| Updated: 17:22, 22 January 2021 Lincolnshire Wildlife Trust has closed Willow Tree Fen in the hope that the newly arrived pair of cranes will repeat the breeding success on the nature reserve last year â the first time the birds bred in Lincolnshire for 400 years. Cranes form lifelong pair bonds and are very site loyal. The pair that nested at Willow Tree Fen were expected to return from their wintering grounds in February but arrived on today (Friday, January 22) and Lincolnshire Wildlife Trust closed the site with immediate effect. Cranes need peace and quiet, and to be socially distanced from people and dogs. Closing the site will give the cranes the best possible chance to nest in Lincolnshire for a second time.