Wildebeest calving in the Serengeti ecosystem is concentrated on the short-grass plains in the early part of the year. Around eight in ten calves are born in a span of two to three weeks, producing several hundred thousand newborns in a matter of days at peak.
The mechanism is predator swamping. Lions, hyaenas and cheetahs in the calving area can only kill and consume so much in a given period, and their numbers cannot rise quickly enough to match a pulse of that size. Spreading births across months would expose each calf to a longer period of adequately fed predators; compressing them means most calves survive the vulnerable first days simply because the predators are already full.
The calf itself contributes. Wildebeest neonates are precocial to an extreme degree, typically on their feet within a few minutes of birth and able to run with the herd the same day, faster than a comparable newborn of most other ungulates. Exactly what synchronises conception so tightly, beyond rainfall-driven grass growth and social cues at the rut, is not fully established.