Big-eyed birds sing early songs: dawn chorus explained. (This Week).

As dawn breaks on a misty Welsh morning, the earliest birds to break into song are likely to include European robins, followed by blackbirds and song thrushes and then a plethora of other species as sunlight crowns the horizon. The last to join the chores, such as chaffinches and blue tits, may not chime in until 100 minutes after the first crooners began.

This pattern is repeated worldwide, and ornithologists have often pondered what determines when a particular species begins its morning singing. Now, scientists say that they've found the explanation: The larger a bird's eyes, the earlier it starts to sing.

The staggering of avian choruses was first documented 70 years ago but has remained unexplained. Now, researchers have revisited an idea first proposed in the 1960s but never tested. It's that visual acuity--determined by eye size--governs when birds start to sing.

Robert J. Thomas at the University of Bristol in England and his coworkers recorded the time and intensity of light at the moment when each species broke into song at seven European woodland sites. The researchers then temporarily captured individuals from 57 of these species and used calipers to take rough measurements of eye-surface diameter.

In the April 22 Proceedings of the Royal Society B, the researchers report a strong relationship between the light intensity at which birds start to sing and eye size. "Birds with big eyes do start to sing earlier," says Thomas. The relationship remained even after the scientists used statistics to discount other factors such as body size.

"Even though the result is expected, it's an excellent example of the way in which visual capabilities affect communications behavior," says Sandra Vehrencamp of Cornell University.

Birds singing at dawn and dusk attract mates and defend their territories. However, this musical interlude can be hazardous since it can draw the attention of predators, such as tawny owls, to a bird's location. The singing also makes it difficult for a bird to hear danger coming, says Thomas. It therefore makes sense, he adds, that singing not begin until the bird can be on the lookout for trouble.