The traditional ship's look-out post was known as the crow's nest because of its precarious position at the top of the mast.

The birds like to make their nests in the highest branches of the tallest trees or perch them on top of telephone poles and pylons.

Now one mating pair has astounded experts by turning the myth into reality and building their new home on a yacht at Brighton Marina.

The birds moved on board the Nicha on Monday, ferrying backwards and forwards to the marina with twigs to add to their nest.

They settled for a commanding position at the tip of the 45ft mast, building their nest on top of its radar for a perfect bird's eye view.

According to the Royal Society for the Protection of Birds, it may be the first time a crow's nest has been recorded on a yacht.

Spokesman Keith Noble said: "They like to nest somewhere pretty secure like a big tree or a pylon.

"I haven't found any other reference to crows nesting on a mast so this is very unusual."

The nest was first spotted by eagle-eyed secretary Louise Webber, who works at Ancaster Boat Sales where the Nicha is moored.

She said: "I was sitting at my desk looking out of the window when I suddenly noticed it and thought, 'That shouldn't be there'.

"The birds have been flying back and forth ever since bringing twigs. They seem quite happy but I don't know how long the nest will last if there is a strong wind."

The yacht's owner, Christian Schambri, knew nothing about the new occupants until he received a call from Ancaster yesterday.

The Surrey-based farmer said: "I am usually plagued by seagulls who make a mess of the boat so this makes a nice change. It would be nice to see the eggs hatch.

"I like to give nature a hand. Sometimes I have been on the yacht when birds have arrived exhausted after crossing the Channel. I usually put a towel down for them to sit on and they stay for an hour before flying off.

"Once birds built a nest in my tractor and I had to wait for them to hatch and for the fledglings to fly before I could use it again."

Mr Schambri has no plans to move his yacht until the end of the nesting season.

He said: "The Nicha is for sale and I am having a new boat built for me. It cannot be moved at the moment so nothing drastic is going to happen to the birds."