Tanzania and loops clockwise through the Serengeti National Park and north towards the Masai Mara reserve in Kenya. This migration is naturally caused, by the availability of grazing. The initial phase lasts from about January to March, when the calving season begins – a time when there is plenty of rain-ripened grass available for the 260,000 zebras that precede 1.7 million wildebeest and the following hundreds of thousands of other plains game, including around 470,000 gazelles. During February, the wildebeest are on the short grass plains of the southeast part of the ecosystem, grazing and giving birth to approximately 500,000 calves in 2 to 3 weeks. Few calves are born ahead of time and of these, hardly any survive, largely because very young calves are more noticeable to predators when mixed with older calves from the previous year. As the rains end in May, the animals start moving northwest into the areas around the Grumeti River, where they typically remain until late June. The crossings of the Grumeti and Mara rivers beginning in July are a popular safari attraction because crocodiles are lying in wait. The herds arrive in Kenya in late July / August, where they stay for the rest of the dry season, except that the Thomson's and Grant's gazelles move only east/west. In early November, with the start of the short rains, the migration starts moving south again, to the short grass plains of the southeast, usually arriving in December in plenty of time for calving in February. About 250,000 wildebeest die during the journey from Tanzania to the Maasai Mara National Reserve in southwestern Kenya, a total of . Death is usually from thirst, hunger, exhaustion, or predation, including by big cats. Ecology The Serengeti has some of East Africa's finest game areas. Besides being known for the great migration, the Serengeti is also famous for its abundant large predators. The ecosystem is home to over 3,000 Lions, 1,000 African leopards, and 7,700 to 8,700 spotted hyenas (Crocuta crocuta). The East African cheetah are also present in Serengeti. African wild dogs are relatively scarce in much of the Serengeti. This is particularly true in places such as Serengeti National Park (where they became extinct in 1992), in which lions and spotted hyenas, predators that steal wild dog kills and are a direct cause of wild dog mortality, are abundant. The Serengeti is also home to a