IT will be trap inspections and leg ringing as usual today for bird warden Paul Baker, as Fair Isle celebrates the golden jubilee of its pioneering bird observatory.

The peak of the late-spring migration is no time to be taking it easy and Mr Baker and his staff have their annual census to complete, possibly adding yet another bird species to the British list. So the big jubilee party will be in August, on the 50th anniversary of the first paying guests staying at the observatory.

Yesterday, a crane arrived on the island, midway between Orkney and Shetland, en route from winter quarters in Spain to the forests of Northern Sweden.

Another arrival was former warden Roy Dennis, now chairman of the Fair Isle Bird Observatory Trust, who will chair the trust's annual general meeting today.

Since the first warden, the late Kenneth Williamson, landed on the three-mile-long island in June 1948, the Fair Isle Bird Observatory has logged 21 new species for the UK and ringed 328,000 birds of 265 different species. Indeed, the leg ring of one bird later turned up in Natal, South Africa, being used as jewellery.

Apart from the scientific work, the observatory and its lodge have provided holidays for thousands of people over the years, with its culinary reputation rivalling the ornithological records. Visitors spend money on Fair Isle knitwear and other souvenirs, helping the local economy.

The observatory was the idea of the late George Waterston, a former Scottish director of the Royal Society for the Protection of Birds. Fair Isle was his first sight of Britain when, as a seriously-ill Army officer, he was being evacuated on a hospital ship from a German prisoner of war camp.

After the Second World War he bought the island - which he was later to hand over to the National Trust for Scotland - and, with the help of some friends, set about opening the observatory in some shacks abandoned by the Navy.

Mr Dennis said yesterday: ''It's marvellous that we've been going for 50 years and it's a great tribute to George Waterston and his friends, who started it all in 1948. Some people thought they were over-enthusiastic and doubted if it could survive. It has survived difficult times, but we've been helped by many, many people, not least the Fair Islanders and the National Trust for Scotland.''

Mr Dennis, who was on the island from 1963 to 1970 and now works as a wildlife consultant in Speyside, said that, in its first 25 years, the observatory was ''at the leading edge of understanding about bird migration and navigation'', with its systematic recording of bird migrants as part of a chain of European observatories.

''In the second part of its existence I think our major contribution has been the detailed study of seabirds and their relationship with the marine environment,'' he said.

With 43,300 pairs of breeding fulmars, 37,500 guillemots, 1000 gannets and 21,000 puffins, the scientists at the observatory have plenty of study material on their doorstep. Among their findings are that seabirds live surprisingly long lives. One Fair Isle puffin clocked up 32 years and a fulmar lived to see its 36th birthday.

Mr Dennis said: ''It would be lovely to go another 50 years, tied in with helping the community. That was one of George Waterston's original ideas and I hope we can continue to do that, and to encourage the wise use of Fair Isle and its seas.''