More experienced bid watchers - or twitchers as they are known - will be asked to observe the arrival of rarer species like wheatear, blackcap and the spotted fly catcher and report them to a specially created website (details later).

Says the BTO: "Many people note these events down on their calendars or in notebooks which might otherwise be lost. The survey will take advantage of increasingly easy access to the web in homes, offices and libraries to collect information and feed back day-to-day changes in the patterns of migration."

Migration Watch is being funded by the BTO's 30,000 members and sponsorship from Northumbrian Water - and it could not possibly have been launched at a more crucial time.

Global warming, whatever its cause, is playing havoc with long-established weather patterns, particularly over the Mediterranean which has to be crossed by many of the estimated 16 million birds which migrate to Britain every year.

This is causing major changes in movements not just amongst birds but also in fish and even butterflies. Sharks and other fish species that would normally be found only off the African coast or in the Caribbean have been spotted in English waters.

Red admiral butterflies which, unbelievably, used to migrate across the Channel to France as winter nears, are now staying on in England - I spotted one in my Yorkshire Dales garden only two weeks ago. And some once migratory birds are taking up permanent residence here.

This may, on the face of it, sound like good news. But there are dangers, too, for some trees - like beeches - and some of our favourite wildflowers, including bluebells, which burst into flower just before the canopy of the woodland they inhabit comes into full leaf.

Should that canopy thicken only a few weeks earlier, thanks to a warmer climate, the bluebell would be denied the daylight it needs for its short but wonderful blossoming. And that would be a tragic loss to the English countryside.

The pattern of bird migration could provide important clues as to why and how these changes are taking place - so information gathered by Migration Watch could have value to scientists far beyond that of birdlife, vital though that is.

So if you are interested in the countryside in general, or birds in particular, you could help put together the bits of one of the biggest jigsaws in nature. You can find out more about Migration Watch on:

www.bto.org