Scientists from a Thetford-based conservation group have attached tiny satellite trackers to five cuckoos to find try to find out why their numbers are declining so rapidly.

The UK population is thought to have dropped by about 65pc – or almost two-thirds – over the past 25 years with fewer and fewer birds migrating back to this country each summer.

Researchers at the British Trust for Ornithology hope by tracking their movements – particularly those outside of Britain – it will help them understand why so few make it back.

Graham Appleton, from the BTO, said: 'The essential thing we need to know is where they actually go to when they are not in this country. We don't know where they spend their winter.

'We also need to know where they stop off along that route. We all have to re-fuel at service stations and knowing about those service stations will help us understand the cuckoos more.'

Five birds have been caught in Norfolk and fitted with mini backpack-style satellite trackers which weigh about five grammes.

The device works for 10 hours each day before going into 'sleep' mode while a solar panel recharges its batteries.

The trackers were fitted between May 19 and June 1 and already the scientists at the BTO are gathering some interesting data.

Mr Appleton said: 'We weren't expecting them to move quite so much or so quickly. One of them is already in France and another is in Sussex, which is outstanding.

'It looks like every day things are going to change and every day is going to be very exciting.'

Each tracker costs about �2,500 and it costs an additional �50 a month to hire the satellite time. BTO has enough money to keep an eye on the five cuckoos – which are all male – already fitted with the devices for the next year but hopes to find additional funds to extend the project.

Mr Appleton said he also hoped to include some female cuckoos as part of the scheme.

The cuckoos given the tags so far have been caught in four different sites in Norfolk.

Clement, who was caught at the BTO's Nunnery Lakes reserve, Thetford, is now in France.

Martin, who was first found at Breydon Water, near Great Yarmouth, and Lyster, who was caught on the outskirts of Martham, have spent the last week flying round Norfolk,

Kasper, also from Martham, has moved the least, remaining in the Horsey and Hickling Broad areas,

While Chris, named after Chris Packham and found in Thetford Forest, stayed where he was for a few days before a sudden dash to East Sussex at the weekend.

The BTO website has a map keeping track of the birds' movements and a blog from each of the cuckoos about their travels: www.bto.org.

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