The RSPB has appealed to the public for information and offered a reward after three Hen Harriers disappeared from the north-west of England in suspicious circumstances.
Three of the birds of prey have vanished from the United Utilities Bowland Estate in the past three weeks, leading to the failure of two nests and meaning only one now remains.
The first male bird disappeared in late April, but fortunately another appeared to take his place and the female was able to remain sitting on the nest in this case.
However, two further males then went missing and the associated females were forced to abandon their nests and the eggs within them because they no longer had partners to provide them with food.
A reward of £10,000 is now being offered by the RSPB for information leading to a conviction relating to the disappearance of the birds, with police also investigating.
"We don't know what has happened to these three birds, but we will find out and we will save our Hen Harriers. This is an awful setback, but it will not stop us," said RSPB conservation director Martin Harper.
Head of nature policy Jeff Knott added in an interview with the Telegraph: "This has been the last stronghold in England for the past few years. That's why it's particularly worrying that three birds have now disappeared. The difficulty increasingly is protecting the adult birds away from the nest. The major problem affecting hen harriers is illegal persecution."
There were only four successful Hen Harrier nests in the whole of the UK last year, while two tagged birds also vanished from the same Lancashire habitat last September.
Meanwhile, the Joint Links coalition has warned that British species and habitats face their "biggest threat in a generation" because of a European Commission 'fitness check' on the Birds and Habitats Directives that may lead to protective laws being watered down.
The law has facilitated the creation of nature reserves for endangered and migratory birds since 1979 and the coalition says weakening these policies would be a retrograde step.