The size of the Bison herds at Yellowstone National Park varies depending on the season. In the winter, the herds tend to be smaller as some of the animals migrate to lower elevations in search of food. The herds also tend to be smaller during times of drought, as the animals compete for scarce resources. The largest herds are typically found during the spring and summer months when grasses and other plants are more plentiful. According to recent estimates, the largest herd contained approximately 4,900 animals. However, the size of the herd can fluctuate significantly from year to year, depending on factors such as weather conditions and disease outbreaks.