A light solution to amorous birds

Oxford University researchers have discovered how birds sense the lengthening days of early Spring and time when they breed, solving a 70-year mystery.

Love in the air? No, just light in the hypothalamus – bird brains are sensitive to light changes that can trigger spring-time reproduction

Light receptors deep in the brains of birds detect the changing day length and trigger the seasonal development of the reproductive system. If all goes well chicks hatch six to eight weeks later.

The research, published in Current Biology, has pinpointed the identity of the light receptors in chicken and Japanese quail in a deep part of the brain called the hypothalamus.

“When you hear birds singing in the Springtime, it’s a light-sensitive molecule deep in their brain that’s triggered this reproductive event,” said Professor Russell Foster of the Nuffield Laboratory of Ophthalmology at the University of Oxford, who led the work. “By timing their mating to the changing seasons, birds can make sure that there will be enough food around for their chicks.”

Exactly how birds sense the arrival of Spring has been a long-standing problem. In the 1930s, it was shown that birds surprisingly don’t use their eyes to measure the increasing number of hours of sunlight. Instead a deep part of their brain registers day length. This is possible because bird skulls and brain tissue let a lot of light through so that a significant amount of light still reaches deep parts of the brain.

“Eyes can respond very well to light in fractions of a second, which is crucial for vision. But detecting dawn and dusk requires measuring changing levels of light over long periods of time, a different thing altogether. This is what the light receptors do in a deep part of the bird’s brain called the hypothalamus,” says Professor Foster.

“We have shown that chickens have a pigment called VA opsin, that it reacts to light, and that it is around in exactly the part of the bird’s brain that we know responds to day length,” says Dr Stephanie Halford, first author on the study. “This evidence is as strong as we can get that we’ve pinpointed the light receptors that tune birds’ responses to the changing seasons.”