Great Wildebeest Migration

Great Wildebeest Migration

by Jim Heck, June, 2008

The greatest wildlife spectacle on earth is the migration of about 1½ million white-bearded gnu (wildebeest) through the Serengeti/Mara/Ngorongoro ecosystem. Wildebeest are joined by another half million zebra and gazelle.

The easiest time to see the migration is when it’s most concentrated, December-April on the southern grassland plains of the Serengeti and NCA. At no other time, anywhere during the year, are the animals as concentrated. The most dramatic times — including river crossings — are in May-July, but this period requires a lot of luck and usually more time for the effort. A large portion of the herds reaches the Mara by August and lingers there through October. The herds are usually most dispersed July-November.

Weather significantly effects these patterns.


Nothing in the wild is ever guaranteed. But during more than a half century of tourism to East Africa, few seekers of the “Great Migration” have been disappointed if they did their homework and looked in the right places at the right time of the year. This is because the migration is very predictable, as predictable as long-term weather has been. The advent of global warming may be set to confuse things a bit, but so far the effect has not been substantial.

The migration follows the new growth of nutrient grasses, which is governed by rain. The wilde need large amounts of grass to eat. Where and when the grass grows is normally easy to predict, presuming the weather is normal.

The best nutrient grasses grow on sunlit plains after it rains. There are three choice areas for this in the massive 7,000 sq. mile Serengeti/Mara/NCA ecosystem. They are (a) the southern third of the Serengeti, widely referred to as the “southern grassland plains”; (b) sections of the Serengeti’s western corridor just above and below the Grumeti River; and (c) Kenya’s Maasai Mara.

The historical rain data is very misunderstood. There is a huge myth about East African rains that confuses the issue. Parts of Kenya and smaller parts of eastern Tanzania have two rainy seasons: the Long Rains (March-May) and the Short Rains (end of the year). Because these were the areas settled by colonial farmers, this weather dynamic was presumed very incorrectly to be true of all of East Africa. East Africa lies astride the equator, an area of one of the most complex weather systems on earth. Tiny microclimates in relatively small areas differ wildly from one another.

For the most part, the wildebeest migration area does not have two rainy seasons. It has a single rainy season from the end of the year through May, and the rest of the year is dry. (The Mara is a small but important exception to this very true, general rule. It can rain in the Mara, at least a small bit, almost every day of the year except in October.) The rain begins in the south and recedes north towards Lake Victoria. And the wilde follow it.

We generally think of the beginning of the migration starting at the end of the year, when all the wilde are gathered on the southern grassland plains. The rain has just begun. The grasses that emerge after the long half-year dry season of dust and debris are the most nutrient of the year. The herds stay here until the rains end sometime in May or June, eating the nutrient grasses and calving early enough that the young wilde can grow strong before they have to move north. This is the best time to experience the grandeur of the migration, because it is the only time during the year that the largest congregation of animals occur in a single area.

During this time large sections of the herds will move to and fro, usually east and west, traveling in migratory files and in migratory behavior from one area of good grass to the next. They aren’t really migrating, but moving around an area that is about 1500 sq. miles, because this is a large enough area that parts become dry and grassless while other parts get heavy rains and blossom with grass. In the equatorial regions weather doesn’t move like it does in the temperate zones. There aren’t fronts that carry massive storms across big areas. Massive thunderstorms do occur and they are impressive, but they simply form, drop their rain, and then dissipate. This can lead to very spotty areas that are quite wet next to near drought areas. The wilde have to continually navigate these areas on the plains.

But come May, the rains begin to end in the south and recede relatively quickly towards Lake Victoria to the north. The veld everywhere dries up and the wilde quickly consume all the last grasses. The young are normally about 4 months old and ready to undertake a frantic movement north.

Almost suddenly, the herds start to run north. Unlike their less focused to-and-fro on the southern plains, now they form large, thick lines, 3 or 4 abreast, and run crazily north, sometimes for hours without stopping. This is the time that wildlife films are made, because when the herds reach rivers or lakes, they often stampede drowning each other. This is the time crocodiles feast. Mothers are separated from their calves and turn back, and sometimes in a river or lake there will be several files going north and several files going south at the same time!

But seeing this is much more difficult than seeing the congregation of animals on the southern plains earlier in the year. This is because it’s hard to predict in the time most visitors give to a vacation, exactly when the rains will stop. Moreover, the wilde run fast and furious at this time, and figuring out when and where the tight, concentrated migratory files are going to be, is not easy. This is a huge area they are running into, 3 to 4 times bigger than the southern grassland plains. So unless you have a month and unlimited resources to follow the herds, it’s a poorer bet trying to see the migration at this time of the year, rather than earlier or later in the year.

As the herds move north, they generally split into several or more large groups. These partial herds may move in completely different directions. The largest tends to move into the Serengeti’s western corridor by May and June, following the rain trends. This doesn’t last very long, though — maybe a month at most. The rains continue to diminish towards Lake Victoria, moving outside the park. When it dries in the western corridor, the herds tend to stop following the northwest movement of the rain, because that would be through heavily human populated areas. So they turn back on themselves slightly, moving east through the corridor and the adjacent Grumeti Reserve. The frantic racing that began a few months previously tempers down a little, but the herds still move quickly. If they could they would move north, but that too is an area with high human habitation. So they continue moving east until they reach the left edge of the northern Serengeti. As soon as the borders between human populated areas and protected wilderness allow, they turn north, and usually it’s raining in this direction. Lake Victoria stretches out a bit towards this area, and the rain dynamic is sustained. This area — just north of where the western corridor meets the northern finger of the Serengeti — is mostly thick woodlands. This makes game viewing difficult if not impossible, and does not provide large areas for grazing grass, so the wilde generally move quickly through this area. About 25k before the border with Kenya, the Serengeti’s top-most ecosystem turns back into savanna grasslands which continue into Kenya’s Mara and is the dominant feature of the Mara. So this is an excellent area for migration viewing for a late season migration experience in Tanzania. But it usually lasts for a very short time in July.

Sometime in late June through mid-August, a huge portion of the herd, maybe a million, reaches Kenya’s Maasai Mara. This is the furthest north and rainiest protected grassland plain in East Africa. That’s why they’re here. It rains in the Mara practically every day in the year, except in October and the first part of November.

The wilde remain in the Mara, unable to follow the rains right into Lake Victoria because of the human populated areas, there. So they remain in these furthest northern areas eating whatever grass they can find, grass which until October is continually regrowing with the new rains.

The Mara has a number of relatively big rivers. So there are many opportunities for “river crossings” as the wilde — unable to constrain their migratory urges — race back and forth all around the Mara and over and back over its several big rivers. The scene may not be as expansive or dramatic as the May/June filings in the south, but it is often easier to plan on seeing, because it lasts for a much longer time (August-October) in the much smaller area of the Mara.

So what happens when the rain stops, everywhere? That is truly the marvel of the migration. Without a cloud or smell of water to be guided by, the wilde know where to go. They dare not run, because there’s no food now to fuel them, and half are pregnant. So they return — often in very small packs or even individually — usually walking slowly southwards through the now parched and dismal veld of November and early December. And at the end of this laborious trek they arrive, again, upon the great southern grassland plains … just as the new rains start anew.

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