Cant claim absolute certainty, but as an educated guess
Who=National Trust(its their land)or a contractor
Why=Trap Mink
Why=Yes, they are a huge problem, the difficulty is though, the traps are almost never sprung as the mink is too sly a critter.
They are a huge problem elseware in derbyshire too, and its not just the birds they go for, anything is fair game.