Earlier birds threaten the cuckoo

Cuckoos have inhabited rural Britain for thousands of years, but climate change is taking its toll on the bird that once dominated other flying species by stealing their nests.

New figures to be released later this year by the RSPB and British Trust for Ornithology are expected to show a continuation of a marked decline that saw its numbers crash by 43 per cent from 1994 to 2004.

Global warming is said to have shattered the delicate timing that the cuckoo needs to pull off its trick of hiding its eggs in the nests of other birds. Milder winters and earlier springs are changing nesting patterns. This means that many birds have already begun breeding by the time cuckoos arrive to try to find empty spaces for their eggs.

Tim Sparks of the Centre for Ecology and Hydrology told The Independent on Sunday: "Global warming is definitely a major factor. It is quite clear that the change in arrival dates is linked to warmer weather and earlier springs."

Meadow pipits and dunnocks foster 80 per cent of cuckoo chicks, but they are now breeding up to two weeks earlier than in the past, while cuckoos are starting to arrive up to two weeks later than usual. The cuckoo is also suffering from the decline in the number of these two species; their numbers have dropped by more than a third in recent years.

Cuckoos cannot simply switch their allegiance to other birds, as their eggs are genetically programmed to mimic those of the species that they normally target. Thinly distributed around the UK, there are now thought to be just 15,000 pairs left. Recording the first call of the cuckoo each year is an age-old British institution that has become an annual fixture in newspaper letters pages.

The cuckoo's distinctive call, made by males as a way of defending their territory and attracting females, is becoming more infrequent, and recorded sightings in 2005 were down by a third on those of the previous year. The overall outlook is bleak, according to Dr Martin Fowlie from the British Trust for Ornithology. "Cuckoos are facing a real crisis and the question is whether we will still hear them in 10 to 20 years' time," he said.

The next edition of the Population Status of Birds in the UK, due in 2007, is expected to show the cuckoo added to the list of Britain's threatened birds.