Well according to one electricity board worker it is the protected birds and they could put a dampener on plans to demolish the giant structures.

Owner E.ON plans to demolish the twin structures because it says they are becoming unsafe and it wants to redevelop the site.

Peter Spooner, who works for Yorkshire Electric Distribution Ltd, heard the bird's distinctive call while he was out checking the security of the site.

When he looked up at the landmark towers, Peter was sure he could see a peregrine falcon – a bird so rare in England they are protected by law.

"I have seen peregrines in Scotland before and I'm as sure as I can be that there was one up the top of the cooling tower," said Peter, from Loxley.

"I'm no expert but they are quite distinctive. I had a closer look through my binoculars and then checked a reference book at home and I really think it is them.

"I mentioned it to some colleagues who said that it wasn't the first time they had been spotted up there."

The birds are offered the highest possible protection in the UK after their numbers fell significantly during the '50s and '60s when the chemical DDT got into the food chain.

The chemical caused peregrines to lay eggs that were so thin they cracked as the mother tried to turn them in the nest meaning that birth rates hit a dangerous low. They are now making a bit of a comeback and RSPB spokesman Margaret Overend said she was not shocked to hear of them nesting in Sheffield's iconic cooling towers.

She said: "The birds' natural habitat is a sheer cliff face – which is not dramatically different from the sides of a cooling tower.

"As long as there is a ledge at the top they would happily build a nest there and while we can't be 100 per cent sure that what Mr Spooner saw was a peregrine falcon, we are certainly not surprised by it.

"It is very interesting news for bird watchers in Sheffield."

The birds could also be a saviour for the controversial cooling towers which are due to be demolished later this year.

Margaret said: "While the nest is in use – when it is being built or being used by the birds – the demolition could not take place.

"The protection clause states that you cannot damage a nest or nesting site and the offence carries a very heavy penalty.

"However, once nesting season is over and the birds have moved on there is nothing to stop the demolition going ahead."

An E.ON spokesman said: "We sent steeplejacks up to inspect the top of both cooling towers late last year and found no evidence of the birds nesting there."