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

This year the annual butterbump survey has recorded another increase with 198 booming male bitterns found at 89 sites across England and Wales, of which 102 were found on RSPB reserves.

Historical Bittern recovery

From historical sources, it is clear that Bitterns once bred across the UK. By the 1880s, however, they were considered extinct as a breeding species in the UK. Following recolonisation early in the 20th Century, initially in the Norfolk Broads, numbers increased to a peak of about 80 booming males in the 1950s, with most in the Broads.  There then followed a steady decline, leading to a programme of monitoring and research to determine accurately the number of individuals at the few sites that still retained Bitterns in the 1980s and to help diagnose the causes of decline and to identify a means of halting and then reversing it.

Surveying booming Bitterns

Annual surveys were established in 1990, using a new method that combined techniques of mapping the territories of booming males using triangulation and the individual identification of these males by the characteristics of their booming songs.

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 is very distinctive, but at a distance can be mistaken for a mooing cow or even a foghorn!

 

These boom recordings, of 3 different males, were made at Minsmere in the 1990s

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.  Although Bitterns do boom at any time of the day, 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. 

From 1990, booming numbers continued to fall until a low point only 11 booming males in the UK in 1997; mainly within Norfolk and Suffolk, with a small outlying population at Leighton Moss, in Lancashire.  Numbers started to increase from the late 1990s, with more than 100 boomers recorded for the first time in 2011, and more than 150 in 2015. In 2019 there was another annual increase in the number of booming males, from 189 in 2018 to 198, following the pattern of a year-on-year increase in each year since 2006. For the first time, over 100 boomers were recorded on RSPB reserves.

Distribution and abundance of booming Bitterns in 1997 (11 boomers at 7 sites), 2010 (87 boomers at 47 sites) and 2019 (198 boomers at 89 sites). The scaled circles range from 1-2 to 15+ booming males per site.

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. Habitat management, restoration and creation has taken place at more than 80 reedbeds throughout the UK since the mid-1990s, and many of these sites are away from the traditional 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 project, from 2002 to 2006, 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. From the distribution maps above, 86% of the occupied sites in 1997 were at risk of flooding by sea water, 53% were at risk in 2010 and 43% in 2019.

 

Photo by Ben Andrew (rspb-images.com)

Thank you to our Bittern survey volunteers

A large number of volunteers now help to monitor Bitterns at many reserves across the country, including coordinated booming surveys 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.

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