Leach's Storm-Petrel, also known as Leach's Petrel or Mother Cary's Chicken, is the most widespread procellariiform breeding in the Northern Hemisphere. More than 6 million pairs nest in burrows or crevices on Atlantic islands from Norway to Massachusetts and on Pacific islands from Baja California to Hokkaido, Japan. Outside the long nesting season, these seabirds disperse widely in the Atlantic and Pacific oceans, well away from land and mainly in the tropics. Millions more nonbreeding individuals, mostly immatures, remain at sea year-round, although many of them visit colonies during the nesting season, often covering vast distances. Small and dark and not usually gregarious or attracted to ships, this species is inconspicuous at sea. Much work remains to determine the nonbreeding distributions of these populations.