The Grey Heron is sometimes mistaken for a stork - or an ostrich!

The three-foot fish eater's restricted diet meant it relied on clean water for a plentiful supply of food.

By the 1950s, leading ornithologist James Fisher was saying it was unlikely they would ever breed again in London.

But now, thanks largely to improvements to the water quality of the Thames, the capital has become something of a heron hotspot.

"Anyone walking along the Millennium Bridge is quite likely to see herons there these days," said Adrian Thomas of the RSPB.

Breeding pairs have been spotted all along the Thames, at the Walthamstow Reservoirs, and Regents and Battersea Park among others.

You get the 'urban hothouse' effect, herons in London breed earlier than any herons elsewhere Adrian Thomas, RSPB

Their striking appearance has attracted so many admiring glances, several groups organised a series of heron-related events this weekend.

Mr Thomas said: "I was there last year when we tried out a smaller version of Heron Day, it was the success of that which made us want to do a bigger one this year.

"There were people coming through saying: 'Oh my God, it's an ostrich!'

"For bringing wild birds closer to people who probably don't have a lot of contact with wildlife, it is proving to be quite an exciting species."

But not everyone is a fan of the herons.

Penguins at London Zoo have found their long-beaked neighbours in Regents Park helping themselves to their fish. And exotic fish owners have reported Grey Herons tucking into the contents of their garden ponds.

The rise of London's Grey Heron 1950: No herons 1985: Three heronries 2000: 16 heronries

Without any natural predators, it is the bird's restricted diet which is its biggest threat.

While the Grey Heron has been known to eat frogs and even the occasional rat, it relies on fish which can be killed off by frosty weather - which may be another reason why they are back in London.

"You get the 'urban hothouse' effect, herons in London breed earlier than any herons elsewhere," said Mr Thomas.

"It is very rare for an urban area to hold such a large proportion of the UK population - one in every 30 or 40 herons are in London."

"They are now becoming tamer than they once were.

"They are completely wild birds but the herons you get in places like Regents Park are probably the tamest birds any where in the country."