Scientists hoping to discover why the number of cuckoos is rapidly declining by using tiny satellite trackers are already collecting vital information – just seven weeks into the project.

Researchers at the Thetford-based British Trust for Ornithology have attached small back-pack-like devices to five birds which were caught in Norfolk in the middle of May.

It is hoped by tracking their movements, it will help to explain why the UK cuckoo population has dropped by 65pc over the past 25 years.

Paul Stancliffe, a BTO ornithologist, said just a few weeks into the project they had already learned a lot thanks to four of the birds beginning their migration to Africa.

He said: 'The first, Clement, left the UK the first week of June. That surprised us a little . We weren't expecting them to start leaving until about now.

'He was followed by three more between then and the first few days of July.'

Mr Stancliffe said it provided key information about how long the birds were in the UK and how long their breeding season was.

He added: 'It's common knowledge among bird watchers that the common swift spends the shortest length of time in the UK of any migrant bird. But it now looks like the cuckoo beats that.'

Researchers had a brief scare at the end of June when Chris, a cuckoo which had travelled to the Netherlands, had stopped transmitting and it was feared he may be dead. But on July 4, his tracker started working again and he is alive and well in an area of France 40km from Strasbourg.

The four migrating birds have each followed one of two routes, with all of them appearing to be heading for a stop-over south of the Alps.

The fifth bird, Lyster, has remained in Norfolk which suggests there are still active female cuckoos in the area which he hopes to breed with.

BTO has funding to track the birds for at least the next 12 months.

To sponsor a cuckoo visit www.bto.org.uk.

victoria.leggett@archant.co.uk