There are really two islands here, plus a pair of rocks some charts call the Watchers. Waldegrave Island is the larger; Little Waldegrave lies about a kilometre to its west, a flat-topped table roughly a kilometre long and 32 hectares in area, its sides dropping abruptly to the swell. Underneath the thin skin of windblown limestone is a crystalline basement of ancient rock, the same hard stuff that once formed a far more prominent headland. The geology reads like a before-and-after photograph taken across millennia. What you see above the waterline is only the part that refused to drown. To the north, the coast is rocky but sheltered from the southerly swells that batter everything else, which is why the islands' northern shores are the only practical way ashore.

Little Waldegrave is a maternity ward. On its ledges, Australian sea lions haul out and pup, part of a colony numbered at 38 animals near the end of the last century. These are among the rarest sea lions on Earth, an endangered species found only along the southern and western coasts of Australia, and every breeding rock matters. The islands are also the second most important breeding ground in South Australia for Cape Barren geese, those handsome grey birds with a startling lime-green cere above the bill. Little penguins come ashore here too, hundreds of pairs nesting in burrows and crevices, slipping out to sea at dawn and waddling home after dark. White-bellied sea eagles patrol overhead. The whole arrangement is fragile enough that the islands have been off-limits to grazing and development for decades.

On Wednesday, 10 February 1802, Matthew Flinders sailed past in HMS Investigator, charting a coastline no European had mapped, and named the group after William Waldegrave, 1st Baron Radstock, a naval officer who had fought at the Battle of St Vincent five years earlier. Flinders' crew, hungry after weeks at sea, killed a few of the sea lions on the larger island. The same name still marks the nearby mainland headland, Cape Radstock. For a stretch of the nineteenth century the islands were stripped for guano, the nitrogen-rich seabird droppings prized as fertiliser, mined under government licence before 1919. Waldegrave Island was grazed by stock until 1967, when grazing finally gave way to conservation.

Since 1972 the islands have formed the Waldegrave Islands Conservation Park, set aside specifically to protect Cape Barren goose breeding habitat and sea lion haul-out areas. The geese don't stay put; in summer the flock spreads inland to the Elliston district, feeding in the swamps around the margins of nearby Lake Newland and, less romantically, on grain in the wheat paddocks. The waters around the islands are clean and unspoilt, productive grounds where Elliston's boats chase King George whiting and crayfish. It is a small place that does a great deal of work: a fragment of drowned headland that became, almost by accident of geography, one of the safest cradles on this coast for animals that had nowhere else to raise their young.

The Waldegraves do not stand alone. They belong to the Investigator Group, the scatter of islands Flinders charted along this coast, and they fall within the Investigator Islands Important Bird Area, a designation that recognises the whole archipelago as critical breeding habitat for seabirds. Surveys on Little Waldegrave between 2001 and 2006 turned up a long roll call of birds: white-bellied sea eagles and peregrine falcons among the hunters, rock parrots and white-fronted chats in the scrub, ruddy turnstones and red-necked stints among the migratory waders that pause here on journeys spanning hemispheres. The plant life is humbler and partly introduced, native juniper and coast daisy-bush tangled with African boxthorn left over from the grazing years. Slowly, with people kept at arm's length, the islands are reverting to what they were before anyone arrived to mine, graze, or hunt them.