Birds know to sing in the spring because of hormones triggered by longer days, researchers have found.
Japanese quail were observed under varying amounts of light
Teams from the Roslin Institute near Edinburgh and Nagoya University in Japan have identified how part of a bird's brain is affected by seasons.
Scientists found cells near the pituitary gland release a hormone in the spring in readiness for mating.
The bird then begins to sing more often to attract potential mating partners, the experts tell the journal Nature.
Prof Peter Sharp of the Roslin Institute said: "While we knew what area of the brain was affected by seasonal change, until now we did not know the exact mechanism involved.
"Now we have identified a key element in the process of the brain's activity when spring arrives.
"Such knowledge would have been impossible in the past but advances in technology enabled us to scan thousands of genes so we could work out which ones are affected by seasonal change."
Researchers used a genome chip - known as a microarray - to scan 28,000 genes from the Japanese quail (Coturnix japonica), which had received varying lengths of light corresponding to short and longer days.
They discovered that genes in cells on the surface of the brain were switched on when the birds received more light.
The result was that cells started to release a thyroid-stimulating hormone.
Researchers found that the hormone, which has previously been associated with growth and metabolism, indirectly stimulates the pituitary gland to secrete further hormones called gonadotrophins.
These cause the birds' testes to grow and as a result they begin to crow to attract partners.