The booming sound of a male bittern is being hailed as a sign of success for a 10-year programme to bring the species to a nature reserve.
Bitterns are said to boom when trying to attract a mate
The call has been heard several times at the Royal Society for the Protection of Birds' Lakenheath Fen reserve on the Suffolk-Norfolk border.
The 267-hectare carrot field bought in 1995 has been converted into reed beds.
Bitterns have been seen at Lakenheath Fen before but this is the first time that its familiar call has been heard.
The RSPB is hoping a female bittern joins the male who they say booms to attract a mate and establish a territory.
Other reed bed specialists such as marsh harriers and bearded tits have already colonised Lakenheath Fen.
Norman Sills, warden at Lakenheath Fen, said: "The bittern was always the prize species because it is very rare and very fussy about its habitat.
"Bitterns need dense reed beds for nesting, and we've designed the reserve with lots of reed edge too, giving access to open, clear water for catching fish."