"They also begin to sing and molt sooner." The researchers had observed 40 male blackbirds (turdus merula).
According to the team, the light conditions in cities can thus significantly influence the seasonal rhythm of urban animals. Because in many animal species the length of the day determines, for example, sleeping habits or even the breeding season. In birds living in cities, these rhythms could be disrupted because artificial light sometimes leads to extreme light conditions, the researchers said.
In order to measure the average illumination to which the birds are exposed at night, scientists led by researcher jesko partecke equipped several urban blackbirds with light sensors. "The intensities were very low at 0.2 lux – only one-thirtieth of what a typical street lamp emits," says partecke. But even such low values were enough to make the gonads of male blackbirds ripen earlier.
The scientists therefore compared captive urban and woodland blackbirds, which they exposed to a lighting intensity of 0.3 lux at night, with a control group for ten months. "The results were astonishing: the testicles of the birds grew on average almost a month earlier than in animals that slept at night in darkness," explains partecke. In addition, testosterone levels in their blood increased, they began singing about an hour earlier, and molted toward the end of the breeding season much earlier than birds with dark nights. The study is published in the "proceedings B" of the british royal society.
Researchers still don’t know exactly where the early reproductive readiness comes from. Accordingly, they want to investigate whether city light is more beneficial or detrimental to blackbirds overall.