Adults and young are vulnerable to predation at the breeding colonies, their only defence being to spit oil. Petrels cannot breed on islands where rats have been introduced, and feral cats frequently kill these birds on Foula in the Shetland Islands. The American mink, a non-native species in Europe, is a strong swimmer, and can colonise islands up to from the mainland. Natural predators of petrels and other seabirds include skuas and large gulls. The yellow-legged gull is a particular problem in the Mediterranean, and great skuas were estimated to kill 7,500 petrels a year on St Kilda, an unsustainable number. Some great black-backed gulls on the Atlantic islands specialise in taking seabird chicks at night, and peregrine falcons hunt adults at sea. Localised predators include Eleonora's falcon on the Columbretes Islands and the nocturnal barn owl on the Balearics; a few owls can wipe out a colony. The little owl is also a predator of both adults and young where it occurs.