Another record year for booming bitterns

Blog by Simon Wotton, Senior Conservation Scientist, RSPB Centre for Conservation Science

The Bittern is an elusive bird seen occasionally skulking through reedbeds looking for fish or flying over a reedbed on its broad, rounded, bowed wings. The males make a remarkable booming sound in the spring, and this year the bittern survey has recorded another increase with 164 booming male bitterns found.

Bittern have made a come back

Bittern were considered extinct as a breeding species in the UK in the 1870s. Following recolonisation early in the 20^th Century, numbers of bitterns increased to a peak of about 80 booming males in the 1950s, but then fell to fewer than 20 by the 1990s, with similar declines witnessed in many other countries in Western Europe.  By 1997, there were only eleven booming male bitterns in the UK; these were mainly within Norfolk and Suffolk, with a small outlying population at Leighton Moss, in Lancashire.

Photo: Bitterns are one of the UK’s biggest conservation success stories

Surveying bitterns

There has been an annual survey of booming male bitterns every year since 1990, with an encouraging increase in booming numbers since the late 1990s, with more than 100 boomers recorded for the first time in 2011, and more than 150 in 2015. 

In 2017 there was a very slight increase in the number of booming males, following the pattern of a year-on-year increase in each year since 2006.  A minimum of 164 boomers were recorded at 71 sites, compared to 162 boomers at 78 sites in 2016.

Image: number of booming bitterns 

Booming sounds

The distinctive booms of territorial males can be heard from as early as January at some sites, most typically following mild and wet winters, and can still be heard into June and rarely July.  The best time to listen out for booming males is from the middle of March to the middle of May.  A booming bittern can be very distinctive at close quarters, but at a distance can be mistaken for a mooing cow or even a foghorn!  Often male bitterns give a grunting call before their booming is fully developed, this grunting can be hard to hear unless you are close to the bird and it can sound very unlike full booms. 

It is possible to hear a booming bittern at any time of the day, however, the best times to hear them are in the two hours around dawn and at dusk.  The time you are most likely to hear a male is about half an hour before sunrise.  There is likely to be much less background (particularly traffic) noise before dawn than at dusk.  There is little point in going to listen for booming bitterns when it is very windy, but they quite happily boom in the rain.

Image: number of booming males and sites

Suitable habitat

Major wetland habitat management, habitat restoration and creation are ongoing for this species and annual population monitoring is the main yardstick with which we can measure its success. 

Some of the best places to see and hear bitterns now are wetlands that were created, from the mid 1990s, for bitterns and other wetland wildlife. Significant works have taken place at more than 80 reedbeds throughout the UK in this period, including through two EU Life projects specifically for Bitterns in the UK.  The first project, from 1996 to 2000, ensured that ‘emergency action’ was taken to restore over 350ha of reedbed across 13 sites, most of which were in the core East Anglia breeding area. The second project, from 2002 to 2006, created more than 300ha of new reedbed, restored a further 350ha, and restored and created nearly 40km of ditches on 19 sites. In 2017, 25% of sites, holding 72 booming males, were part of the EU Life projects. 

Most of these sites were away from the core East Anglia breeding areas, to encourage Bitterns to reoccupy their former range and give the species a sustainable long-term future in the UK. The second EU Life-Nature-funded project was the first and largest project of its kind in the UK aimed at safeguarding a species’ habitat in the face of imminent changes due to climate change, as most of the UK in the 1990s were all along the East Anglian coastline, where many sites are highly vulnerable to sea water inundation during storms, which climate-change models predict will increase in severity and frequency as our climate changes.

Thank you to our bittern survey volunteers

A large number of volunteers now help to record booming bitterns at many reserves across, such as Minsmere, Lakenheath, Ham Wall and Shapwick Heath, including coordinated listens involving a number of surveyors to ensure that the whole site is covered from several different listening points at the same time. It is now only possible to achieve a full national survey each year with help from volunteers, landowners and conservation site staff.  

Natural England and the other partner organisations have played an important part in the bittern success story of recent decades. Natural England has done lots of work on their National Nature Reserves supported management, and also with restoration/creation work on sites managed by other organisations through the Species Recovery Programme. Natural England has also co-funded the monitoring/research work with RSPB through Action for Birds in England.