More than 200,000 zebras migrate through the Serengeti-Mara ecosystem, constituting Africa’s largest zebra migration, according to the Tanzania Wildlife Research Institute. Zebras in the greater Serengeti-Mara ecosystem migrate primarily in response to seasonal rains. During the wet season, family groups (usually a stallion plus two or three mares and their offspring) merge with others to form large herds and migrate to the southern grasslands. While on the plains, the zebra migration is mostly constrained to areas with large amounts of grass biomass. Although they occasionally foray onto the high-quality short-grass plains in the extreme southeast, their movement tends to focus on grazing the long-grass plains. As the dry season advances, the herds move northwards and westwards into the woodlands while avoiding the thicket patches, which often conceal predators. Some family groups spend the dry season in the large, grassy savanna meadows of the Western Corridor, while others head north. Other family groups migrate as far as the Masai Mara in Kenya. The migratory routes of individuals are often similar between years, with animals crossing rivers at nearly the same place. Foaling is year-round, although there tends to be a small peak in foaling rates associated with the rainy season.