In one of the world's most amazing animal spectacles, an immense herd of wildebeest (also known as gnu) approaches cautiously, hesitates and finally stampedes across the ford from Tanzania's Serengeti National Park into the Kenyan nature reserve of Masai Mara.

The same scene has been repeated every day since the beginning of August, in an annual, impressive and immutable migration of hundreds of thousands of wildebeest and zebras moving from south to north in search of pastures.

This year, however, the animal trek is not quite like all the others. It started about a month later than usual because of the tardy start of the rainy season and it is threatened by major brush fires in the north-western part of the Masai Mara.

So far, says Bernard Ngure, a researcher for the Kenya Wildlife Service, "the migration of the gnus has not been affected by these brush fires, which have only killed the odd stray animal, and which are in the western part of Mara, opposite to the zone where the migration begins.

"But the more these fires gain ground, the more they will disrupt the migration by depriving the gnus of their pastures."

He said guards and county officials had mounted a coordinated plan of attack against the fires which, although they have ravaged vast extents of grassland, have not yet menaced the many tourist camps established in the national park.

The migration of the wildebeest is one of the attractions that draws people here. They come from all over the world to watch the classic crossing of the crocodile-infested Mara river.

This year, according to travel agents' blurb, was supposed to have been the biggest animal migration for 100 years, with some tourist brochures speaking of two million animals. "Some figures are nothing more than speculation," says Ngugi Gechaga, spokesman for the National Wildlife Service.

But some tourists came away extremely disappointed that they hadn't been able to see the wildebeest after all. For some reason, the animals avoided their normal crossing point at the Mara river and instead forded the remote Sand river at the extreme southwestern point of the nature reserve.

Perhaps the zebra knows something. With their quivering nostrils and ears cocked for danger, the zebras act as guides for the wildebeest.

"The zebras are smart. They've got excellent long-range vision. The gnus see only the ground in front of them and they are idiots," says David, a tour guide.

But the zebras and wildebeest have good relations comes to sharing food resources. While the lumbering wildebeest munch on the tops of the plants, the zebras prefer the tasty shoots at the bottom. "Gnus and zebras live together without rivalry," says Bernard Ngure.

But the crocodile with its jaws locked around the head of a zebra who made the mistake of stopping for a drink shows that even long-range vision has its limits.

The annual migration of the wildebeest, with their clumsy air, their large bearded heads and their long manes, is an important attraction in Kenya, which is trying to rebuild its tourist industry in the face of terrorist threats.

On the flight here, the Air Kenya pilot warmly thanks passengers for having ignored warnings by the US and British governments to stay away from Kenya.

"The Masai Mara is no more dangerous than Central Park" in New York, the pilot says.

TERRA.WIRE