The big question: Should the sighting of a rare auk in Britain give us cause for alarm?

Why are we asking the question now?

A long-billed murrelet has turned up off the coast at Dawlish in Devon, exciting feverish interest among the UK's birdwatchers. Coach and train loads of people have descended on Dawlish from as far afield as the Netherlands in recent days to see and photograph the murrelet diving and swimming offshore near the town.

This is because this bird - initially thought to be a much more common little auk - is a very, very long way from home: 4,000 miles off course, in fact. It is the first time a long-billed murrelet has been spotted in the United Kingdom; indeed, it is the first live one ever found in western Europe. In 1997, a drowned long-billed murrelet was found in a fishing net off Lake Zurich.

What is the long-billed murrelet and why is it special?

The long-billed murrelet (or Brachyramphus perdix) is a small (about 25cm long) member of the auk family of seabirds, which includes guillemots and puffins.

Its natural habitat is on the eastern seaboard of the Asian landmass, around the Kamchatka peninsula and the Sea of Okhotsk - almost diametrically on the opposite side of the world to where it is now.

Until 1998, this particular species of murrelet was considered merely a subspecies of the marbled murrelet, which it closely resembles. It is distinguished from the latter by being slightly larger and longer billed, with a distinctive pale throat during breeding.

Its most unusual attribute is that, unlike most seabirds, it does not breed in colonies or even close to the sea, instead often going far inland and nesting on branches of conifers. It is considered officially threatened, with its habitats eroded by logging and oil development.

Is its arrival in Britain a sign of global warming?

To use an appropriate metaphor, one swallow does not make a summer and one stray murrelet is not necessary an indicator of environmental changes. Long-billed murrelets are in fact known to make occasional appearances all over the continental United States and further afield, especially in winter. The Devon bird may have arrived in Britain from almost any direction, including flying across the Arctic ice or Siberia.

Since it is normally a cold-water bird from Asia, there is no suggestion it has been attracted by the warmer waters around the UK, in the same manner that some warm water Atlantic fish are moving northwards; if it was a seabird normally found around the Azores or the Canaries, it might be evidence of birds following a similar pattern, although even then it would need more than one to be evidence of a trend.

However, it is known that the migratory patterns of some birds are changing because of warmer air over Britain. Blackcaps, birds normally found in central Europe and once a rare visitor, are now being regularly sighted in British gardens in the winter, when they should have migrated south in search of warmth.

Are migration patterns changing for other reasons?

Not necessarily. Many birds in Europe, North America and the northern Asian landmass tend to be migratory and, as a result, some end up in Britain on their travels. Scientists believe that a very small percentage of migratory birds are genetically programmed to do the complete opposite to the normal pattern of movement - an evolutionary failsafe button designed to ensure the species constantly seeks out new feeding and breeding grounds.

If, as is the case with the blackcaps, they find somewhere to their liking on their visit to Britain, then they begin to return in numbers as the information is passed down in the genes through breeding.

Other cases of rare or unusual birds are much more to do with isolated flukes - a strong airstream, high winds and storms, or even simply hitching a ride on a passing ship. Weather events are most often the cause - birds from central Europe or even the Asian steppes, such as the sand grouse, have been known to turn up in Britain in search of food during particularly harsh winters. Birds from the tropics are almost unheard off, since they do not tend to be migratory.

If the phenomenon of vagrant birds seems more common now, it could be because technological advances in telescopes, mobile phones and digital cameras, as well as access to internet databases, have made rare or unusual visitors to our shores easier to spot, photograph and identify. The chances of birds such as the long-billed murrelet being logged as soon as they set down in and around Britain are greater than ever before.

How do the birdwatchers come to hear so quickly?

Firstly, there are more birdwatchers per head of population in Britain than anywhere else and they outnumber other wildlife enthusiasts - such as butterfly hunters - by about 10 to 1. Two-thirds of the population regularly feed garden birds while the RSPB estimates that up to three million people watch birds regularly.

This is partly due to an explosion of interest in conservation and environmental matters. And this newspaper has also done its bit by highlighting such phenomena as the still unexplained decline in sparrows. The hard-core enthusiasts of the sort who turned out in Dawlish at the weekend are thought to number up to 10,000.

The public interest, combined with advances in equipment plus the fact there is much kudos to be gained among senior twitchers by being the first to record a sighting of a rare bird, make unusual arrivals easier to keep track of.

All interesting sightings are recorded on websites and immediately transmitted by subscription services to fellow twitchers by pager, text or e-mail. The Dawlish bird was originally believed to be a little auk until it was posted on the Surfbirds website and its true identity established.

This is the kind of thing that leads to the phenomenon of bands of excited twitchers charging around the country to try to get their 'scopes and cameras on birds such as the long-billed murrelet before it decides to take off back to the Sea of Okhotsk.

Such sightings are known as "cripplers" because of the crippling costs involved to some in pursuing their hobby.

What about other rare sightings?

The most exciting event in recent years was the spotting in 1997 of an extremely rare slender-billed curlew in Druridge Bay, Northumberland; normally found only in central Russian and Kazakhstan, there are believed to be only about 50 in existence. Other recent firsts include a red-billed tropic bird, from the tropical Atlantic, found off the Scilly Isles in June 2001, a black lark from Kazakhstan, in Anglesey in 2003, and a still-to-be-confirmed sighting of an olive-tree warbler, from the eastern Mediterranean, in Shetland in August this year.

So, how many British birds are there now?

Currently, 280 species are classed as native to the United Kingdom - that includes common birds such as robins and crows, and migratory birds such as swallows and nightingales

Adding other species that have been occasionally recorded takes the total to more than 570, including the rarities listed above and, when it is eventually officially confirmed, one long-billed murrelet.