The RSPB said a male hen harrier vanished from a Forest of Bowland nest three weeks ago, with males at two others not seen for a week.
Male hen harriers disappearing while part of an active nesting attempt is "exceptionally unusual", the charity added. Police are investigating.
The reward is for any information which leads to a successful conviction.
The hen harrier is listed as a red status species, meaning that it is threatened and that populations have suffered a severe decline in numbers.
In the absence of males to hunt for food, females at two of the nests were forced to abandon their eggs, the charity said.
Last year there were only two breeding pairs in the Forest of Bowland.
Martin Harper, from the RSPB, said: "Hen harriers are hanging on by a thread in England and the disappearances of the past few weeks have made a desperate situation even worse.
"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."
Source: BBC Nature
A £10,000 reward has been offered after three rare birds of prey disappeared in Lancashire.