Blackbirds born in urban environments have developed ways to keep their stress levels down when compared to their forest-dwelling counterparts, a new study suggests.

The research shows, for the first time, that city life changes how wild animals respond to stress.

For instance, having to build a nest on the side of an apartment building might not upset a second-generation city blackbird, while the same scenario could cause anxiety chemicals in rural birds to skyrocket.

The study, performed by researchers at the Max Planck Institute for Ornithology in Germany, is detailed in the August issue of the journal Ecology.

Stressed out

The researchers captured two groups of week-old blackbird nestlings — one from a forest habitat and the other from the center of Munich — and hand-raised them in controlled laboratory conditions for one year.

The researchers measured levels of glucocorticoids, stress hormones that help the birds survive under difficult environmental conditions.

Compared with the forest-born nestlings, the urban birds showed reduced levels of stress hormones after the researchers had stressed the birds by capturing them in cotton bags and handling them.

"The main thing is catching the birds and handling them. That's a relatively rough stressor," study team member Jesko Partecke told LiveScience.

The researchers suggest that the reduced stress response in urban birds has a genetic basis and that it developed as a way to cope with anxieties unique to city life, such as light pollution, noise and close proximity to humans.

"They have to cope with completely different environmental conditions compared with forest birds," Partecke said.

In birds and other vertebrates, including humans, prolonged elevation of glucocortoids can impair reproductive, immune and brain functions.

To survive amidst humans, the blackbirds have decreased the release of stress hormones that are only beneficial on a short-term basis.

Other urban animals

Whereas previous studies have reported behavioral differences between birds born in urban settings and those in natural habitats, changes in physiological stress responses have not been documented.

For instance, scientists have found that city blackbirds are often tamer than their forest-living relatives.

The results could explain the impact of city stresses upon species other than the European blackbird.

"Urbanization is not only happening in European blackbirds, but all over the world a lot of animals are colonizing these urban areas," Partecke said.

After the one-year study, Partecke and his colleagues released the birds into the wild. They hope to receive grant money to pursue lingering questions.

"We don't know anything about the consequences of reduced stress levels," Partecke said.

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