Counting the birds

Like many of you, I expect, I took part in the RSPB* Big Garden Birdwatch (BGB) last weekend.  It involved recording the largest number of each species seen at one time during one hour of birdwatching.  Although it has limitations the BGBW gets a lot of publicity and encourages many people to take note of the birds in their gardens.  Despite its size, I have reservations about the way this survey analyses its results.  It prioritises abundance so it is biased in favour of birds that go around in groups and sit around on feeders long enough to be counted, such as finches, rather than birds that dash in, grab a seed, and take it off to eat in cover, such as tits.  In my garden I expect there are more coal tits than greenfinches, yet I rarely see more than two coal tits at one time whereas it is easy to see six greenfinches sitting on the feeder together.

I wrote previously about the differences between the RSPB BGB and other surveys such as the BTO* Garden BirdWatch, which is a weekly survey which has been running since 1995.  The main difference is that while the RSPB BGB prioritises abundance, the BTO GBW prioritises presence, so they reach different conclusions about which is the "commonest" garden bird.  For example, if in ten gardens a robin was seen in eight and six starlings were seen in each of two the RSPB BGB would decide that starlings are the commonest garden birds whereas the BTO GBW would choose robins.

In the RSPB BGB in 2020 nearly half a million people recorded almost eight million birds and the "top ten" were

1. House sparrow
2. Starling
3. Blue tit
4. Wood pigeon
5. Blackbird
6. Goldfinch
7. Great tit
8. Robin
9. Long-tailed tit
10. Magpie

This is despite the fact that house sparrows have declined by 53% since the survey began in 1979 and starling numbers have fallen by 80%.  Both are on the Red List of birds of conservation concern.

This chart shows the BTO GBW results for 2019 (green), the latest available, compared with 1995 (yellow), the first year of the survey.  It shows that the birds you are most likely to see in the garden are blue tit and wood pigeon, with house sparrow in 7th place and starling 13th.

This chart shows the winners and losers over 25 years.  House sparrows, starlings, greenfinches and song thrushes have fared badly for various reasons whereas goldfinches and wood pigeons have done well.

I suppose it doesn't matter all that much whether presence or abundance is given priority so long as it is clear what people are talking about.  However, the 2021 results in a few weeks' time will also get a lot of publicity and it will be a pity if it isn't made clear how much trouble these birds are in.

Last weekend I managed a few photos through the kitchen window when there was a brief break in the cold, wet, sleety, windy weather we are having.  These are biased in favour of birds that sit on my "woodpecker" feeder, a dead branch with suet in a hole.  If the weather improves I'll also take some of the birds on the seed feeders and post them later.

*For readers not from the UK RSPB = the Royal Society for the Protection of Birds and BTO = the British Trust for Ornithology.