The bittern is one of our rarest birds
The cold weather has brought a rare surprise for visitors to the RSPB's Rye Meads nature reserve, near Hoddesdon.
A pair of bitterns, amongst the country's rarest birds, are wintering the Hertfordshire reserve.
In the early 1900's, the bittern was declared extinct as a breeding species in Britain and Ireland.
However, they've since re-colonised and there are now an estimated 75 breeding males in the UK.
The bitterns plumage is one of nature's greatest examples of camouflage.
They have pale feathers with dark stripes, allowing them to melt into the background of their favoured habitat; rendering them virtually invisible in reedbeds.
Staff at the reserve were able to capture an image through a telescope of one of the pair, standing on the edge of a reedbed near an unfrozen section of one of the reserve's lagoons.
The difference in size suggests they may be a male and female. It's hoped they will stay and mate on the site, but bitterns do not normally form strong bonds and do not "pair-up".
Joan Childs, Site Manager for the RSPB's Rye Meads nature reserve, said, 'We're delighted to be hosting two bitterns on site this winter. We've been working hard to attract bitterns by managing our reedbeds specifically for them, and extending the area of reedbed on the reserve.
"We've also recently introduced a thousand small fish to the lagoons to make sure there's plenty for bitterns to eat. We've done everything we can to offer them the perfect home for the winter and it's fantastic that they have decided we are up to scratch! Hopefully lots of visitors will see them over the forthcoming days.'
During the spring mating season the males make a remarkable booming noise to attract a mate, this can start as early as January. It's this call that has helped researchers monitor the population size of this secretive and difficult to spot species.
Bittern are members of the heron family. They feed mainly on fish but also eat amphibians and insects. They are a red listed species, meaning their numbers are so low, there is concern for their long-term future.