One of five Norfolk cuckoos fitted with tracking devices to monitor their migration patterns is believed to have died.

Tiny 5g back-pack-like devices were fitted to the birds by a Thetford-based conservation group last year which followed their progress as they headed from breeding grounds in Norfolk, south for the winter.

The British Trust for Ornithology is now 'almost certain' one of the cuckoos, Clement, has died.

His last transmission was received on February 25 and, although other tagged birds have gone missing only to reappear at a later date, temperature data received via satellite suggests he is no longer alive.

Director of the BTO, Andy Clements, who is the bird's namesake, said: '[It is] sad news of Clement's demise, but already we have gathered extraordinary data about cuckoo migration thanks to Clement, and all his sponsors.

'Let's hope his colleagues all make it back to the UK safely.'

Clement, the most sponsored of all the birds, was the first British cuckoo known to take a western migration route, crossing from Spain instead of from Italy.

From North Africa he made his way to Senegal, again the first British cuckoo to be recorded there, before joining the other four tagged cuckoos in the Congo rainforest.

According to the latest data from the tags, of the four remaining birds – Lyster, Kasper, Martin and Chris – who were all caught within kilometres of one another, Chris and Martin were in the Ivory Coast, while Lyster and Casper were close to one another on the north-west shore of Lake Volta, Ghana.

The red-listed species has seen numbers halve in recent years, but experts have very little information on what happens to them once they leave the UK, with even their basic migration routes something of a mystery.

BTO experts said it was previously thought all cuckoos left for North Africa or Europe direct from their southern hemisphere wintering locations.

The new information however will allow them to assess what determines number of cuckoos making it back to Britain each spring and why they arrive early or late.

The project is funded by the BBC Wildlife Fund, Essex and Suffolk Water, BTO supporters and individual sponsors. The BTO is also hoping to tag cuckoos in Scotland, Wales and small number of females in spring this year.

The birds' progress can be followed on their blogs at the BTO website, www.bto.org.