amber, ffs where have these people been looking. Err ..... At the entire length and breadth of the British Isles, mate.
You, or I might glance out our window and get a snap shot of what we Think is going on out there ~ like my own, famous 'six or seven great tits'
But, the BTO employs a vast network of dedicated observers, right across the land, who specifically go out and head count every single specimen and send in their official reports, to be number crunched by specialist scientists at HQ.
That's what they do. It's what I do. It's why I ring birds. I also head count and rep