Blackbirds exposed to low-level nighttime light levels such as those in most cities sing, moult, and develop their reproductive systems earlier, scientists from the Max Planck Institute for Ornithology in Germany report in a new study.

After tagging urban birds to determine how much light they were exposed to, the researchers set up an experiment: They exposed wild-caught city and forest blackbirds to lighting intensities of 0.3 lux at night (similar to what city birds are exposed to) for 10 months.

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“The results were astonishing: The birds’ gonads grew on average almost a month earlier than those of animals that slept in the dark,” researcher Jesko Partecke said in a press release.

They also found that testosterone rose earlier in the blood of birds exposed to nighttime light.

“All of this indicates that, from a seasonal perspective, the animals are ready to breed earlier,” Partecke said.

While the cause is not clear, the scientists have several hypotheses: The extra light could confuse day length for the birds, for example. Or the light might allow birds to hunt more and gain more energy to reproduce earlier. Or, it could alter the metabolism of the birds.

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The next step, the scientists say, is to study how night-time light affects the fitness levels of the birds.

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