What is actually happening
One herd, one loop, two countries
The herds calve on the short grass plains of the southern Serengeti between January and March, move west and north through the Serengeti from April to June, cross the Mara River into Kenya from late July, spend the dry months in the Maasai Mara, and turn south again around late October.
The crossings themselves are unpredictable. Herds gather at the bank, hesitate, sometimes for a day or more, and then go, often triggered by nothing a person can see. Crocodiles wait in the channels and lion and hyena work the far bank.
This is why we push people towards three nights rather than one, and towards camps on the western side of the reserve where the main crossing points are reachable at first light.