Yesterday 31st July, we had a great view of the wildebeest migration at masai mara as they cross the mara river in thousands. However, as this was happening something crossed my mind; how does they determine it's time to move? Having done the serengeti and masai mara, i tend to think the vegetation on both side isn't the main reason for the migration. What do you think?

This is what we were told last year in August after seeing two major crossings in the northern Serengeti:
The wildebeests are completely dependent on the availability of grasslands to graze, and they must migrate to find food and water. They follow the rains and graze on the lush grasslands that are rich in nutrients. The wildebeests mainly migrate from the dry plains of the Serengeti to the greener pastures of the Masai Mara.
The Great Migration also plays a crucial role in the ecosystem of the Serengeti. The migrating wildebeests help in the distribution of nutrients across the grasslands.