The Great Migration is the largest overland movement of animals on earth: roughly 1.5 million wildebeest, 200,000 zebra, and 350,000 Thomson's and Grant's gazelles moving in a continuous circuit around the Serengeti-Mara ecosystem in search of fresh grass. There is no single starting point and no true beginning or end — the animals are always moving, and the "migration" is simply the name given to the annual pattern.
The calving season on the southern Serengeti plains (roughly January to March) is one of the most intense wildlife spectacles: hundreds of thousands of calves are born within a few weeks, attracting an extraordinary concentration of predators. The crossing of the Grumeti River in June and the Mara River crossings from July through October — where thousands of wildebeest plunge into crocodile-filled water in a wall of noise and panic — are the scenes most shown in wildlife documentaries. To time your visit for these events, plan for the Grumeti in June, or the northern Serengeti and Masai Mara in July to October.