The number of birds migrating to Britain from Africa each spring is falling sharply, according to an alarming new study.

Well-loved migrants, such as the spotted flycatcher and garden warbler, are increasingly failing to turn up in parts of Britain where they were once common visitors.

Even the cuckoo, whose distinctive call heralds the start of the spring, has suffered a 60 per cent decline, the Royal Society for the Protection of Birds found.

Wildlife experts are baffled by the decline but warn that the annual system of migration form Sub Saharan Africa to Europe could be under threat. The scale of the problem has stunned ornithologists.

Since the late 1960s, the number of willow warblers – our most common summer visitor – has fallen by 70 per cent.

Turtle doves have experienced an 82 per cent decline over the same period, while cuckoos are down 59 per cent and spotted flycatchers are down by 84 per cent.

Dr David Gibbons, of the RSPB, said: "We are seeing some very marked declines in migrants, which are giving us real cause for concern.