Mandø sits within the Wadden Sea National Park, a UNESCO World Heritage Site and one of the most important bird habitats in Europe. The tidal flats surrounding the island support enormous concentrations of wading birds during the spring and autumn migrations, when species including dunlin, red knot, curlew sandpiper, grey plover, and bar-tailed godwit gather in numbers that can be counted in the tens of thousands.
The island also supports breeding populations of rarer species. The little tern nests here, as does the white-breasted pratincole on occasion. Seals haul out on the sandbanks visible from the seaward side of the dyke, and harbour porpoises are sometimes seen in the channels.
The best vantage points are from the top of the dyke that protects the island's eastern side, and from the flat terrain near the causeway where the flats are exposed at low tide. Bring binoculars — the distances are significant. The late summer and autumn migrations are the peak season for numbers, but spring brings the spectacle of birds in full breeding plumage.