This wildlife phenomenon, which may be seen by safari-goers visiting these parks, is called 'the migration' or 'the great migration'.

Season summary

A million wildebeest
The herds number between 1 and 1.5 million wildebeest and a few hundred thousand zebras. The herds vary in size from year to year, depending on rainfall and availability of grazing, and sometimes also on disease.

Seasonal wildebeest and zebra migrations do occur also in other parts of East Africa, although not involving near as many animals. Tarangire National Park in Tanzania, for example, receives large numbers of these animals during the dry season from June to October. Other species migrate, too, such as the gazelles in the Serengeti-Masai Mara ecosystem, but not as far and as the wildebeest and zebras.

River crossings
You may have seen nature documentaries where wildebeest and zebras in thousands cross rivers, challenging currents and waiting crocodiles. These documentaries were shot in Masai Mara or Serengeti, where the migration has to cross the Grumeti River and the Mara River to reach the pastures it is heading for.

The documentaries are shot in areas where you can go, too. The migration usually crosses the Grumeti River in the Western corridor in Serengeti during June, the Mara River in Masai Mara during August to October, and the Grumeti River in northern Serengeti during November to December. The timetable isn't fully predictable, though, as the movements of the migration are determined by rainfall and the availability of grazing and drinking water. If the normal seasonal pattern of rainfall is disrupted, the migration may not behave as it usually does.

You also need some luck and timing to see river crossings, even when there are a lot of wildebeest and zebras around. Crossings are not continuous, but rather intermittent bursts, after herds have gathered on the riverbanks and hesitated for hours or even days. Once they start to cross, they all want to get to the other side as quickly as possible.

Not one single herd
The migration rarely appears as one single big herd, but is normally divided into many smaller herds. These may be compact at times, but may also be quite scattered in the landscape. Sometimes, it may be difficult to find the wildebeest and zebras at all, as they may be moving through areas where there are few or no roads.

All in all, you can't book the migration. It is always on the move, and reacts to weather and rainfall. The best you can do is going to an area where the migration usually is seen during that time of the year, and then, if the herds are not present, be prepared to improvise, for example by doing long day-tours to other parts of the parks. At worst, you may see just a glimpse of the wildebeest and zebras. At best, you may see them in hundreds of thousands.

Predators
Crocodiles that live in the rivers where the migrations passes each year benefit a lot from this superabundance of food. These crocodiles are known to be big. Other predators may also benefit from the turbulent river crossing, where it's possible to find exhausted, injured or less observant prey.

Some prides or groups of lions follow the migration, to have a good supply of food all year round. Stationary prides may only benefit from the herds during some months every year.

Hyaenas, cheetahs and leopards may also prey on the migrating herds, and during the calving and foaling seasons in January and February, which the herds spend in southern Serengeti, also jackals and large eagles may join in.

When to see the migration
If you want to see the migration, you should choose the time of year and the destination (Serengeti or Masai Mara) accordingly. The map below shows where the migration is normally staying throughout the year.

The map shows a general pattern, from which the herds may deviate because of rainfall, availability of quality grass etc. It's not possible to predict the exact whereabouts of the migration beforehand, for example when you book your safari. All you can say is where it is usually found.

In August to October, the migration is usually in Masai Mara in Kenya, and in December to June, it's usually in Serengeti in Tanzania. In July and November, it's usually on the move between these parks, and its whereabouts are even harder to predict.

The main factors that control the migration pattern are the onsets of the rainy seasons. The long rains in April to May initiate the move from southern Serengeti towards Masai Mara, while the short rains in November initiate the move from Masai Mara to Serengeti.

More about where to go
More about safari seasons

Going at the "wrong" time of the year
If you go to Masai Mara or Serengeti during the "wrong" time of the year, i.e. when the migration is normally not present in the park, you will probably not see more than few straggling wildebeest and zebras, as the big herds will be elsewhere.

This doesn't mean that you will have a poor safari, it just means that you won't see the migration. Both parks have a rich mammal wildlife, and most species don't migrate.