In the bleak mid-winter, a group of travellers from Britain is keeping warm and well-fed in one of the world's least-known regions - the Congo rainforest.

They are five cuckoos caught in East Anglia last northern summer and fitted with miniature satellite transmitters, enabling their migratory journeys back to Africa to be followed by scientists for the first time.

The five birds, named by researchers as Clement, Martin, Lyster, Kasper and Chris, have been separated on occasions by thousands of kilometres during their great odysseys. But to the amazement of researchers from the British Trust for Ornithology, they have now all recongregated in the same, little-known part of the continent - the Congo River basin.

It is an astonishing revelation about the birds whose two-note call is Britain's best-known sign of spring, as although the destinations of a few British migrants are known - UK swallows fly to South Africa, and nightingales fly to west African countries such as Guinea - no one had any idea where British cuckoos spend their winter.

The knowledge is vital in the effort to halt the precipitous decline of the birds, which dropped in numbers in Britain by 65 per cent between 1984 and 2009. They may be vanishing because of problems in Britain, problems on their African wintering grounds, or problems on their 4830km migratory journeys. No one knows.

Just as remarkable a discovery has been the fact that while Chris, Martin and Kasper flew through Italy and straight across the Sahara desert, Clement and Lyster went to Spain and down the Atlantic edge of the continent, more than 1600km to the west.

Yet they are all now relatively close, with three about as close to each other as they were when they were caught in Norfolk and Suffolk in May and June.

Clement, Martin and Lyster are wintering on the Teke plateau north of the Congo capital, Brazzaville, a sparsely inhabited area of grasslands with forests along the rivers. Clement is about 80km northeast of Lyster - when their tags were fitted they were 77km apart - and about 95km northwest of Martin. Kasper is on the Teke plateau's southern end, about 48km north of Brazzaville, while Chris is farthest to the northwest, just over the border in the Democratic Republic of the Congo.

Chris occupies the wildest region, the Western Congolian swamp forests, an area characterised by impenetrable marshes and forest flooded for several months of the year.

"We have gained completely new knowledge which will be invaluable in understanding the migratory cycle and planning ways to help the declining populations of these amazing birds," said the trust's Dr Chris Hewson.