Thrushes are dying out and no one knows why. This man is determined to change that

Nicholas Roe meets Roy Taylor, RSPB super-sleuth

We are sitting in a pub garden, halfway through a pint of lime and soda, talking about the death of the song thrush. Suddenly, my companion says, "Listen!" The sounds of drowsy fields and hedgerows fill the air. "Come on," he says, leaps up and hurries down the long, rough-cut lawn towards open ground where, for the next 10 minutes, we scrabble around the bushes until ... "There it is," he whispers.

A big, fat, goggle-eyed baby song thrush peers nervously at us from the thicket. So what, you want to say. But you do not, partly because my companion's enthusiasm is infectious, also because he is out here in the countryside doing an extraordinary job for which everyone should be grateful.

Note the incongruity: Roy Taylor is making an awesomely detailed, nest- by-nest study of a tiny patch of land in the heart of West Sussex - to find out why birds are not dying here.It is a sad mark of how groggy our environment has become: the good bits stand out. All over the country, song thrushes are in decline and no one knows why; but this 1,500-acre spot in Sussex is one of the few areas left where for some reason that just is not happening.

Roy Taylor, 25, has been sent here by the Royal Society for the Protection of Birds to examine this strange circumstance. By comparing this good area with ones where the birds are dying out, they might just learn what is going wrong and therefore how to save one of Britain's most popular, yet most threatened, songbirds. So Roy is gumshoeing his way round the fields as part of a research project that is likely to cost pounds 200,000 and take five years to complete.

The 40 farmers and ordinary home-owners who have agreed to give him access to their land and gardens call him "the bird man" because he is so obsessed with his task. Ask if his 60-hour-a-week job, which began in January, has involved personal sacrifice and Roy Taylor pauses. "Sacrifice? No. I have a long-term relationship, but that doesn't get in the way of what I do. My partner has to make sacrifices, yes."

He is on site by 6.30am most days after driving 20 miles from his home on the Sussex coast (the RSPB asks that the site remains secret to protect itsresearch). Then it is a scrabble through thickets and bushes until about 6pm, with a short break for lunch.

It is the detail of the job that bends the mind: he has already mapped 200 pairs of song thrushes in the study area, ringed 240 individual birds, studied 163 nests, spent three hours at a stretch observing single birds. He has even gathered a huge box of bird droppings to analyse at home during the long winter evenings.

To sympathise with any of this you need to grasp the whole Bird Picture, and the nub of it is that songbirds are dwindling fast. Figures produced by the British Trust for Ornithology show that song thrush numbers have dropped by 54 per cent since 1969, linnets by 56 per cent, bullfinches by 67 per cent, tree sparrows by 80 per cent and so on.

Intensive agriculture is the most likely cause, but picking out exactly which bits are doing the most harm is a problem. Once you know that, you can set about putting things right: with targeted subsidies, say, or simply by asking farmers to help. But we do not know - certainly not for the song thrush.

"Birds have been an interest of mine since I was five," Roy confides, while shinning up a wall to examine a nest where three young song thrushes wriggle helplessly. He weighs them tenderly, measures their growth, puts them back. "You get hooked."

There are at least four possible reasons why the song thrush is dying - reasons that may become clearer through Roy Taylor's work. One is the obvious loss of nesting spots. Everywhere fields have grown bigger and in the process hedgerows, rough ground and copses have vanished. Intriguingly, in this little patch of Sussex edging the South Downs, the land has an old-fashioned feel to it: lots of small paddocks, clumpy woods, unkempt verges and quiet village greens. If you had to guess a good area for birds - and humans - this would be it.

Song thrushes might be dwindling nationally because farmers have been switching from grassland to crops, which reduces worms, the staple diet of the song thrush and many birds. This area is horsey with lots of grazing.

The use of pesticides might be important, particularly "molluscicides" which kill slugs and snails but also take out worms. Roy's observations have confirmed that song thrushes resort to snails and even slugs when drought sends worms diving for cover.

Finally, nests get raided. This is considered a longshot as a cause of major decline, but certainly cats, magpies, foxes and jays all have a go at young song thrushes, partly because the parents build nests in easily accessible spots. The point is important because once song thrushes grow to adulthood, after a year, they survive fairly well. It is while getting to that age that they suffer such terrible losses.

This is why the study has focused with such wearying attention on young birds. When Roy met me he was spending 45 hours a week just visiting 15 nests every three days, counting how many fledgelings survived for how long after leaving home.

He had also spent three hours at a time monitoring every feeding flight made by adult pairs to see what kind of land they avoided. Soon he will implant tiny homing devices on a handful of birds so he can track them more easily. Next year, he will collect all the old nests (163), fill them with Plasticine eggs, and place them near new nests as a lure: any marks on the Plasticine might show which animals most attack the song thrush.

It goes on and on; and after next year all this data must be compared with other sites where thrushes fare badly, so everything must be done all over again. Then at last they may have the X-factor; the thing that is doing for all those lovely birds.

Roy Taylor's money is on a mixture of reasons, most importantly the loss of small paddocks that have accompanied agriculture's march towards progress - but he is not jumping to conclusions. Instead, he is scrambling over walls and stumbling through ditches. And just as well.

The RSPB needs donations for its Song Thrush Appeal. Contact: RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL.