Marsh harriers have been found nesting at a nature reserve threatened by airport expansion, the RSPB has said.
Marsh harriers are nesting at Dungeness, the RSPB has said
The charity said the birds at Dungeness were one more reason why London Ashford Airport, in Kent, should not expand.
Reserve manager Bob Gomes has said more flights and larger aircraft will cause a "huge disturbance" to birds there.
The airport said it had carried out environmental surveys, adding it was proven that birds and aviation could "co-exist in the same surroundings".
Expansion plans at the Lydd-based airport are being considered by Shepway District Council.
Managers have said it will be able to attract two million passengers yearly by 2015.
The plans are to extend the runway and build a new terminal.
Airport planners want a longer runway and new terminal building
But Mr Gomes said: "Airport expansion could not come at a worse time for Dungeness.
"It has long been an invaluable site for wintering, breeding and migrating birds."
The RSPB said this was the first time harriers had nested at Dungeness, with three birds known to have made maiden flights from reed beds there.
But the airport's managing director, Zaher Deir, said: "We believe that the RSPB is panicking for no reason.
"We have invested a huge amount of time and resources in an environmental impact assessment, and as part of this we have carried out three wintering bird surveys and this data has been included as part of our submission to the planning authority."
The reserve on the Dungeness shingle peninsula was bought by the RSPB in 1931.
It is said to attract 120,000 birds in winter, including large flocks of gulls, ducks and lapwings.