Male bitterns begin to boom as early as late January to establish territories and attract mates.
Some males are polygamous, and occasionally several nests, each built by a different female, are found within the territory of one calling male. There is no pair-bond as such, and the male normally takes no part in nest building or raising the young.
The nest is a shallow platform of reed stems within the reedbed close to the water-level. If the water level rises during the nesting period, the female will add material to the nest to keep it above the water level.
Bitterns normally have one brood a year. It is suspected that a replacement clutch is laid if the first one is lost, though there is no firm evidence on this. In 1998 RSPB research discovered that bitterns are capable of double brooding with the discovery that one female raised two successful broods. However, it is not known how exceptional this record is.
A clutch of 5-6 olive-brown eggs are laid at 2-3 day intervals. Clutches can be started between end of March and mid-July. Incubation starts with the first egg, lasting for 25-26 days and hatching is stretched out over a period of several days. The young are cared for by the female, who feeds the young directly with regurgitated food.
By 8 days of age the young are able to adopt the typical bittern stance when alarmed, and when 15-16 days old they will leave the nest at times, clambering about the surrounding vegetation. The young fledge at 50-55 days of age, and become independent soon afterwards. Females are able to breed successfully in their first year.
Little is known about chick diet, chick survival or fledging success, though new research by the RSPB is starting to shed light onto these aspects.