Extraordinary rendezvous of the migrating cuckoos

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 the five cuckoos caught in East Anglia last 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 miles 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 our best-known sign of spring, as although the destinations of a few British migrants are known – our swallows fly to South Africa, and our 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 here, problems on their African wintering grounds, or problems on their 3,000-mile migratory journeys. No one knows.

Just as remarkable a discovery has been the fact that while three of the birds, Chris, Martin and Kasper, flew down through Italy and straight across the Sahara desert, the other two, Clement and Lyster, went to Spain and down the Atlantic edge of the continent, more than 1,000 miles 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, Lyster are all wintering on the Téké plateau north of the Congo capital, Brazzaville, a sparsely-inhabited area of grasslands with forests along the rivers. At the moment, Clement is about 50 miles north-east of Lyster – when their satellite tags were fitted in the UK, these two birds were 48 miles apart – and about 60 miles north-west of Martin. Kasper is at present on the Téké plateau's southern end, about 30 miles north of Brazzaville, while Chris is currently farthest to the north-west, just over the border in the Democratic Republic of the Congo.

Chris now 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 BTO's Dr Chris Hewson.