What Is the Great Migration? Wildebeest of the Serengeti

Which animals take part?

The main travelers are western white-bearded wildebeest, a subspecies of the blue wildebeest. They are joined by other grazers:

Zebras often lead the way into new areas. They eat the taller, coarser grass and leave the short, fresh shoots that wildebeest prefer. Gazelles follow, nibbling the lowest growth. Because each species eats a different part of the grass, they can share the same plains. Thomson’s gazelles move a shorter distance than the wildebeest and do not follow the full loop.

Why do the wildebeest migrate?

The migration is driven by rain. The Serengeti has a strong pattern of wet and dry seasons, and rainfall is lowest in the southeast and highest in the northwest.

During the wet season, from about November to May, the herds gather on the short-grass plains of the southern Serengeti and the neighboring Ngorongoro Conservation Area. Grass on these volcanic soils is especially rich in minerals such as phosphorus and nitrogen, which nursing mothers need. When those plains dry out, the herds move north and west toward areas with more reliable rain and water, ending in the north of the Serengeti and Kenya’s Maasai Mara, where the Mara River flows all year.

Research comparing different models found that this mix of rainfall and grass quality explains the route better than either alone. The herds have no single leader; individuals respond to rain, grass and each other, and the whole mass moves like a huge, slow-flowing river.

The yearly cycle, month by month

The timing changes from year to year, but the general pattern looks like this:

• December to March: southern plains. Herds spread across the short grass around Ndutu and the southern Serengeti. Calving peaks in February, when roughly half a million calves are born within a few weeks. See When do wildebeest give birth?.
• April to June: heading north and west. As the long rains end, the herds move through the central Serengeti into the Western Corridor near the Grumeti River. The rut, or mating season, takes place on the move, mostly in May and June.
• July to October: the north and the Maasai Mara. The herds reach the northern Serengeti and cross into Kenya. This is the season of the famous Mara River crossings.
• November: returning south. With the short rains, the herds drift back south to the plains, and the cycle starts again.

The river crossings

The most dramatic part of the migration is the crossing of the Grumeti and Mara rivers. Thousands of wildebeest gather on the bank, sometimes for hours or days, until a few animals plunge in and the rest follow in a rush. Steep banks, strong currents and crowding make crossings dangerous.

Nile crocodiles wait in the rivers and catch some animals, but drowning kills far more. A 2017 study estimated that more than 6,000 wildebeest drown in the Mara River in an average year, leaving over 1,000 tonnes of carcasses. These feed crocodiles, vultures, fish and insects, and the slowly dissolving bones release nutrients into the river for years.

Predators along the way

The migration supports one of the densest concentrations of large predators in Africa. Lions and spotted hyenas take the most wildebeest, while cheetahs, leopards and African wild dogs hunt calves and gazelles. Resident predators cannot follow the herds far because they must stay near their dens and territories, so many lions and hyenas face lean months when the herds are elsewhere.

Overall, predators kill only a fraction of the wildebeest each year. Hunger, disease, exhaustion and drowning also take a large toll, especially in drought years. Figures of about 250,000 deaths a year are often quoted, but these are rough estimates.

How big is the migration, and is it growing?

In the early 1960s, the Serengeti held only around 250,000 wildebeest. Their numbers had been held down by rinderpest, a viral disease spread from cattle. When rinderpest was eliminated through cattle vaccination, the herds grew rapidly and reached well over a million by the late 1970s. Since then, aerial counts have generally found between about 1.2 and 1.5 million.

The Serengeti migration is one of the largest movements of large land mammals on Earth, though not the longest. Caribou in the Arctic travel much farther; see longest animal migrations. A 2023 aerial survey in South Sudan also counted around 6 million migrating antelopes, mostly white-eared kob and tiang, suggesting that migration may be larger in total numbers.

Threats to the Great Migration

The migration depends on space and water. Threats include:

• Roads and fences. Proposals for a highway across the northern Serengeti have raised concern that traffic and fencing could block the herds’ path.
• Farming and settlements around the edges of the ecosystem, which take over grazing land.
• Changes to the Mara River. Deforestation in the Mau Forest, where the river begins, and water extraction upstream reduce dry-season flow.
• Illegal hunting for bushmeat.

Elsewhere in Africa, many wildebeest migrations have already been cut off by fences and farms. Protecting the Serengeti-Mara system as one connected landscape, across the border between Tanzania and Kenya, is key to keeping this spectacle alive.