Three peregrine falcon chicks are thought to have been stolen from a nest in the wild for the trade in illegal falconry birds, the RSPB said.
The wildlife charity said six men with ropes and rucksacks were seen on cliffs above the well-known nest site at Beeston Castle, Cheshire, at around the time the chicks vanished.
With no evidence of peregrines being trapped or killed, it is thought they were captured for use in falconry.
Taking birds from the wild to do this is illegal, and the theft on May 20 is being investigated by police. Proposals to remove the need for captive peregrines to be registered with the Government could lead to more being snatched from their nests, the RSPB warned.
The Department for Environment, Food and Rural Affairs is considering whether to cut the number of birds of prey species for which owners have to register individuals in captivity.
The RSPB fears the peregrine, red kite, hobby, merlin and hen harrier could all be taken off the list of species which have to be registered and ringed under Schedule 4 of the Wildlife and Countryside Act 1981.
Duncan McNiven, of the RSPB, said: "The news these chicks have been stolen is very upsetting and illustrates why we are so concerned that the law protecting these incredible birds be maintained. The bird registration scheme, in conjunction with DNA testing, allows stolen birds to be traced.
"These measures have a proven track record in reducing nest robberies and the laundering of wild birds into captivity to be sold for profit."
The peregrine falcon saw numbers plummet in the 1960s, largely due to poisoning by pesticides such as DDT, but have recovered in recent years to around 1,300 breeding pairs in the UK.
According to Mr McNiven, they still face a number of threats including illegal killing and nest robberies. "To remove the need for captive peregrines would be to remove one of their main guarantees of their protection," he warned.
>>In Pictures:Birds of prey under threat
