Not an event with a date
The Great Migration is the single most-sold thing in East African travel and the thing most often misunderstood at the point of booking. It is not a happening you attend. It is a continuous, year-round circuit of grazing animals moving with rainfall, and at every moment of every year it is somewhere. The question a traveller actually faces is not whether the migration will be running but which part of the Serengeti to sleep in during a particular month, and how much slack to leave.
The mechanism, the published numbers, the disagreement between them and the reasons a calendar cannot be trusted are set out in the great migration safari guide. This page covers where it sits as a feature of the park.
The published figures, and why they differ
Tanzania National Parks describes more than 1.5 million wildebeest, accompanied by hundreds of thousands of zebra and half a million gazelle as well as eland, moving in an annual pattern in search of fresh grazing and water. UNESCO describes over one million wildebeest plus hundreds of thousands of other ungulates on a roughly 1,000-kilometre annual circular trek spanning Kenya and Tanzania.
Both figures are published by serious bodies and neither is a current census. Recent counts have come in lower than the long-repeated numbers and the matter is not settled.
The ground it uses
The circuit is bigger than the national park. TANAPA defines the Serengeti–Mara ecosystem as roughly 30,000 square kilometres comprising the national park, part of the Ngorongoro Conservation Area, the Maswa, Grumeti, Ikorongo and Kijereshi game reserves, the Loliondo Game Controlled Area, and the Masai Mara National Reserve in Kenya. The herds cross all of it without reference to any boundary, including an international one.
In broad terms the herds calve on the southern plains and around Ndutu, move north and west through the western corridor and across the Grumeti, cross the Mara in the north, and return south down the eastern side through Lobo. Which of those is happening in a given week in a given year is not something any operator can guarantee.
What honest planning looks like
Three things follow, and none of them appear in the marketing.
A river crossing is a chance, not a booking. The herds arrive at a river when they arrive, mill about for days, and cross when something triggers it. Three nights in the right place in the right month gives a reasonable chance. One night gives very little.
The timing moves between years. The Serengeti straddles the boundary between Tanzania’s bimodal and unimodal rainfall zones, which is why the position of the herds in a given month shifts by weeks from one year to the next.
The park is excellent when they are elsewhere. Lion, leopard, elephant, buffalo, giraffe and hippo are resident and do not migrate, and TANAPA records more than 27 carnivore species and over 540 bird species. A Serengeti trip in a month when the herds are three hundred kilometres away is still a very strong safari and a quieter one — see game drives and wildlife safaris.
Common questions
When is the Great Migration?
Continuously. There is no off season for it; there are only different places it is. What changes month to month is which region of the ecosystem the herds occupy.
Can a river crossing be guaranteed?
No. Crossings are unpredictable in both timing and location, and the herds may wait at a river for days. Building in more nights in the right region is the only thing that improves the odds.
How many wildebeest are there?
TANAPA gives more than 1.5 million; UNESCO gives over one million. Neither is a current census and recent counts have come in lower. Both figures are widely repeated as though settled.