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BiologyA gregarious species, the northern rockhopper penguin breeds in large colonies that may comprise over a hundred thousand nests. Breeding pairs are monogamous, and usually return to the same nest every year (2). Egg-laying commences around August (3), with the female usually producing a clutch of two eggs of unequal size (2). Usually only the chick from the larger egg survives to maturity. Incubation takes around 33 days, with both parent birds taking it in turns to sit on the eggs for extended periods of a time, whilst the other forages for food. Incubation is aided by a bare patch of skin on the lower abdomen (known as a 'brood pouch') that allows greater heat transfer to the egg. Once hatched, the male will remain to brood the chick for the first 25 days, whilst the female regularly brings food back to the nest. After this time, the chick is able to leave the nest, and will congregate with other chicks in small groups known as 'crèches' whilst the parent birds forage (2). In order to maintain its waterproof coat, the northern rockhopper penguin engages in frequent grooming, which helps to flatten the feathers and to spread a waxy substance that is secreted just below the tail. Grooming is also an important social bond between pairs. After breeding the northern rockhopper penguin forages extensively in order to build up fat reserves in preparation for its annual moult. It takes around 25 days for the penguin's coat to be fully replaced, at which point it leaves the land and spends the winter months foraging at sea, before returning to shore to breed in the following spring (2). The diet of the northern rockhopper penguin is composed of a variety of oceanic species, such as crustaceans, squid, octopus and fish (4). Groups may often feed together and dives may be to depths of up to 100 metres (2).