Tanzania is part of East Africa and is known for large herds of ungulates which include wildebeest, zebra, Thompsons gazelles, elands and topi. The largest hers that live in Tanzania is the Serengeti- Maasai Mara Plains Ecosystem with more than two million ungulates The species that migrate includes Thompson Gazelles (Eudorcas thomsoni), Elands (Taurotragus oryx) and Grant’s Zebra (Equus quagga boehmi). However the Western White Bearded Wildebeest (Connochaetes, Taurinus, Mearnsi) has the biggest migration.
While the migration is extremely hard to predict as their movement consist of three things green grass, water and rainfall patterns. However there are certain times of the year that the herds do return to the same location for birthing which is in January and February and this applies to both species. After the calving the movement can change depending on the environmental factors. There are instances where massive herds of wildebeest have arrived in the Maasai Mara Plains in early June due to the fact the calving grounds didn’t provide enough grass or water and there are abnormal rains that occur in Christmas and there are already massive herds gathering on the short grasslands. 

There are herds in Kenya Maasai Mara Plains, the Loita Herd that do their calving in the Loita Plains which is north of the Maasai Mara Plains, they remain on the Loita plains until the end of May and will start migrating to the Maasai Mara Plains during the dry season. 
The female do form the large herds, especially in hundred of thousands or even ten of thousands there group size can also be determine by environmental. It’s not common for gregarious mammal species to disperse if the environment does provide them with food or water. If conditions are to dry the large herds can divide into smaller groups, it’s only after the thunderstorms and rainfall that brings the individuals back to the herd. 

Serengeti Migration Route.
Wet Season (January to May): Majority of the herds are settling on the short grasslands which includes southern Serengeti National Park, Ngorongoro Conservation Area and Loliondo Controlled Area. Majority of the females in February will start to give birth on the short grasslands. 
Transition Season, Wet to Dry(June to May): This is when the dry season commence, majority of the herds move west, passing through the western corridor. There are large herds of wildebeest, that move directly from Loliondo Controlled Area to the Maasai Mara Plains and literally avoiding the Western Corridor. The Loita Herds also start moving south west to the Maasai Mara Plains as well. 
Dry Season(July to November): Majority of the herds are ranging in the northern Serengeti to Kenya’ Maasai Mara Plains and some herds are even in Loliondo, between the border of Tanzania and Kenya. 
Dry to Wet(November to December): The herd start the migration south back towards the southern grasslands by moving through the Western Corridor and other herds moving through Lobo and Loliondo Mountains. 
Crossing Seasons: Mentioned before the wildebeest follow the rainfall and thunderstorms. Depending on how much water and food is available the river crossing can occur as early May to December. There are two major river crossings that occur along on the western route, one at the Grumeti River and one in the Mara River. This crossings occur when the herds are heading north and south. Generally the peak of the Mara River crossing season occurs in June to November, as the herds start entering their dry season range this is where animals will start crossing back and forth the river. Some will face death by either mass drowning or being killed by crocodiles. 
The Grumeti River has two crossing season in early May to late July when the herds are moving north  and one in early October to late December when the herds are moving south. Not the biggest crossing, but considered to be the second biggest.