conditions, urban birds’ circadian rhythms were clearly altered, running faster by 50 minutes than forest birds and being clearly less robust. There seems to be a different beat to city life. City clocks were also less persistent, especially in the business district.”
Previous research undertaken by other researchers has suggested strong links in humans between disrupted sleep patterns and an increased incidence of depression and diseases including obesity and some types of cancers. The work shows for the first time that that when sharing human habitats, a wild animal species has a different internal clock. “We’d be keen to find out the costs and benefits of modifying biological rhythms in blackbirds and other animals commonly found in our cities. This may help us to better understand the challenges of coping with urban life.”
The researchers have raised the possibility that the differences in the biological rhythms could be the result of micro-evolutionary changes in response to the stimuli of urban life such as artificial light and increased levels of noise. Davide Dominoni, of the Max Planck Institute and an affiliate of the University of Glasgow, added: “For songbirds, early risers may have an advantage in finding a mate and thus a greater chance of successfully producing offspring and passing along their chronotype to the next generation.” Other research has shown that chronotypes are highly heritable, so the process of natural selection could mean that city birds are evolving to favour early risers. “The speed of urbanization is accelerating around the world, but we don’t have anything close to a complete understanding of the effects of urban living on humans and animals.”