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.
An interesting and thoughtful post Chris. We got 19 too! Which is just slightly below our average, we missed some usual suspects completely - eg magpie. But saw bullfinch (relatively unusual for us) and siskin and goldcrest which we don't always. We must do the comparsion with our garden too!
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