The story of the Bittern in the UK is often heralded as a conservation success story. From a low point of just eleven booming males in 1997, the most recent counts in 2023 saw 236 males booming mating calls resonating across their reedbed habitats.
However, a new report out this month warns that we should not rest on our laurels when it comes to the future of these enigmatic birds. Bittern – Booming or Boom And Bust published in British Birds, reports on an audit of 29 key sites for Bittern in the UK and has identified the main risks posed to the recovery of the species, as well as some key habitat management actions to maintain and increase the population in the UK.
I have been fortunate enough to work on some of the RSPB’s amazing wetland reserves, so it was with great interest that I dived into this report by Graham White.
What is the issue?
Bitterns are a secretive member of the Heron family. They are reedbed specialists and the males’ deep, resonating boom is the loudest of any UK birds call. Their streaked tan and brown plumage, once described to me by a colleague as "a heron that has been in the toaster", makes them incredibly well camouflaged, meaning they are often easier to hear than to see.
Bittern in the reeds. Ben Andrew (rspb-images.com)
Like much of our wildlife, Bittern, and the reedbed habitat on which they are dependent, is threatened by climate change. Coastal reedbeds are at risk of salt water contamination during coastal flooding. Reedbeds are also in danger of drying out and ‘scrubbing up’ where trees and shrubs colonise as leaf litter rises, or water levels fall, and natural succession occurs. Wildfires can also be an issue during periods of drought. This can lead to a lack of food availability as the variety and quantity of fish necessary for Bitterns to feed on are not able to survive.
With the threats to the Bittern well known, a concerted effort began in the early 1990’s to reverse their decline. A recovery programme, led by the RSPB and Natural England, as part of their Action for Birds in England (AfBiE) partnership, and with contributions from organisations in the private, voluntary and public sectors was launched, with two projects attracting EU LIFE-Nature funding. These LIFE funded projects allowed for the restoration of 700 hectares of reedbed, and stretched the core reedbed habitat out of East Anglia across England. Increasing the available habitats has allowed the Bittern population to recover to the levels we see today.
The number of booming Bitterns and occupied sites 1990-2023
Managing reedbeds for Bittern has also provided benefits for other species that share the habitat. Mammals including water vole and noctule bats, birds including Bearded Tits, Reed Warbler and Marsh Harrier as well as a range of invertebrates including spiders and moths benefits from this habitat improvement.
Importantly following these interventions we now see the majority of booming males are on inland sites and so safe from the threat of saline inundation due to sea level rise. However this is not the only risk that Reed beds and therefore Bitterns face.
Booming Bitterns under threat of saline incursion, showing the spread to inland sites (blue).
So what do we need?
Bitterns generally require large sites greater than 20 hectares with wet reedbird and deep pools, a significant proportion of open water, a sustainable fish population and wet nesting areas, secure from predation. The majority of our Reed beds are separate from the rivers that feed them and therefore vulnerable to successional pressures including colonisation by water loving plants such as willow, and clogging up of water channels in the reed beds by the reeds themselves. This means that programme of management is necessary to maintain enough high-quality habitat for this species to not just survive but to thrive.
One of the interesting things revealed by this audit is that it is possible for a site to ‘punch above it’s weight’ in terms of Bittern productivity. It is generally accepted that Bitterns require 20 hectares of reedbed, however this has been brought in to question in this paper, with a much stronger correlation to fish density being discovered. One site that was singled out, and one that I’m very fond of is RSPB Old Moor in South Yorkshire. With just 11 hectares of reedbed, the site supports three boomers, and three nests. The high fish density, high amount of reed edge as well as overall wetland quality contributing to this site’s overachievement.
Density of booming Bitterns compared with fish weight (<300gm)
The paper suggests the following actions which will require buy in from a range of actors;
This report serves to remind us that in the UK’s managed landscapes, where we need to balance the needs of people with wildlife, these outcomes don’t happen by accident. Our ‘wildest’ spaces need careful management with the investment of time, money and expertise, as well as collaboration in order to get the best results for nature.