Birds lose color vision in twilight

It has long been known that birds have highly developed color vision that vastly surpasses that of humans. Birds see both more colors and ultraviolet light. However, it was not known what amount of light is necessary for birds to see colors, which has limited the validity of all research on this color vision to bright sunlight only.

"Using behavioral experiments we can now demonstrate that birds lose their color vision in the twilight and show just how much light is needed for birds to be able to interpret color signals," says Olle Lind, a doctoral candidate at the Department of Cell and Organism Biology.

For humans and horses, color vision ceases to work after dusk, at light intensities roughly corresponding to bright moonlight. However, the light threshold is not the same for all vertebrates. Geckos, for instance, can see colors at night. In the experiments performed by the Lund University Vision Group, the color vision of birds stopped working at light intensities corresponding to what prevails shortly after the sun goes down. Birds need between 5 and 20 times as much light as humans to see colors. Among all the vertebrates tested thus far, birds are the first to lose their color vision in the twilight, even though they are the vertebrates that probably see colors best of all in the daylight.

With these findings it is now possible to start to draw conclusions about how birds use their color vision at dawn and dusk. The findings also direct our focus to previous research about how important color is when it comes to eggs or begging baby birds in enclosed nests. Inside enclosed nests it is dark even when the sun is bright outside.

"Against the background of our new discoveries, we should now re-evaluate earlier research about how birds perceive the color of their eggs and their young in the nest," says Olle Lind.

The research findings were recently published in Journal of Experimental Biology 2009, 212: pp. 3693-3699.