DYK
Did you know that there are six million breeding pairs of blackbirds in the UK, each pair capable of producing up to four broods each year? No, neither did we.
Common blackbird song recorded by Beatrix Saadi-Varchmin (CC BY-NC-SA 4.0) xeno-canto.org
Southwick Country Park Nature Reserve 
I originally posted that blackbirds are the most numerous breeding bird in the UK – and I attributed the claim to the RSPB – but I was wrong on both counts. I have corrected the post; my apologies to you and the RSPB with the advice to always do your factchecking before you press publish, not after.
I checked with the RSPB and the commonest bird is a wren.