Vikings named it Skalmey, the cleft island, after the way two bays nearly slice its eastern end clean off the main body. That eastern bulb, called the Neck, is connected to the rest of Skomer only by a thin isthmus of land, and visitors are forbidden to cross it. The whole island sits low in the water, plateau-flat with a single rise to Gorse Hill, and you could walk its length in a long morning if the paths permitted. They mostly don't. Skomer is a working nature reserve, and the puffins, shearwaters, and storm-petrels dictate where human feet may fall. The volcanic bones beneath the turf are Silurian basalt and rhyolite around 440 million years old, including a rock so locally distinctive that geologists named it skomerite, an altered andesite found here and traceable across the Marloes peninsula on the mainland.

Walk Skomer in the right light and the ground reveals what the bracken usually hides: faint rectangles of stacked stone, the outlines of fields a Bronze Age farmer once worked. Airborne laser scanning combined with excavation by archaeologists from Sheffield, Cardiff, and the Royal Commission for Ancient and Historical Monuments in Wales pushed the date of human settlement back to roughly 5,000 years ago. Iron Age boundaries, prehistoric house remains, and standing stones survive across so much of the island that much of it has been designated an ancient monument. Then, in the 14th century, someone introduced rabbits, and the new arrivals burrowed and grazed the place into a different shape entirely. The puffins later inherited those burrows for their own nesting, which is one of the small ironies of Skomer: a Norman warrener's livestock project, eight centuries on, became infrastructure for the island's most charismatic birds.

The shearwater number is the one that stops people. Around half of the entire global population of Manx shearwaters breeds on Skomer and its near neighbour Skokholm, with surveys estimating hundreds of thousands of pairs across the two islands. The birds are extraordinary navigators. A young shearwater leaves its burrow at five or six weeks old, fledges into the Atlantic dusk, and flies thousands of miles south to overwinter off the coast of South America. Five years later, the survivors return to within metres of the hole in which they hatched. The puffin colony, meanwhile, has been climbing steadily out of a mid-twentieth-century slump: 24,108 birds in 2019, 34,796 in 2020, and 43,626 counted in the most recent 2025 survey. Add guillemots, razorbills, kittiwakes, storm-petrels, oystercatchers, shags, and patrolling peregrines and short-eared owls, and Skomer in May is less an island than an aviary.

Skomer has its own mammal: the Skomer vole, Myodes glareolus skomerensis, a subspecies of bank vole that exists nowhere else. Without ground predators, the vole population swells to around 20,000 in the summer bracken, which is enough to feed an entire breeding colony of short-eared owls quartering the centre of the island at dusk. Stranger still is the island's lichen flora, which a 1995 survey put at 248 species. They paint the rocks in coloured bands you can read like geology: a black crust of Hydropunctaria maura where the waves hit, a brilliant yellow-orange stripe of Caloplaca and Verrucoplaca above it, then grey-white maritime Ramalina higher still. Where seabirds perch and enrich the rock with guano, nitrogen-loving Xanthoria parietina blazes orange. Among them, Roccella fuciformis reaches its northern limit here, surviving on this Welsh island as a Mediterranean exile.

Getting to Skomer is rationed. The Dale Princess crosses from Martin's Haven in about fifteen sheltered minutes, but the boat runs only April through September, only when the weather agrees, and never on Mondays. Visitor numbers are capped at 250 a day, with advance booking strongly recommended since a new management system came in during 2020. Sixteen lucky people each night can stay in the converted farm buildings at the island's centre, restored in 2005 with solar power for hot water and lights. The 1996 Sea Empress oil spill, which fouled much of the Pembrokeshire coast, came close enough to put the lichen monitoring teams on alert; Skomer escaped the worst of it, but the spill is the reason researchers now track those coloured rock bands so carefully.