The great migration has started early this year. Tom Watts sizes up one of Africa's wild wonders...


The number of major river crossings which the herd negotiates during the course of its migration. The first is the Grumeti River in Tanzania, where the wildebeest congregate on the south bank in order to build up a high density before crossing – the aim is to avoid the lurking crocodiles. Those that survive must then cross the Mara River, which straddles the Tanzania-Kenya border, where crossings form bottlenecks in which thousands perish by drowning or trampling. Several riverside camps offer dramatic vistas, such as the Grumeti Serengeti Tented Camp. Prices start at £426 per person per night, full board.


The distance, in kilometres, covered during the migration in its 12-month cycle. In November and December, rains bring the herds south to the grass plains of the Serengeti and Ngorongoro Conservation Area, where they feed until April before heading north again to the Masai Mara. Acacia Africa has an overland 13-day Kenya and Tanzania Adventure from £689 per person, with camping, most meals and services of a local guide; flights not included.


The number of million years ago that life in the form of the hominid (great ape) species is thought to have inhabited Tanzania's Ngorongoro Crater, the vast volcanic caldera that makes up part of the Ngorongoro Conservation Area, through which the herd passes. The Ngorongoro Crater Lodge comprises three luxurious camps with crater views, including a treehouse camp, where prices start at US$685 (£457) per person per night, full board.


The estimated number of mammals that undertake the yearly mass migration. Exodus has an eight-day Classic Kenya Safari from £1,649 per person that takes in the Masai Mara.


Area in square kilometres of Liuwa Plains in the far west of Zambia, the site of central Africa's second largest migration behind the Serengeti-Mara pilgrimage. This lesser-known park, the size of Sussex, witnesses an annual influx of around 37,000 wildebeest over several months. Stay at Matamanene Camp in Liuwa Plains to witness the unique natural phenomenon. Prices start at £375 per person per night full board.


The average number of wildebeest calves born, usually during three to four weeks at the start of the year. Ndutu Safari Lodge claims to be the only lodge from which you can witness the migration during calving season. In the south-eastern Serengeti ecosystem, the lodge's 34 cottages all have verandas with lake views. Prices start at £299 per person per night, full board.


The flying time, in hours, from London to the Kenyan capital, Nairobi. From here, travel 270km south west to the plains of the Masai Mara to witness the tail end of the great passage. Kenya Airways, Virgin Atlantic and BA compete from Heathrow non-stop to Nairobi, from where Air Kenya has connections to the Masai Mara.


The starting price, in pounds per person, of the eight-night Great Migration Safari with Somak Holidays, which follows the route from Tanzania to Kenya. Includes flights, full board and activities.