(1.9 per square yard). Both sexes fiercely defend the area around their nest. Where space allows, the distance between nests is double the reach of an individual. Nests are made from seaweed, plants, earth and debris from the sea. The males usually collect the materials. Nests are compact cups typically in height. The area which a nest occupies grows throughout the breeding season as the breeding pairs throw their excrement outside the nest. Over years, nests can reach in height. Northern gannets lay one egg that on average weighs , which is light for such a large seabird. The egg is around long by wide and the shell is pale blue and translucent initially before fading to a chalky white surface that is easily stained. M. bassanus was among those birds most affected by the shell-thinning effect of DDT, which contributed in large degree to its ban. Where two eggs are found in a nest this is the result of two females laying an egg in the same nest or one egg being stolen from another nest. Northern gannets will lay a replacement egg if the first is lost. Incubation takes 42 to 46 days, during which time the egg is surrounded by the brooding bird's warm, webbed feet. Just before hatching begins, the brooding bird releases the egg from its feet to prevent the egg from breaking under the adult's weight as the chick breaks it open. This is a frequent cause of death for chicks of birds that are breeding for the first time. The process of breaking the eggshell can take up to 36 hours. The webbed feet are also used to cover the chicks, which are only rarely left alone by their parents. Chicks that are left unattended are often attacked and killed by other northern gannets. Newly hatched chicks are featherless and are dark blue or black in colour. In the second week of life they are covered in white down, replaced over the next five weeks by dark brown feathers flecked with white. Young chicks are fed regurgitated semi-digested fish by their parents, who open their mouths wide for their young to fetch the food from the back of their throats. Older chicks receive whole fish. Unlike the chicks of other species, northern gannet chicks do not move about the nest or flap their wings to ask for food: this reduces the likelihood that