Bird experts in Devon are baffled over the virtual disappearance of the Dartford Warbler in part of the county.
Dartford Warbler numbers increased after the 1960s
The birds had fought their way back from near extinction to be commonplace at sites such as Lympstone Common and the Venn Ottery nature reserve.
But a summer RSPB survey has failed to record a single one in east Devon, but numbers are up on Exmoor.
Wildlife wardens said it was possible that the birds suffered the effects of the cold winter.
The species is regularly monitored on the heaths, but it was surveyed by an RSPB team this spring and summer as part of a national count of the birds.
The preliminary results from the survey show a dramatic drop in the warbler population at sites including the RSPB's own Aylesbeare Common nature reserve. At some locations surveyors found none of the birds at all.
Aylesbeare warden Toby Taylor said it was "really worrying news".
Some sites in neighbouring Dorset have also seen declines. The official findings of the survey will not be ready for publication until next year.
Mr Taylor said: "It's possible that the birds have suffered the effects of the cold winter we had, although in the recent hot weather it seems like a distant memory."
During prolonged periods of cold Dartford warblers find it difficult to forage for enough food, especially if the ground is covered in snow. They mainly eat insects and spiders which die in cold weather.
Dartford warblers nearly became extinct in the UK, in the mid-20th Century, and were hit hard by the severe winter of 1962/3 when the population dropped to just 10 pairs.
The RSPB's work to restore lowland heaths in both Devon and Dorset contributed towards the recovery of the species. Numbers increased after the 1960s to nearly 1,700 pairs in 1994.
Mr Taylor said: "It is too early to be sure of the reason behind the disappearance of the birds from some of the east Devon sites, but the results of the national survey are definitely going to make interesting reading."