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BiologyThe breeding season of Cook's petrel extends from October to April, with eggs laid in early November on Little Barrier Island and a month later on Codfish Island (8). They dig burrows in the soil in colonies, into which one egg is laid and incubated for 47 to 51 days. Adults forage at sea for periods of 2 to 13 days whilst feeding chicks (9), and chicks fledge after 88 days (2). Migration to the east Pacific occurs over winter, when they will spend their nights feeding mainly on squid, crustaceans, and small fish. Cook's petrels are most frequently seen feeding by surface-seizing; grabbing prey with their sharp bill from the ocean surface (2). Like all other petrels and closely related albatrosses, Cook's petrel has a special digestive system consisting of an upper and lower stomach. Any oil from the petrel's diet can be stored in the upper stomach, whilst the water, fat and proteins are digested as normal in the lower stomach. This oil acts as a constant source of energy as small amounts trickle into the lower stomach to be absorbed, but can also be regurgitated to feed their chicks. The oil is a sticky and foul-smelling liquid which also makes it an effective form of defence. If threatened, chicks will eject large quantities of the oil, which sticks onto the fur or feathers of a predator. Not only does this make the predator smell awful, the coated fur or feathers lose their insulating and waterproofing properties, which can be fatal (10).