This is organised by the RSPB every January. The idea is simple. You count the maximum number of each species seen at the same time in your garden, park etc. for any hour over the weekend and submit this. From these returns, the RSPB can learn a great deal about the birds around.
Certainly here, there are fewer smaller birds around. It is not uncommon to see 6-12 different species, but it is unusual to see that many different small birds. There are far more pigeons that in the past.
I do it every year. It is surprising what appears over the hour.
UPDATE 1625z: Just 6 species seen! Way down on previous years. Most of these were larger birds. Some very common birds were missing. Overall? January!! Dull and cold.
See https://www.rspb.org.uk/get-involved/activities/birdwatch/ .