Cranes have been found breeding in the fens of East Anglia after 400 years.
The Suffolk wetland which the birds are nesting in was a carrot field until the RSPB turned it into Lakenheath Fen Nature Reserve 11 years ago.
Large-scale drainage of fens for agriculture had led to the birds' disappearance in the 1600s.
RSPB chief executive Graham Wynne said the arrival of the birds came ahead of plans to make further wetland reserves in the Norfolk and Cambridgeshire fens.
RSPB staff discovered the nest by chance while carrying out a routine survey of the site. It is believed the eggs are near to hatching.
COMMON CRANE (Grus Grus)
Global population: 220,000
The RSPB plans to create 20 sq miles (51 sq km) of wetlands in the fens over the next 20 years.
Mr Wynne said: "The arrival of cranes at Lakenheath Fen is fantastic news for the future of this species in the UK.
"It is also testimony to the truly inspirational work done at the site - we have gone from carrot fields to cranes in 11 years.
"Their decision to nest on the reserve was totally unexpected, very exciting and completely wonderful."
A new visitor centre at Lakenheath Fen Nature Reserve opened on Thursday.
The common crane is one of the biggest European birds. It nests in marshy vegetation, and populations all over Europe have suffered historically from the disappearance of wetlands.
Its wingspan can reach 240cm (7.8ft) and has a loud bugling call.
Adults have a grey body and black, white and red markings on the neck and head.
The reserve at Lakenheath was restored from a carrot field
The bird has a long history in England: it appears on medieval illuminated manuscripts and was on the menu for a feast held in York by Henry III in 1251.
Their presence is also recorded in place names such as Cranfield and Cranbrook.
The fens of East Anglia used to cover an area of roughly 3,336 sq km (1,300 sq miles). But the draining of these lands in the 17th Century, coupled with persecution by people, led to the crane's complete disappearance as a breeding bird.
Lakenheath Fen was created to act as a wetland habitat for another bird, the bittern. Between 1996 and 2002, the RSPB converted over 200 hectares (500 acres) of arable land into reedbeds and damp meadows.