Big Garden Birdwatch 2019 - Feral Pigeons - only 16.1% of gardens!

Looking at the Big Garden Birdwatch results for 2019, I was surprised to see that feral pigeons were observed in only 16.1% of gardens in England.

In my South London garden, they are regular all-day 'hooverers' of every type of bird food that falls to the floor.

Any thoughts?