Warmth leads to earlier birds laying earlier

A WARMER climate is causing Britain's birds to lay eggs significantly earlier in the season, according to the most detailed study yet of nesting habits.

Data collected over the past 57 years by the British Trust for Ornithology has identified a link between rising temperatures, especially in spring, and earlier breeding times for 31 of 36 bird species.

Dr Humphrey Crick, senior ecologist at the Trust, said the study analysed nearly 93,000 records gathered between 1939 and 1995 and found that temperature is the crucial factor determining when a bird lays its first clutch of eggs. "We looked at the country as a whole and found evidence to suggest that changes in the climate are having an impact. It shows you can explain the changes purely in terms of temperature," he said.

The scientists found that egg-laying occurred later in the season in the Sixties and Seventies but has become progressively earlier as spring temperatures have increased during the Eighties and Nineties. "Trends towards earlier laying in recent years are due to increased numbers of early nests and were never due solely to fewer records of late nests," the scientists say in the journal Nature.

Nesting data on species such as the wren, blackcap, willow warbler and long-tailed tit, which are all widespread in Britain, showed that they were matched to changes in temperature in central England, the scientists found. Dr Crick said that early nesting may give chicks a better start but there could be disadvantages if they are born out of step with their major sources of food.

"We should be worried that there is such a marked effect. We don't know what the consequences might be," he said.