The RSPB says people taking part in the world's biggest wildlife survey should look out for rare visitors in gardens and parks

The deep winter freeze across northern Europe and Russia has driven many exotic and unusual birds into Britain's back gardens on a weekend when more than half a million people are taking part in the world's biggest wildlife survey.

Amateur ornithologists are being told to "expect the unexpected" as they turn out for the annual RSPB Big Garden Birdwatch, including spectacular flocks of rarely seen waxwings that have been forced into towns and cities across the UK by a shortage of berries in their native Scandinavia and Russia.

Other unusual visitors to bird tables include fieldfare and redwing thrushes, winter migrants who prefer the countryside but are now seeking food and refuge in residential areas.

Experts said it was the best year for almost half a century to spot the exquisitely marked bohemian waxwing, slightly smaller than a starling, with a prominent reddish-brown crest, black eye mask and yellow-tipped tail. In normal years fewer than 100 visit these shores, mainly wintering on the east coast of northern England and Scotland. Occasionally larger flocks, or irruptions, are evident during "waxwing winters" when the population outgrows traditional breeding grounds.

This year's invasion is said to be exceptional, driven partly by severe weather in northern Europe and Russia. "This is probably the best for 30 to 40 years," said Richard Bashford, the garden birdwatch project manager, who two weeks ago awoke to 26 waxwings trilling from a tree in his Cambridgeshire garden.

Flocks, which usually number between 10 and 100 and are rarely seen inland, have been reported in many parts of England and Wales, including Berkshire, Buckinghamshire and Oxfordshire. They are attracted to the berries on shrubs and trees, especially rowan, found in gardens and around town centres and car parks.

Residents in Woburn, Bedfordshire, witnessed a 300-strong flock recently. On the Isle of Lewis in the Outer Hebrides, 622 were recorded. "These are unprecedented numbers. It's just a spectacular year for them. So if you wander round, listen, and hear a high-pitched trilling noise, there is a very good chance of seeing them," said Bashford.

The large, colourful fieldfare and smaller redwing thrushes, also Scandinavian migrants wintering in Britain, should be seen in larger numbers than even last year when cold snaps in January and February forced them from fields to gardens, prompting a flood of calls to the RSPB from baffled town dwellers unable to identify them.

The bird census, which entails volunteers spending one hour this weekend noting all the birds in their gardens, is also expected to reveal the fate of small-bodied birds, including the goldcrest, wren, long-tailed tit and coal tit, many of which are feared to have perished in the harsh weather before Christmas.

Now in its 33rd year, the garden birdwatch is the world's largest example of "citizen science", with results proving extremely accurate and tallying with more rigorous and structured surveys.

It was this "unofficial" survey that, between 1979 and 2010, pinpointed the alarming decline in house sparrows, and also in starlings, whose numbers have dropped by three-quarters over the same period. Theories as to why this has happened include modern housing designs and changing patterns of farming, including increased use of pesticides and the removal of hedgerows.

Thirty years ago there were an average of 10 house sparrows in a garden. Last year the figure was only 3.8. Starling numbers have fallen from an average of 15 to three. Both are now afforded the same conservation status as much rarer birds. "It is astounding these commonplace, everyday birds have the same status as red-listed birds like the corncrake and golden eagle," said Bashford.

In contrast, the wood pigeon has enjoyed an 855% rise in numbers since 1979, and the collared dove is up 375%.

The RSPB is expecting higher than usual numbers of sightings this year because a record amount of birdseed has been bought by householders. Dr Mark Avery, the society's conservation director, said birds had been feeding on this for three to four weeks and had made themselves at home in gardens.

"The really cold weather began quite early in December, and this would have been when natural food sources became scarce," he said. "Birds that wouldn't usually be found in gardens had to adapt their behaviour and look closer to home in our gardens. By now, these birds could have been making the most of our hospitality for over a month, meaning even more unusual sightings this weekend."

People without gardens are being urged to visit open spaces such as parks instead.