The 1824 reclamation was a major piece of Georgian civil engineering. The Cob shut out the sea; the canalised Afon Cefni drained the resulting freshwater marsh through a controlled channel down to Malltraeth Sands. About 230 acres in the parish of Llangaffo alone went into private ownership under the act, and over the next century farmers steadily improved drainage with more embankments and flood-mitigation works. During the First World War the work was pushed harder still, then neglected after the war ended. By the inter-war years the reclamation was in poor condition, and the underfunded catchment board was locked in heated disputes with the drainage engineer and the County War Agricultural Executive Committee. The marsh kept doing what marshes do when humans stop fighting them: it crept back.

On the east side of the Afon Cefni, the wetland habitat has now been deliberately re-created. The RSPB took over part of the marsh and reflooded it, restoring reed beds, shallow pools, and wet grassland. The reserve is officially called RSPB Cors Ddyga - Cors Ddyga meaning Tygai's marsh in Welsh - and it has become one of the best places in Wales to find bittern, that secretive, deep-throated heron of the reedbeds. Marsh harriers nest here too. In winter the mudflats and shallow lagoons host over one per cent of the British population of pintail, that elegantly long-tailed duck. The geology underfoot is Carboniferous limestone, millstone grit, soft coal-measures shales, sandstone, and coal seams - and the valley of the Cefni runs almost exactly parallel to the Menai Strait, like a smaller sibling separated by a low ridge.

In 1947 Charles Tunnicliffe moved into a farmhouse called Shorelands on the marsh edge at Malltraeth. Tunnicliffe, born in Lancashire in 1901, had already established himself as Britain's leading wildlife illustrator - his wood engravings illustrated Henry Williamson's Tarka the Otter in 1932, and his work was familiar from countless Ladybird books, Brooke Bond tea cards, and Royal Society for the Protection of Birds publications. From Shorelands he had the marsh, the estuary, and the sands of Malltraeth on his doorstep. Tunnicliffe filled hundreds of sketchbooks and "measured drawings" - meticulous studies of dead birds laid out with rule and compass, recording every measurement and feather pattern. He worked from Malltraeth for thirty-two years until his death in 1979. The bulk of his archive is now held by Oriel Mon in Llangefni, where it sits next to the paintings of Kyffin Williams - the two great visual interpreters of Anglesey under one Welsh roof.

Modern conservation here is a constant low-grade negotiation between water levels, predators, and plants. Clay-lined ditches are maintained to keep the water table where the reeds and the bitterns need it. Specific swamp areas are kept reserved for bittern breeding. Shallow water ditches are cut along old water courses so waders can feed in their preferred ankle depths. In spring the water control structures lift the groundwater high enough that snipe and lapwings nest in the wet grass. Invasive species - fairy fern, Australian stonecrop, Himalayan balsam - are pulled out wherever they appear. Hedges are kept dense to baffle predator crows and mink. Cattle graze in winter and are pulled off in summer to let ground-nesting birds settle. The marsh that 19th-century engineers tried to turn into farmland is now, in part, deliberately farmed for its wildness.