The bittern is distinctive for its booming call
The bittern, a member of the heron family and known for its unique low booming call, is one of the most elusive birds of the British Isles. Only around 20 males are thought to be left in the UK, as their reed bed habitats have disappeared in recent years.
But staff at London's Wetland Centre, a converted reservoir up the Thames from Westminster, were amazed to see three bitterns emerging from the reeds at the weekend.
Harsh winter
John Arbon, grounds and facilities manager at the centre, told BBC Radio 4's Today programme the two wardens were left "open-mouthed" with surprise.
"They were walking down the path by the reed bed and the bird just jumped out right in front of them.
"They were really surprised. It's a bird we have always hoped to see and to just catch one like that, unexpectedly and so close... they stopped still in their tracks," he said.
The three birds have been seen by staff each day since then.
Experts believe it is possible they may have flown over from the European mainland to escape harsher winter conditions there.
The last time a bittern was spotted so close to central London was when one was reported to have landed in Oxford Street at the start of the 20th Century.
Revival hopes
It is hoped the trio's return may signal a revival in the fortunes of this rare species.
Martin Senior, head of the Wild Fowl and Wetland Trust, told the Today programme: "The longer they stay - they have been here for over a week now - the more likely they are to come back in future years.
"A week ago, we would not have said we would have had bitterns on site so soon - who knows what will happen in the future."
In the meantime the trust is expecting a few wildlife enthusiasts to come visiting, hoping to catch a glimpse of the rare bitterns in the early mornings.