Did you know that 80% of bison calves have April and May birthdays? This is because of something called birth synchrony. Birth synchrony is considered an evolutionary adaptation where adult females give birth during a focused period of time during a specific season, often called the birthing season. This phenomenon is seen globally for many wildlife, such as corals, fish, birds, and some ungulates. For bison, the birthing season is often referred to as the "calving season." Research has documented that the bison calving season often occurs between mid-April to early-July, across multiple herds in different locations, and with many births concentrated within only several weeks. For example, one study showed that at Wind Cave National Park, 95% of bison calves were born within a 40 day time-span from mid-April to late May.
Why would this happen? Some scientists think birth synchrony occurs in bison due to strong seasonal limitations on food availability and quality, so that females enter estrus as a group depending on when their physical condition recovers from a long period of food stress that they all go through together. In this case, when green grass is limited to only a relatively short period of time, birth synchrony would be expected to be more focused than when green grass is available for a longer period of time. This reason could also explain why birth synchrony occurs just prior to peak green grass availability, when the mother would have access to the best food of the year, and thereby optimize her milk production following birth, thus benefiting her calf nutrition and growth.