The 800-kilometer trek of the immense wildebeest herd is the largest mammal migration on earth. The timing of the migration coincides with greening of nutritious grasses on the short-grass plains during the wet season. These areas are safer because predators can be easily spotted making it an ideal place for calving. However, the plains dry and the wildebeest are forced to move in search of greener pastures in the western corridor. The northern extension of the ecosystem has the highest rainfall, but the grasses are least nutritious. This is the dry season retreat for the wildebeest, at least until the south becomes green again. The result is a clockwise movement from the south, west, north, and back to the south.