A visitor trail and wetland nature reserve which is the size of 100 football pitches has been completed on farmland across the River Trent from Gainsborough.
The RSPB's Beckingham Marshes project – in partnership with the Environment Agency – features 100 large wet ponds, 4km of ditches and three wind pumps to circulate the water.
"Dragonflies, tree sparrows and lapwings are settling into the new reserve," said RSPB site manager Michael Copleston.
"It's also been fantastic to see new records of curlew, lapwing, water voles, barn owls and brown hares."
RSPB volunteer Chris du Feu said: "The reserve is already different with all sorts of species returning.
"A pair of curlews seeing off a hunting buzzard was a sight unimaginable here, even five years ago."
Environment Agency area manager Lee Rawlinson said: "The work which the RSPB and the local community have done is really exciting and continues to bring lots of additional benefits for the area.
"Beckingham Marshes is an excellent example of how we can together improve the environment to be enjoyed by people and wildlife."