The incubation period lasts forty days, and the fledging period from 60 to 130 days. Young penguins have blue-grey backs and white fronts, without the black and white markings of their parents. Originally the African penguin nested in guano (hardened bird droppings, in the past several meters thick) but when this was mined for fertiliser in the nineteenth century they were forced to adapt to other conditions. Now they nest in crude shallow burrows dug out of the sand or under beach vegetation.