The Medium-billed Prion (Pachyptila salvini), also known as Salvin's Prion, is a species of seabird in the petrel family Procellariidae. This small prion breeds colonially on a number of subantarctic islands in the southern Indian Ocean, principally Īle aux Cochons in the Crozet Islands, where four million pairs are thought to breed. Other breeding colonies include Prince Edward Island, St Paul Island and Amsterdam Island. At sea they range from South Africa eastwards to New Zealand. There are two subspecies, P. s. salvini from the Crozets and Prince Edward Island and P. s. macgillivrayi from St Paul and Amsterdam. The species is named for the British ornithologist Osbert Salvin.
The Medium-billed Prion is a small (29 cm) petrel with grey and white plumage. Like the Broad-billed Prion it has lamellae in its bill in order to filter seawater for food. The main components of its diet are amphipods and krill, although it will also take fish and squid. In addition to filter feeding food is obtained by seizing and hydroplaning.
The colonies of Medium-billed Prions are attended nocturnally in order to avoid predation by skuas. The nests are concealed in burrows usually dug into soil. Nests are attneded regularly for several months prior to breeding. A single egg is laid in November or early December, which is incubated for around 50 days. Both parents share the incubation duties and feed the chick once it is hatched. The chicks fledge around 60 days after hatching.
The Medium-billed Prion is not considered threatened. Although numbers have declined on some islands where rats and feral cats have been introduced, the world population is estimated at around 12 million birds.