Concerns are growing for a satellite-tagged cuckoo who has been providing scientists with data for more than four years after he went missing in action.

Chris the cuckoo, named after TV presenter Chris Packham, is the longest-serving of the British Trust for Ornithology's (BTO) study birds, having been tagged in July 2011.

But scientists now fear the intrepid bird has gone missing over the Sahara desert.

Phil Atkinson, BTO scientist, said Chris's tag shows he has been stuck in northern Chad for ten days.

'This is very worrying as he normally crosses the desert in a day or two, stopping on the shores of Lake Chad in the south of the country and on the southern edge of the desert.

'At this stage we are very concerned about him and wait with baited breath for further signals which will tell us whether he is still alive or not,' he said.

Chris Packham, the BTO president, said: 'Chris has rewarded the BTO with its highest honour - important new reliable data - and has revolutionised the way we understand migration.

'And, if he has fallen in action then 'In some corner of a foreign desert that is for ever Cuckoo. There shall be in that gold sand a richer dust concealed'. But let's hope not!'

Click here to track Chris' progress.