Pied avocets stopped breeding in Britain in the nineteenth century, driven out by drainage of coastal marshes and by collecting of eggs and skins. Their requirement is narrow: shallow brackish water over soft mud, with bare islands for nesting and few ground predators.
During the Second World War, stretches of the Suffolk coast were flooded as a defence measure, and grazing marsh reverted to shallow saline lagoon. Avocets bred at Minsmere and Havergate Island in 1947, and the sites were subsequently protected and managed. The bird was adopted as the RSPB's emblem, and the British population has grown substantially since, entirely on habitat that is actively maintained.
The usual telling makes this a happy accident, which is only the first half. Lagoon habitat silts up, becomes fresher and grows over without intervention, so the population depends on continuing water-level management and predator control. That dependency, rather than the wartime origin, is the durable point: the avocet's British range is a managed outcome, not a recovered natural one.