Thirteen kinds of primate live in Nyungwe, a quarter of all the species in Africa, in a thousand square kilometres of mountain rainforest in southwest Rwanda. Cloud sits on the trees for most of the year. Chimpanzees are what most people come for, and finding them is a morning's work. You set out before dawn with a guide, push through wet undergrowth, and hear them long before you see anything: the calls come first, then a dark shape moving in the branches overhead. That is how the forest works here. It gives up what it has in pieces, and almost all of it is above your head.
The Igishigishigi trail is named for the giant tree ferns that crowd it, and it climbs gently through drifting mist and birdsong to East Africa's first canopy walkway. You edge out along a swaying suspension bridge with the treetops level with your feet, the forest floor dropping away under the planks and birds passing beneath you. The green runs unbroken on every side. Lower down, the main road through the forest is the birding place, and cars pull over at first light while the mist still hangs in the trees. More than three hundred species live here, dozens of them found only along the Albertine Rift, and the open verge gives a clearer line into the trees than the trails do. At the edge of the forest the trees stop and tea bushes climb the hillsides in neat emerald rows, with the dark wall of forest standing behind the pickers. A spring high on Mount Bigugu is counted among the most distant sources of the Nile; rain falling a few metres the other way goes to the Congo. The Rwenzori colobus are the exception to all this patience. They are black-and-white monkeys with long capes, travelling in troops of three or four hundred, the largest of any tree-dwelling monkey in Africa. When one crosses overhead it can take minutes to pass.