LONDON (Reuters) - Birds start singing in the spring because of a biological response to longer days, researchers said on Wednesday.

A crow is silhouetted against the rising sun in New Delhi April 24, 2007. REUTERS/Vijay Mathur

When birds are exposed to light for longer periods, certain brain cells trigger a series of hormonal reactions telling them to find a mating partner, which they do by singing, a team of Japanese and British researchers reported in the journal Nature.

“While we knew what area of the brain was affected by seasonal change, until now we did not know the exact mechanism involved,” said Peter Sharp, a researcher at the Roslin Institute near Edinburgh, who worked on the study.

The researchers, led by Takashi Yoshimura of the Nagoya University in Japan, scanned 38,000 genes present in brain samples taken from Japanese quails to see which of the birds’ genes were affected by varying degrees of light.

Genes in cells on the surface of the brain switched on when the birds received more light and began releasing a thyroid-stimulating hormone.

The genes activated 14 hours after dawn on the first day of sufficient length, the researchers said.

“Such knowledge would have been impossible in the past, but advances in technology enabled us to scan thousands of genes so that we could work out which ones are affected by seasonal change,” Sharp said in a telephone interview.

This hormone, previously associated with growth and metabolism, helped to stimulate the pituitary gland to secrete other hormones. In turn this caused the birds’ testes to grow, which eventually resulted in crowing to attract a mate.

The findings could also one day lead to better treatments for infertility because humans have the same cells in the same part of the brain, Sharp added.

“It is sitting there and standing there with the same characteristics as in birds,” he said. “The big question is whether these cells are involved in the reproductive system.”