The timing and extent of the migration [V 007 / are dependent on rainfall] and subsequent food availability which can vary from year to year. However, the herds are confined to the same physical limits today as they were a million years ago: the Isuria [G 17 / Escarpment] to the northwest, the forested hills of Loita and Gol in the east, the Eyasi Escarpment and Crater Highlands in the south and southeast, respectively, and Lake Victoria to the west.
Within these physical limits, the migrating wildebeest herds instinctively go south and east when it rains and west and north when the rains end. During the rainy season, typically January to May, the wildebeest herds cover the 3,500 square miles of the central and southern short grass plains, moving to areas of rainfall to take advantage of the new growth it brings. They give birth between late January and mid-March while on the plains.
When the rains end in May or June, the herds move to areas that have green vegetation and permanent sources of water. [G 42 / Rut] occurs while the herds are moving off the plains. Some herds move to the Western Corridor before migrating to the woodlands later in the dry season. Other herds move north to the woodlands from the central plains.
