Cuckoo Migration is one of the great mysteries and to date there is still no agreement on how Cuckoos find their way to the Congo for the winter starting from different locations in Europe.
The team at Copenhagen University under Prof Kasper Thorup have been able to tag fledgling cuckoos to follow their migration. The link shows their results. The British trust for Ornithology (BTO) has been forbidden from tagging young birds as these are deemed too small to carry the extra load of a transmitter. But you will see their results on older birds which seem similar to the fledglings.