As the season of dawn chorus intensifies to its crescendo in May, a group of scientists has been investigating why some birds rise earlier than others, writes Caroline Davis.

In a typical Welsh woodland, start times can differ by as much as 100 minutes. The common redstart begins to sing before the light of dawn is detectable to humans. As light breaks, robins, blackbirds and song thrushes join the medley. By the time chaffinches and blue tits sing, the redstart is on his final bars.

It has been suggested that birds delay their song until they can see well enough to carry out song-related behaviours such as courtship. And since singing may give away its location, the bird needs to ensure it is light enough to detect predators.

A team of scientists and volunteers, led by Rob Thomas from Bristol University's Centre for Behavioural Biology, looked at 38 species of the passerine order of birds, which includes jays, blackbirds, finches and sparrows.

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They visited seven woodland sites in the United Kingdom, Portugal and Switzerland, recording the start of each species' song. This time was calculated relative to the "civil dawn twilight", when the sun is six degrees below the horizon. They also measured the maximum diameter of the birds' pupils and light intensity.

Birds with larger eyes start singing earlier than those with smaller eyes. They also found that smaller birds begin singing at lower light intensities than larger birds with equivalent eye size. The team said this may be because of the large overnight loss in body mass - up to 15 per cent in small species. An earlier start would give more time to replenish fuel stocks.

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"The dawn chorus is linked to when it's best to start feeding," Dr Thomas said.

"The research fits with wider studies on how birds make decisions," he added.

The research appeared in the Proceedings B of the Royal Society.