The main threat to the species is the reduction in availability and quality of Phragmites-dominated swamps and other marshes due to drainage and abandonment of traditional uses for reedbeds.
In the past, continued drainage of land for agricultural uses and excessive water abstraction in the catchment of reedbeds reduced the area of reedbeds in the UK, while the main current threat to the habitat comes from neglect and lack of management, which allows reedbeds to dry out and become unsuitable for bitterns.
Excessive water abstraction is still a potential hazard to existing reedbeds through reduction in the summer water levels within the reeds, and hampers efforts to establish reedbeds in areas where they have been lost in the past.
Reedbeds represent an early successional stage from open water to woodland, and without careful management and control of water levels they will naturally dry out as leaf litter and silt accumulate within the reed, eventually allowing the establishment of shrubs and trees. Currently, 85 per cent of the British population is within reserves where better habitat is created for bitterns by re-creating the early successional stages of reedbed, and carrying out other specific habitat management.
Water pollution, especially eutrophication, has reduced reedbed viability and food availability within them. This can result in reed dieback, development of anoxic sediments and algal blooms, all of which can adversely affect feeding opportunities and food availability. Salt water intrusion caused by sea level rise and breakdown of sea defences can be very damaging because of the resultant tidal fluctuations in water levels and reduced food availability.
In recent decades bitterns have been exposed to several potentially damaging pollutants such as organochlorine pesticides, PCBs and mercury, and the birds accumulated some of these at high levels into their bodies. This threat has now declined, and is no longer considered to affect the population.
Hard winter weather can increase mortality and hence seriously deplete the bittern population. This is particularly noticeable if freezing weather continues for a month or more. Declines of up to 40 per cent have been observed. The icy weather at the beginning of 1997 probably contributed to the halving of the number of booming bitterns from 22 to 11.