Light Pollution Sparks Early Sexual Maturity For City Birds
February 14, 2013

City Lights Speed Up Sexual Clock For Some Birds

Enid Burns for — Your Universe Online

The light that emanates from cities may be causing birds to mate earlier. That's according to a study conducted by Max Planck Institute for Ornithology in Radolfzell, Germany. The study found that nighttime light in urban areas causes birds and other animals to develop their reproductive systems earlier.

In the study researchers at the Max Planck Institute studied European blackbirds (Turdus merula), to see how they develop in urban and rural areas. They found that for urban-dwelling birds, their testosterone levels rose earlier in the year and their testes matured at a younger age compared to their sylvan counterparts. City birds also began to sing and molt earlier.

The researchers attribute these precocious developments to the ever-present light pollution found in cities, which they say can "exert a major influence on the seasonal rhythm of urban animals.” Seasonal changes in day length are responsible for adjusting the biological clocks in animals, and light levels help to dictate their sleep-wake cycles. The day length is also important for determining the breeding season.

"People have long been exploiting this in areas such as agriculture: egg production in battery faming can be increased by altering the length of day with the help of artificial lighting," the authors say.

Birds that live in urban areas are subject to similar affects. Street lights and other ambient light sources contribute to changes in the day-length clock, that can change cyclical developments among the population.

"City-dwelling animals are now not only exposed to natural light conditions, at times they also experience extreme levels of lighting as a result of artificial light. But what effect does artificial light have on the time-of-day and seasonal organization of these urban animals?" the researchers initially questioned. "To answer this question, it is first crucial to know what light intensities the birds are actually experiencing at night."

A group of scientists working with ornithologist Jesko Partecke from the Max Planck Institute and the University of Konstanz tagged several urban blackbirds with light loggers to measure the average light intensities they were exposed to. "The intensities were very low — 0.2 lux. That's just one-thirtieth of the light emanating from a typical street lamp," the scientist said in the report.

Such low levels of light are nonetheless sufficient to make the gonads of male blackbirds mature earlier than their rural counterparts. "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.

Testosterone levels were also measured in the birds' blood, which served as another indicator of the animals' readiness for breeding. Levels rose earlier if the birds had been exposed to light at night. The scientists also discovered that song activity fell out of rhythm as a result of nighttime lights. The blackbirds began singing on average an hour earlier than their countryside relatives.

"All of this indicates that, from a seasonal perspective, the animals are ready to breed earlier," said Partecke. Birds also molted earlier, a phenomenon that usually happens toward the end of a breeding season.

"These findings are clear evidence that the artificial light we find in towns and cities can dramatically change the seasonal organization of wild animals," the authors concluded.