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 BGB 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.

RSPB BGBW & BTO GBW

Last weekend I took part in the RSPB* Big Garden Birdwatch (BGBW), along with half a million other people.  I saw 19 species in my garden in one hour, compared with a recent average of 23 per week.  I have been submitting weekly counts of birds in my garden to the BTO* Garden BirdWatch (GBW) since 1995 so I thought it would be interesting to compare my own GBW recordings with the national results from the BTO GBW and the RSPB BGBW.

The data sets are obviously different and this makes them a bit difficult to compare. The data I can get on my garden from the BTO GBW website are total rankings for all four seasons over 23 years whereas their national data are for the first quarters of each year for 2003-2015.  And the RSPB numbers are for the (one hour) 2017 survey only.  All are presented as rankings.  The BTO results for my garden and for the UK as a whole are of presence rather than abundance.  In other words, the likelihood of seeing a species rather than the number of individuals present.  The RSPB results, on the other hand, are for total numbers of birds (I think) so are for abundance rather than prevalence.

My garden is a bit different from the average.  It is just about one acre and has dozens of large mature trees.  Despite being just inside the city boundary it is more rural than suburban and the numbers above reflect that.  Much as I would like more house sparrows, I am very happy that the great spotted woodpecker is my third most reliable garden visitor. My top nine birds were all present in more than 90% of weeks recorded.

As mentioned above, the RSPB numbers are collected in a one hour observation period once a year.  The strength of this survey is the huge number of participants.  I suspect many people would have chosen to record numbers in the first half of the morning as I did and this may introduce a bias.  I was surprised to see that sparrow and starling ranked so highly (1st and 2nd) until I realised they are counting numbers of birds rather than % of gardens.  (Obviously sparrows and starlings tend to go around in gangs, whereas woodpeckers and nuthatches usually go around in ones or twos.)  I am not sure this is the best way to present their findings.

The BTO GBW started in 1995 and has about 6600 participants, down from a peak of 12,500 a few years ago.  Its recorders are more committed (they provide the data and pay a subscription to fund data collection and analysis), and probably more expert.  It now has over 1200 weeks of recordings and so is very valuable for looking at long term trends.  It does also collect numbers of birds but ranks birds by the likelihood of being seen rather than total numbers.

My own BTO GBW numbers are only retrievable as totals for the 23 years but in that time a lot of things have changed. House sparrows have been absent from my garden since 1997 and tree sparrows only arrived in 2012.  Woodpigeons, woodpeckers and goldfinches are all more common than they used to be, while my rookery gradually got smaller and was then abandoned after the 2011 season (averaged over 23 years, rooks still rank 15th on my list).

Birds are under threat from habitat loss, climate change, industrial farming, etc so all data on bird populations are valuable.  Both the BTO and the RSPB run many different surveys, so these two are just two pieces of the jigsaw.

*  For non UK readers, RSPB = Royal Society for the Protection of Birds and BTO = British Trust for Ornithology.