Watch the birdie - he may be gone next year

As the winter deepens, more gardens than ever have bird-tables and other suitable surfaces bedecked with netted peanuts, sunflower seeds, pieces of energy-rich bacon rind and sundry other foodstuffs. The British are a nation of avid bird-feeders.

Ravenous magpies and wily grey squirrels, their hibernation sometimes broken by mild weather, may polish off more than their fair share of this culinary bonanza. But households countrywide will rise to the excitement of spotting their first bird-table nuthatch or jay of the winter.

However, for 247 gardens scattered across Britain - from inner-city London and suburban Milton Keynes to remote enclosures on Rannoch Moor and the Uists - feeding birds is not simply done out of kindness and in the hope of spotting a rarity. These bird-feeders have a seriousness of scientific purpose. They are the sources of information for the British Trust for Ornithology's (BTO) Garden Bird Feeding Survey.

The survey is 25 years old. It has charted the numbers of birds feeding each winter, weekly from October to March, in a cross-section of gardens countrywide. Some gardens have been observed since the inception of the BTO's scheme in 1970.

Last winter, 81 different species were recorded. Urban gardens attracted between six (Ramsgate, Kent) and 32 bird species (Crewe, Cheshire). Rural gardens nurtured between eight (Dromore, Co Down) and a staggering 47 (Ottery St Mary, Devon), the latter including such unusual garden feeders as water rail, black redstart and firecrest.

According to the BTO's analysis, suburban gardens for the first time last winter supported more species (slightly) on average than rural gardens, 19.3 compared with 18.7. The BTO's David Glue thinks this might be because of several factors: a wider array of food put out in many suburban gardens; the lack of severe weather so that "rural birds" could feed well enough in the countryside, and the decline in several farmland birds because of habitat loss.

But apart from a few mildly interesting statistics - and some golly-gosh records of rarities - what use is all this feeding and recording? Does it tell us anything more about Britain's more common birds? The answer is a resounding yes.

What it shows is that the frequency with which certain species visit gardens is changing, in many cases very rapidly over a short period of time. Wood pigeons, for instance, are now recorded in 52 per cent of gardens compared with an average of 19 per cent between 1970 and 1980. Siskins, diminutive greenish finches, have increased more than eightfold, and goldfinches sevenfold, during the same period while pied wagtails have become much less common.

Some of these garden changes reflect widespread trends. Take song thrushes, once more familiar to gardeners and farmers than blackbirds. Only 61 per cent of gardens hosted them last winter, an all-time low reflecting their decline throughout Britain. Although their breeding success has remained undiminished, populations appear to have been affected by agricultural changes, possible poisoning by slug pellets, and slow recovery from mortality caused by past cold winters.

Other declines in garden use are the result of mild winters - which in themselves may, or may not, reflect a warming climate. Pied wagtails are now seen less often on the patio because in mild winter weather they can find insects in fields and on roads. These little, long-tailed birds need to find a food item every few seconds of daytime to survive cold weather. Much the same is true of redwings, fieldfares and also Scandinavian thrushes, a couple of million of which overwinter here. "Last winter they failed to grace most gardens in the survey simply because there were more ice-free pastures to peck invertebrates in and plenty of hawthorn and rowan berries left in the countryside," says Mr Glue. "They didn't need to raid garden hollies and cotoneasters."

With wood pigeons it is different. They are adapting quickly to the benefits of garden visits. It is the food supply - or rather the lack of it - between January and March that determines how many of them survive. If sown cereals, stubbles from the autumn or crops of oil seed rape are covered with snow or ice, then wood pigeons - often a farmer's number one pest - are in trouble. Gardens, many with plenty of tree cover reminiscent of their original habitat (before they became exploiters of field food), can provide berries while their owners frequently supply nuts and seeds, all of which a few wood pigeons will clear in an afternoon.

Mr Glue thinks they have become adapted to garden life in winter. Even last winter, when it was generally mild, they were recorded in more than twice as many gardens in the BOT's survey as 20 years ago.

With great spotted woodpeckers - those magnificent black, white and scarlet tree lovers - the story is a little more complicated. They have been on the increase for much of this century, living off woodland seeds, pine cones and insects in winter. They are about three times as abundant now as they were in the Sixties. Much of this increase is fortuitous, one of the few benefits of Dutch Elm Disease which has put paid to up to 40 million trees. Feasting on the abundance of invertebrates burrowing under the bark of dead and dying elms, their numbers rocketed. Now that the supply has dwindled, these spectacular woodpeckers have turned to gardens, regularly raiding bird-tables where they have become a very welcome sight.

Siskins were rarely spotted in gardens until around 1970. But they have been spreading south out of Scotland and forsaking their winter dependence on conifers and alderwoods (where they eat the trees' seed), increasing their populations substantially in northern England, Wales and parts of southern England.

"They have discovered peanuts in plastic nets, maybe because they mimic the larger conifer cones they often feed on," says Mr Glue. "Peanuts in gardens may even be at least partly responsible for accelerating the siskins' southward spread," he adds. Seeds on the ubiquitous cypresses used as screening conifers in plenty of gardens may have been what first enticed them in. Then they got a liking for peanuts. The BTO found that they visited 59 per cent of surveyed gardens last winter. The average in the Seventies was just 7 per cent.

Goldfinches, too, seem attracted to peanuts, even though they are by nature eaters of small seeds such as thistles and teasel. In the 1970- 80 decade, on average, goldfinches visited just 3 per cent of survey gardens. Last winter it was 22 per cent. "It may be that seeds on garden plants have attracted them, then they, too, have taken a fancy to peanuts," says Mr Glue. "It may be like blue tits which, years ago in Southampton, started pecking their way into milk bottles through the foil caps. Within a few decades the habit had spread countrywide. It could be the same with goldfinches."

While many of the commonest winter garden feeders have changed little (see box), a great number of bird species are now much more frequent in gardens than they were even a couple of decades ago. What this means is that more species are becoming more tolerant of people, a modification of their behaviour which has, perhaps, been forced on them because of habitat decline in the countryside.

Maybe the countryside we have so substantially modified by agricultural intensification and urbanisation is no longer capable of nurturing the birds it once did. Or is it that more and more of Britain's gardeners are heeding Mary Poppins after all?

High-flyers among the garden feeders

Top 12 in 1994/95 (percentage of gardens recorded)

Blue Tit 100 Blackbird 99 Robin 99 Great Tit 98 Chaffinch 96 Dunnock 95 Greenfinch 94 House Sparrow 93 Starling 92 Coal Tit 88 Collared Dove 83 Magpie 70

Source: British Trust for Ornithology's Garden Bird-Feeding Survey