In May or early June over 3 million wildebeest and zebra come to the Serengeti
Plains in search of water and fresh grass. From July to October the countless
herds congregate along the swollen Mara River, the final barrier cutting them off
from the sweet green grass of the Masaai Mara. In November, and the arrival of
the short rains, the herds begin their migration south, to the southern Serengeti.
The migration follows the long rains and is therefore seasonal, dependant on
weather conditions. Remember: The migration is controlled by nature, and of
course by weather patterns, so timings can vary month by month and year by
year.