Given that they nest on the sort of terrain where grouse are reared and shot that's probably where nasty things will happen to them.
While there's no disputing there is illegal killing of Hen Harriers they have also been preyed on by Eagle Owls in the past. If anyone knows the true numbers of them they'll keep quiet, because the pro Eagle Owl faction would be as unhappy about it as the pro Hen Harrier faction would.
How inconvenient would it be for the Hen Harrier supporters (or shooting opponents...) if it turned out Eagle Owls were responsible for bumping off a great proportion of the birds that 'go missing'?
I'm not suggesting that is the case, just putting forwards an alternative narrative. I wonder if White Tailed Eagles have a taste for harrier?
Everyone has an agenda and spins the facts to suit their cause.