Birdwatchers of Britain, your hour has come

Of all the weekends in the year, none is so uniquely, madly, and wonderfully British as the last weekend in January. Next Saturday and Sunday, well over half a million of us will settle by a window for an hour, with notebook, binoculars, and field guide to hand, ready to record and count every bird that enters our gardens. Some chests swell to the sound of martial music, but ours do so at the thought that no other nation would be so magnificently devoted to wildlife – and daft.

The occasion, as it has been since 1979, is the Big Garden Birdwatch (BGB). It began, and continued for about 20 years, as an exercise for junior members of the Royal Society for the Protection of Birds. Then, noting that increasing numbers of adults were joining in, the RSPB threw it open to all in 2001, and added online filing for their birdwatchers. The numbers of participants exploded: from 55,000 in 2001, to more than 500,000 today. Altogether, the BGB has meant Britons have spent, over the history of the project, around 3 million hours (the equivalent of 380 years) counting the birds in their gardens.

The result is not only the world's largest mass participation wildlife survey, but a data set stretching back over 31 years, and providing a remarkable snapshot of what is happening to our lowland birds. The weather can have a marked effect on sightings. A warm winter here, such as in 2007, can mean fewer birds foraging in gardens, while a cold one on the Continent can bring a sizeable influx. The RSPB's Richard Bashford also points out the influence of proprietary bird feed, now a very sophisticated market. Teasel and thistle seeds, designed to attract goldfinches, for example, may be responsible for increased sightings.

This year, the RSPB hopes for a bigger participation than ever, especially to monitor the effects of the cold November and December. Take part by going to rspb.org.uk/birdwatch/. To show how the sightings, and species' fortunes, have changed since 1979, we give here the top 10 birds spotted over a number of years, together with a summary of that winter's weather, and the number of spotters.

1979

Average per garden:

1. Starling 15

2. House sparrow 10

3. Blackbird 4

4. Chaffinch 3

5. Blue tit 2.44

6. Robin 2

7. Greenfinch 1

8. Great tit 0.90

9. Dunnock 0.8

10. Song thrush 0.56

Number of spotters: 30,000

Weather: The most severe winter since 1962-63, January averaging below freezing.

The first year, and the striking thing is the quantity of birds per garden, the species most frequently spotted turning up in gardens in numbers three to five times greater than today, despite the harsh winter. The totals for 1979's house sparrows and starlings are often compared with those in succeeding years to point up the extent of decline, but they are so high that there are grounds for scepticism about their accuracy. It takes some believing that numbers of starlings, house sparrows, blackbirds and chaffinches all fell by two-thirds in the eight years to 1987. Just missing the 1979 top 10 are the magpie, at 11; collared dove, at 12; coal tit, at 14; and wren at 15.

1987

Average per garden:

1. Starling 5.52

2. House sparrow 3.75

3. Blue tit 1.25

4. Blackbird 1.2

5. Chaffinch 1.03

6. Greenfinch 0.66

7. Robin 0.55

8. Great tit 0.51

9. Dunnock 0.45

10. Song thrush 0.33

Number of spotters: 15,000

Weather: Cold, dull January, the 12th being the coldest day of the century in many places.

The severe weather of the days leading up to the BGB could go some way to explain the decline since 1979, but, as that was cold, too, the startling decline is deeply puzzling. The robin, for example, whose numbers are a mere quarter of what they were eight years before, is a bird better able than many smaller birds to survive winters, very rarely suffering more than a 50 per cent decline in even the coldest years. Incidentally, the magpie, whose numbers roughly trebled in the 20th century, is, in 1987 (as almost every year), hovering just outside the top 10. In 2010 it will fall to 14th.

1998

Average per garden:

1. Starling 3.3

2. House sparrow 3.1

3. Blue tit 2.4

4 . Blackbird 2

5. Chaffinch 1.9

6. Greenfinch 1.3

7. Great tit 1.1

8. Robin 0.9

9. Collared dove 0.9

10. Wood pigeon 0.7

Number of spotters: 11,687

Weather: Mild, but with a very windy January.

The top two birds appear in the garden in smaller numbers, but, curiously, the totals for birds in other positions rise. The top half dozen are almost the same species as 1979, although the robin falls from sixth to eighth, with only half as many per garden as then. The real interest is in the new entries, with the collared dove, a bird not seen in Britain until the mid-1950s, and wood pigeon, entering the top 10. The poor old song thrush, at No 10 in 1979, now falls to 14th, and the dunnock, despite more of them being seen than in the first year, goes down to 12th. The crow, which made a brief appearance in 1993's top 10, drops out again.

2003

Average per garden:

1. Starling 4.89

2. House sparrow 4.85

3. Blue tit 3.11

4. Blackbird 2.72

5. Chaffinch 2.19

6. Greenfinch 1.87

7. Collared dove 1.69

8. Great tit 1.47

9. Robin 1.37

10. Wood pigeon 1.32

Number of spotters: 314,645

Weather: Mild December, and exceptionally sunny January.

The rise of the collared dove continues, as does that of the wood pigeon, with nearly twice as many of the latter per garden as in 1998. Fluttering just outside the top 10 are the dunnock (11th), long-tailed tit (13th), coal tit (14th), and goldfinch (15th), all of which are benefiting from a run of mild winters. The song thrush is 17th. For the first time in the years we have selected, all the top 10 have averaged sightings of at least one per garden. But there can be sharp changes in a few years. Between 2004 and 2008, the average number of birds seen per garden fell from 34.8 to 28.4.

2010

Average per garden:

1. House sparrow 3.87

2. Blackbird 3.28

3. Starling 3.13

4. Blue tit 2.58

5. Chaffinch 2.19

6. Wood pigeon 1.91

7. Robin 1.49

8. Great tit 1.39

9. Collared dove 1.33

10. Goldfinch 1.29

Number of spotters: 529,076

Weather: January was the coldest month since 1987.

The second tough winter in a row sees the house sparrow take top spot (as it did in 2004 and 2005), with starlings slipping to third. Also down sharply, are smaller birds such as the coal tit, long-tailed tit, and goldcrest, victims of cold weather. Wood pigeon climbs to sixth (it was 13th in 1979), and goldfinch makes an appearance in the top 10, which it has only begun to do in recent years. The song thrush continues its decline. Once in the top 10, it is now down at number 21. It is one of seven of the more common birds which have shown a fall since 1979, while eight have risen, the wood pigeon and collared dove by more than 800 and 300 per cent respectively.