Bitterns feed on fish, especially eels, amphibians and invertebrates. They usually hunt along the reed margins in shallow water and on the edges of dykes. Males are polygamous with each mating with up to five females. The nest is built in the previous year's standing reeds and consists of a platform some 30 cm across. The eggs are laid in late March and April, usually four or five in number, and incubation and care of the chicks is provided by the female bird. After hatching, the chicks spend about two weeks in the nest and then disperse amongst the reeds. They adopt the camouflage posture of the adult when threatened but little else is known about the chicks' feeding habits and behaviour. The birds usually leave their nesting grounds in winter but the UK population is often joined by over wintering birds from the continent. Due to its secretive nature, bittern numbers are usually monitored by counting the booming calls of males heard at the main sites.