The migration follows a vague pattern year by year which tends to be influenced by rainfall and food availability. The herds, accompanied by several hundred thousand zebras, congregate on the short grass plains in the south just after Christmas to give birth and then make their way north via the west of the ecosystem towards Lake Victoria. During the journey they may travel 40 miles in one day in a particular direction and then do the same in the opposite direction the following day. This is often the case during the rainy season when thunderstorms may be very localised but also quite heavy.