Thrills and trills in your backyard

This weekend, thousands of people will be taking part in the 33rd Big Garden Birdwatch. You could be one of them, says Michael McCarthy

One morning two or three winters ago I glanced out of the kitchen window into the garden and saw, on the feeder that hangs from the apple tree, two bright green birds. I was momentarily taken aback.

Bright green tends to be a tropical colour; we generally don't see such hues in British birds, apart from the green woodpecker and one relatively new arrival, the ring-necked parakeet. But these weren't parakeets or woodpeckers.

They were small, sparrow-sized birds, immediately eye-catching because the green of their plumage seemed to be glowing, as if lit from within. What were they? Not greenfinches, whose green is dullish until they open their wings. As I looked at them properly I realised the glow was actually green underlaid with yellow, and there were black and yellow wings bars, and black on the heads. Then the features all resolved themselves into an identity and my bird brain said: "Of course. Siskins."

The siskin is a finch whose normal home is in the high tops of pines and fir trees, in the mountains, and here it was five yards from a suburban back door close to London's South Circular road. There seem to be more and more siskins in the suburbs in winter, not least because the bird food we are putting out is becoming increasingly specialised and energy-packed; but this was the first time I had seen them on my own patch, and I was thrilled.

Is that too strong an emotion? Not for many people, not in Britain. The delight we take in our wild birds is astounding, and I'm not just talking about the pleasure we get from seeing something spectacular and rare – a barn owl hunting in the dusk like a great white moth, or gannets diving in an artillery bombardment on a mackerel shoal. We love the common birds just as much: the ones we see in the garden, the birds at the back door ("backyard birds", as the Americans call them).

If ever we needed proof of this fact, it can be found in abundance today and tomorrow, when across the country more than 600,000 people – that's more than the population of Luxembourg – will be attentively watching those garden birds, and tallying up their numbers.

They'll all be taking part in the RSPB's Big Garden Birdwatch, which is now in its 33rd year and has become what is thought to be the biggest "citizen science" exercise anywhere in the world. Anyone can join in. You set aside an hour, and you watch your garden (although you can carry out the exercise anywhere from a park to a prison yard, as long as you're watching the same space). As you do so, you record all the birds, noting down the largest number of each species you see at any one time (to avoid double counting).

What you're doing is helping create a gigantic snapshot of Britain's shifting populations of winter birds at a given moment – and the more people who take part, the more likely we are to get a true picture of what's happening. The value of the survey lies in its ability to indicate, measure and confirm change. And what has been most fascinating in the table of birds seen in the gardens of Britain (which last year stood at 73 species) is what has gone up the list, and what has gone down.

Consider, for example, the results of the first Big Garden Birdwatch, in 1979. The top 10 birds recorded, with average maximum numbers per garden in square brackets [x], were: (1) starling [15], (2) house sparrow [10], (3) blackbird [4], (4) chaffinch [3], (5) blue tit [2.4], (6) robin [2], (7) song thrush [1], (8) greenfinch [1], (9) great tit [0.9], and (10) dunnock, or hedge sparrow [0.8 ].

Contrast those figures with the results of last year's survey, in which the top 10 birds were: (1) house sparrow [4.2], (2) starling [3.9], (3) blackbird [3.3], (4) blue tit [3.2], (5) chaffinch [2.4], (6) woodpigeon [1.9], (7) great tit [1.6], (8) goldfinch [1.5], (9) robin [1.5], and (10) collared dove [1.3].

Some things are immediately obvious. The same birds make up the top five, though some of the positions have changed slightly; but while the blackbird, the chaffinch and the blue tit are present in roughly similar numbers, there have been enormous declines in the two commonest birds.

The starling has crashed from 15 birds

per garden to fewer than four, and the house sparrow has tumbled from 10 to 4.2; these figures reflect the severe declines in these two species measured in other surveys. The reasons behind them are not fully understood. Thirty years ago starlings and sparrows would be flocking on your lawn, a dozen or more at a time; now they're hopping around in twos and threes.

When we look at numbers five to 10 in the latest list, we see something even more interesting. Three species from 1979 – the song thrush, the greenfinch and the dunnock – have gone completely, and in have come three different birds: the woodpigeon, the collared dove and the goldfinch.

The song thrush's disappearance from the Big Garden Birdwatch top 10 was one of the first indicators of a widespread decline in this well-loved species; the greenfinch has similarly declined because of a disease, trichomoniasis. The dunnock has not fallen greatly in numbers but it has been overtaken by the two members of the pigeon and dove family: numbers of woodpigeons and collared doves are booming, perhaps because these birds seem able to thrive in areas subject to intensive agriculture – a practice that has decimated other bird populations.

The recent, more common presence of goldfinches in our gardens – they arrived in the top 10 in 2008 – tells a different story again. The main reason these birds are thriving is the increasing sophistication of the modern bird-food market (probably worth more than £200m a year now), and in particular the arrival of one specialised type of nutrient: nyjer (sometimes spelt niger) seed. This black grain, exceedingly rich in oil, comes from an Ethiopian plant related to the daisy, Guizotia abyssinica. Goldfinches find it irresistible.

You don't believe me? I can only tell you that a few years back I was told that if I put nyjer seed in the bird feeder, goldfinches would come; I did so – and, for the first time, come they did.

Having goldfinches in your garden is up there with having swallows in your barn or house martins under your eaves. They are the loveliest of all our small songbirds (although bullfinches run them close), for the bright contrasting flashes of their plumage – black, white and yellow – all crowned with that astonishing, bright scarlet face.

In medieval and Renaissance Christian iconography, the goldfinch was symbolic of Christ's passion, partly because it eats the seeds of thistles, associated with the crown of thorns, and partly because its scarlet face was held to symbolise Christ's blood. The bird figures in a whole series of religious paintings, the most famous of which is Raphael's Madonna of the Goldfinch.

The resonance of this, of knowing that this is the bird of Raphael, adds greatly to my enjoyment of it; but I have to say that, even without that knowledge, I would take as much pleasure in those sparkling colours and that fluttering flight and long, silvery twittering song, which seem to be the very essence of the charm of birds (indeed, the collective noun for a flock of goldfinches is "a charm").

Join with the 600,000, take part in the Big Garden Birdwatch, today or tomorrow, and you may well see goldfinches; spend that full hour watching and you'll almost certainly see a round dozen of colourful species. Maybe, if you're lucky, you'll even have the experience I had last winter, when I looked out once more from the kitchen window, and again saw a pair of glowing-green siskins on the feeder.Except that this time they were not alone: four goldfinches were fluttering about them, trying to get at the nyjer seed.

My eyes popped out on stalks, in astonishment and delight. Gordon Bennet, I said, or words to that effect, it's only the bleedin' back garden, yet my birdy cup runneth over.

1 BLUE TIT

Parus caeruleus

Feeding at any angle, even upside down, is the speciality of the diminutive blue tit, now perhaps the most familiar bird on our garden feeders and bird tables. It is so small it needs to have big broods of up to 14 chicks at a time to maintain its population level, as the mortality rate in winter is enormous and most of the birds will not survive a prolonged period of freeze. But the blue tit bounces back in spring.

2 STARLING

Sturnus vulgaris

Aggressive, noisy and bullying, the starling has suffered a huge decline in numbers over the last 40 years.

3 ROBIN

Erithacus rubecula

Our best-loved garden bird appears tame and friendly to people, but is actually a ferocious defender of its own territory against rival robins.

4 CHAFFINCH

Fringilla coelebs

The striking song of the chaffinch, a descending scale ending with a flourish, is one of the characteristic signs of spring in England.

5 COLLARED DOVE

Streptopelia decaocto

An elegant, small dove that has arrived in Britain only in the last 60 years, after a spread westwards across Europe from its original territory in the Balkans.

6 BLACKBIRD

Turdus merula

The garden bird with the most beautiful song of all. Many people find the rich, liquid fluting of the blackbird to be heart-stopping, especially in the dawn chorus, when it is highlighted by the quiet.

7 GREAT TIT

Parus major

The most boldly marked of the tit family and noticeably larger than the blue tit, but not quite so acrobatic as its smaller cousin.

8 GOLDFINCH

Carduelis carduelis

The exciting new addition to many British gardens, attracted by the specialised new bird food, nyjer seed. The brilliant colours, especially that scarlet face, identify it at once.

9 WOODPIGEON

Columba palumbus

One of the success stories of the natural world in Britain in the last half-century, this bird has increased in numbers by several hundred per cent, perhaps because it can live with intensive agriculture.

10 HOUSE SPARROW

Passer domesticus

The urban cheeky chappie of the bird world has virtually disappeared from town and city centres where it once flourished, but it can still be seen in many gardens.