Marsh Harrier

That's a tricky one, I think the only certain answer is, when they are hungry! Over the last few weeks I've been there a couple of days when they have been out hunting, both times mid morning. I think mornings are more likely, but there is a lot of luck involved, and even when they are hunting if they catch a decent meal they will just go to ground and vanish. As all the winter visitors are stating to arrive, I would say arrange a visit anyway, as if the marsh harriers are not around there should be plenty of other stuff to see.