Berlin - All-night lighting in cities is prompting baby birds to grow up faster and mate younger, a study of European blackbirds shows.

Scientists have often suspected that artificial light could affect plants, animals and humans too. Farmers already know that egg production in battery farming can be increased by altering the length of the hens' day with the help of artificial lighting.

Street lamps, light from traffic and lighting from homes are causing a rise in night-time light levels round the globe.

A team at the Max Planck Institute for Ornithology in Radolfzell, Germany checked how light in urban areas affects male blackbirds, scientific name Turdus merula, and found the city birds develop their reproductive systems earlier than forest birds do.

They tagged several urban blackbirds with light loggers to measure average light intensities the birds were exposed to during the night.

“The intensities were very low - 0.2 lux. That's just one-thirtieth of the light emanating from a typical street lamp,” said scientist Jesko Partecke.

The result: the birds' testosterone levels rose and their testes matured earlier in the year than among birds that grew up in dark forests. The city birds also begin to sing and to moult earlier.

The effect was confirmed by catching forest birds and exposing them to artificial light of 0.3 lux at night for 10 months.

The scientists do not yet know what causes the advanced onset of breeding.

It could be that artificial night-time light has the effect of extending the day length for the animals. Or perhaps the light makes the birds continue hunting for food at night and put the additional energy into reproduction. Or light may alter the animals' metabolism. - Sapa-dpa