The wildebeest choose Ndutu as their calving ground, because the soil of the Ndutu area is very rich in essential minerals for growing calves and lactating mothers.
But this depends entirely on the rainfall in an area which stimulates the growth of the new grass. Ndutu's soil drains quickly in the absence of rain, which leads to fine dust and dryness. Hence the migration's move south to the woodlands, where the grass is greener.