One of Nature's most fascinating mysteries is how pigeons find their way home over vast distances.

No matter how far away they are taken, they almost always return to their lofts.

Now German scientists believe they have discovered how the birds do it. Research has revealed that tiny iron structures in their beaks allow them to analyse the earth's magnetic field - much like a compass.

Through the signals picked up, the birds can work out where they are and set out on the best course home.

As well as pigeons, many migrating birds display a remarkable ability to fly thousands of miles to return to a specific garden or tree year after year. Scientists are suggesting they may have similar iron- containing cells in their beaks.

The amazing abilities of homing pigeons made them invaluable during both world wars, with both sides using them to send messages over enemy lines.

Thirty-two of the 250,000 pigeons used by UK forces in World War Two were even awarded medals for valour.

In 2005, the film Valiant recorded the exploits of a group of fictional wartime homing pigeons.

In the past, experts have suggested the birds use the sun and stars to navigate, although in 2004 researchers found that many follow roads rather than their internal compass to plan their route.

Italian scientists also recently found that the birds can create 'odour maps' of areas they fly over, which may help them find their way. However scientists have long believed that they can in some way use the natural magnetism of the earth to navigate. The recent study by German scientists has revealed how this may be possible.

The research, published in the latest edition of the journal Naturwissenschaften, used X-rays to examine the upper beaks of pigeons. They found that within the skin lining are tiny ironcontaining particles in nerve branches which are arranged in a 3D pattern.

The team, led by Gerta Fleissner, concluded that this allows the birds to react to the external magnetic field of the planet and work out their precise location.

She pointed out that similar ironcontaining cells had been found in the beaks of robins, warblers and chickens so it may well turn out to be the way that other species also navigate.

"We expect that the pigeon-type receptor might turn out to be a universal feature of all birds," she said.

Scientists are still discovering more about the incredible abilities of the pigeon.

Last year a French team found that they can memorise 1,200 pictures.

The researchers concluded that while birds and other animals are different in so many ways, our divergent evolutionary paths have had little impact on the basic processes of our memories.

However, despite such impressive memories, pigeons are not the most intelligent birds, according to researchers.

A team in 2005 judged the intelligence of a range of birds and concluded that crows, rooks, jays and ravens topped the IQ league, while the New World quail earned the dubious honour of being the most stupid.