The island is a Special Protection Area (SPA), particularly because of its great cormorant colony of over 750 birds, which is over 10% of the national population, making it one of the largest colonies in the British Isles. It also has good numbers of guillemot, razorbill, shag and kittiwake nesting, and in recent years small numbers of common eider and black guillemot. The Atlantic puffins from which the island gets its modern English name bred in considerable numbers at one time, with up to 2,000 pairs recorded. The brown rat was introduced accidentally to the island, probably in the late 19th century, and reduced this population to a very few pairs. A programme of poisoning these rats started in 1998 by the Countryside Council for Wales appears to have eradicated them, and the puffin population has shown an increase since that date up to 300 birds. Both Manx shearwater and storm petrels are also thought to breed on the island .