Mmm... I think it is a bit of a grey area, as it is not widely recognised as airgun quarry- more for shotguns. Ask an expert (he said unhelpfully) Even so, I see no reason why you can't- if you are in sensible range, a head shot should finish one off very humanely.
Simple answer here is DON'T shoot them.
They are Game birds and you need a game licence to shoot them.
You'll be breaking the law if you do - be warned.
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