Basically, under licence, when hen harriers and their nests reach a certain density in a grouse moor area, eggs may be removed, hatched and reared in captivity and then released into other areas where there are no hen harriers. A proven method that has been used around the world for other species, including used by the RSPB who object so strongly to this (having first agreed to it).
https://www.gov.uk/government/news/i...lp-hen-harrier

I take it from your name you are an ex Llanidloes boy? We lived there for 25 years and now a bit further south.