Witnesses said the young woman, who did not want to be identified, was going past the pond at Rickmansworth Aquadrome when she noticed the stricken Grebe.
Chris Wicks, founder of the non-profit CW Wildlife Rescue, said the woman swiftly stripped to her swimsuit before diving in.
“A young lady was passing, who is a wild water swimmer, and she stripped to her swimsuit and promptly entered the freezing water", Mr Wicks said.
“She swam out to the stricken great crested grebe all the way out she was breaking the ice with her right hand and she grabbed the bird in her left hand and swam back to shore.”
In the video the woman can be seen using her bodyweight to break the ice as she wades towards the bird.
A crowd of onlookers break into applause when she reaches the riverbank with grebe in hand.
