The bittern was common in west and central Europe up until the 19th century, when many breeding areas were abandoned due to drainage and persecution. There were further considerable declines in both population size and range during the early 1900s throughout Europe. The trend has continued, and there have been significant declines over much of its range between 1970 and 1990, even in many of its strongholds in eastern Europe. Many countries with good census data report continuing declines.
In the UK, widespread declines caused by drainage and particularly persecution, led to the extinction of the bittern around 1885, having already disappeared from Northern Ireland by 1840. The species returned to Norfolk in 1900, and was proved to breed in 1911. Since then the breeding population slowly built up, though never reaching Scotland or Ireland.
Having reached a peak of about 80 booming males in the 1950s, the species started to decline again shortly after, beginning in the Norfolk Broads, and despite a slow recovery during the 1990s, reached a low point of 11 booming males in 1997. Since then, the numbers have steadily increased.
This reduction in numbers was accompanied by a contraction in the range of the species, and during the 1970s and 80s it ceased to breed in several counties. Today, only a handful of birds survive, mainly in Norfolk, Suffolk and Lancashire.
The root cause of the sustained decline was loss and impoverishment of the reed habitat. This was exacerbated by high mortality during cold winters. Traditional management of reedbeds for cut reed maintained the reedbeds in good condition for bitterns. As the need for reed declined, many reedbeds were lost as they dried out through neglect or were drained for other agricultural uses. In the continental Europe extensive reed cutting severely reduces the area of reedbed suitable for bitterns. E.g. 90% of Dutch reedbeds are harvested every year, removing the winter cover essential for bitterns.