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

In the latest mishap, US citizens Bob Weise and Steve Shapiro, both 71, had to seek help in Hayle Harbour, Cornwall, when their sailing boat tipped over, causing a fire on board.
Veteran sailor Sir Robin Knox-Johnston is among those criticising the pair.
The duo previously told BBC News they were experienced enough for the trip.
The boat, named Nora, left Norway in July and is aiming for the US state of Maine.
Sir Robin said: "This is no longer a joke.
"It costs between Â£6,000 and Â£8,000 every time a lifeboat is launched. These guys are costing the RNLI a fortune.
"They need to frankly pack it in or, I hate to say it, get the hell out of our waters."
James Instance, from Falmouth Coastguard, said he was not aware of anyone being rescued so many times in a short space of time.
Peter Haddock, Hayle harbourmaster, said he was worried about the sailors' safety and that of emergency services.
Mr Haddock said he raised the alarm on Tuesday after seeing smoke coming from the vessel's forward hatch and described the pair as a "catastrophe waiting to happen".
Fire crews extinguished a "small fire" on the 18-tonne yacht, which Mr Shapiro said was caused by a candle, and that "otherwise there was no real damage".
Mr Shapiro, a screenwriter and author from California, said he had been sailing since he was nine and "people had a right to be angry".
"But it's not justifiable, we have the right to the services as much as anybody else.
"The volunteer organisations volunteered to come out, it was their decision."
Mr Shapiro is accompanied by Mr Weise, who is an ex-US Army helicopter pilot and Vietnam veteran.
The RNLI and Maritime and Coastguard Agency (MCA) refused to comment on how much the rescues would have cost.
A spokeswoman for the MCA said: "Search resources are always used in incidents where people report seeing someone in trouble.
"We would always urge people to call 999 and ask for the coastguard if they're in danger or think someone else might be in distress.
"We never put a price on human life. If we get a 999 call we respond and treat it as an emergency."
A £10,000 reward has been offered after three rare birds of prey disappeared in Lancashire.
[NEXT_CONCEPT]
Two yachtsmen who have been rescued nine times in seven months have been described as a "catastrophe waiting to happen" and urged to "leave UK waters".