Bittern egg-laying takes place from early April to June. The nest, of dead reeds, is just above water level in dense reed in areas where the water is about 10 cm deep. Males may be polygamous but as females do not boom or display it is difficult to know how many are associated with each male. In winter they are more widespread and may be encountered in a wide variety of places where there is reed and open water. Between 30 and 190 birds are recorded each winter.
Continental birds fly in to escape severe weather, increasing the UK population.
Some wintering birds are young British ones dispersing, and these may be crucial to the expansion of the population. So reedbeds too small for breeding but large enough for over-wintering may still be vital. Most will be visited by a wintering bittern from time to time. The decline of the bittern population Bitterns were once quite common in Britain, but wetland drainage and then sport and trophy hunting drove them to extinction as a breeding bird in the late 19th century. Breeding was first recorded again in 1911 and in 1954 it was estimated that there were about 80 'boomers'.