"The Masai Mara and adjoining Loita Plains form the northermost part of the 25,000km Serengeti-Mara ecosystem. The Mara receives the highest rainfall (1000mm average, 1200mm a year at Musiara) of the entire ecosystem. Rain falls here throughout the year, with peaks usually in December, January and April.
Plenty of grass remains after the Serengeti plains to the South have dried up. This beneficient ecosystem supports a wildebeest population of at least 600,000. Together with the associated herds of 200,000 zebra and 350,000 Thomson gazelle, they form a vast assemblage of ungulates whose annual movements trough the ecosystem is known as "The Migration". The sight of hundreds of thousands of these animals moving together through the seas of grass must rank as the greatest wildlife spectacle on earth.
The wildebeest herds congregate during the wettest part of the year in the short-grass plains of the Serengeti ecosystem, where there is sweet new grass and rainwater pools. There they give birth - most of the females calving within a few week in what has become known world over as "The Rut". Early in the dry season, the pools in the short-grass plains dry up and the wildebeest stream en masse through the longer plains and o n to the Western Corridor. As their food supply diminishes, the herds move into the northen Serengeti woodlands and the Masai Mara. Zebra follow similar, but not quite identical, movements. Thomson's gazelle also migrate, but o nly as far as the edge of the woodlands. The routes taken by the herds vary from year to year but the general pattern of the migration remains the same.
Once in the woodlands the herds spread out but keep moving in response to rainfall and the availability of forage. The first wildebeest usually arrive in the Mara in June or July and most remain there until late October or early November. Slowly at first, but increasing momentum, the wildebeest leave the Mara by various routes as they follow the rains back south. The annual incursion of the great herds into the Mara is a comparatively recent phenomenon. Prior to 1969, a few wildebeest "spilled over" from the Serengeti in very dry years, but most of the wildebeest found in the Mara belonged to a completely separate population, the Loita population. The Loita wildebeest, commonly referred to as the "residents", perform seasonal movements between the Loita plains in the wet season and the Mara in the dry months. Following the tremendous increase in the Serengeti herds in the 1960s and 70s, the Mara is now dry season refuge for up to 600,000 Serengeti wildebeest as well as about 25,000 Loita animals."
(From The Official Guidebook to the Masai Mara Ecosystem, by Friends of Conservation)