Parts of the north of England are "black holes where endangered hen harriers simply disappear", Cumbria Wildlife Trust has said.
It follows the disappearance of "Blue", a satellite-tagged male whose transmission suddenly and inexplicably cut out near Longsleddale, Cumbria.
The bird's two brothers also went off the radar in what are described as similarly unsettling circumstances.
A police investigation is under way as hen harriers are a protected species.
They are also one of the UK's rarest birds of prey with just three successful nests recorded in England in 2017.
Neil Harnott, from Cumbria Wildlife Trust said: "The English population of hen harriers is on the verge of extinction.
"Cumbria should have far more of these magnificent birds breeding in our uplands but the evidence would suggest that continued illegal persecution by grouse shooting interests means that the whole of England only had three breeding pairs last year, with none of them in Cumbria.
"It is heart-breaking that with the loss of Blue, all three siblings raised in the Scottish Borders in 2017 have now disappeared.
"Not one managed to survive to breed."
Amanda Anderson, director of the Moorland Association, said there were many initiatives to restore England's hen harrier population.
She said: "This year has seen many hen harrier breeding successes on moorland managed for grouse, such as the recent discovery of eight hen harrier chicks in Bowland estate, Lancashire and a further nest in Derbyshire.
"What is needed now more than ever is a collaborative approach and a commitment to working with each other to restore hen harrier numbers to healthy levels."