Birds living in cities develop their reproductive systems earlier than rural birds.
A new study from the Max Planck Institute for Ornithology in Radolfzell, Germany has found that city birds, exposed to light around-the-clock, develop faster sexually. Birds exposed to low intensity light at night also begin to sing and molt earlier in the year.
The new research, published in the journal Proceedings of the Royal Society Series B, focused on European blackbirds, a species known for massive populations in urban areas. The scientists, led by Jesko Partecke from the Max Planck Institute tagged several of the birds with light loggers. "The intensities were very low - 0.2 lux. That's just one-thirtieth of the light emanating from a typical street lamp," Partecke told Phys Org.
These low levels were very capable of initiating early maturation in the birds. Over the course of a 10-month period, the scientists exposed wild city and forest blackbirds to 0.3 lux of light at night.
"The results were astonishing: the birds' gonads grew on average almost a month earlier than those of animals that slept in the dark," said Partecke. Physical growth was not the only change, either. In birds exposed to low levels of light at night, testosterone levels rose earlier. The birds sleeping in low light even caused the birds to sing about an hour earlier than those sleeping in the dark. The study found that in addition to an early onset of breeding season, light at night also caused the birds to molt earlier at the end of the season.
"All of this indicates that, from a seasonal perspective, the animals are ready to breed earlier," said Partecke. Any advantages to this early onset breeding are still unknown.
"Blackbirds in the city are able to breed earlier in the year due to the artificial light and can produce more young in a year as a result," said Partecke. "But only if the nestlings have access to enough food." Light exposure at night may extend the length of the day for the birds, causing them to hunt for food at night. The birds may be putting this extra food energy into increased reproduction.
