Guest blog by Simon Wotton, 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 to attract a mate. This year the Bittern survey has recorded a bumper 150 booming males – an encouraging increase on previous years.
Photo: Bittern by Andy Hay (rspb-images.com)
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 20th 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 the counties of Norfolk and Suffolk, with a small outlying population at Leighton Moss, in Lancashire.
Figure: Bittern population change from 1900 to present.
Surveying bitterns
There has been an annual survey of booming male bitterns every year since 1990, with an encouraging increase in booming numbers to 140 in 2014 and, so far, just over 150 this year. 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.
Booming sounds
The distinctive booms of territorial males can be heard from as early as January at some sites (often following mild and wet winters) and can continue into June and July. The best time to listen out for booming males from the middle of March to the middle of May. A booming bittern can be very distinctive at close quarters, but beware mistaking a distant boom for a mooing cow or even a distant 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 the final full booms.
When to hear booming bitterns
The breeding season is about the only time of year when it is possible to positively identify a male bittern, as the bare skin around the eye and base of the bill turns a pale blue in males that are in breeding (ie booming) condition.
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 (ie 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.
Thank you to our bittern survey volunteers
A large number of volunteers now help to record booming bitterns at a number of reserves, such as Minsmere, Lakenheath and Ham Wall, 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.