Penguins are social animals; they travel, feed, breed, nest, and winter in large groups. On several Antarctic and sub-Antarctic islands, colonies number in the millions. For example, up to two million royal penguins congregate on Macquarie Island, 750 mi (1,200 km) southwest of New Zealand. There are about 10 million birds living on one of the South Sandwich Islands, located north of Antarctica.
There are several potential reasons for penguins' highly social behavior. First, mature penguins tend to return to breed to the area where they were born. Second, in large groups they are safer from predators, such as skuas, sharks, killer whales, and especially leopard seals. Third, they learn about the location of food from each other. Fourth, group living provides better care for their young and protection against the cold.
Within the social structure, there are two levels: the family and the breeding group. Within the family, which consists of the parents and usually two chicks, the young are cared for and defended against other penguins. Within the breeding group, group defenses are used against skuas and vocal communication causes the birds to breed at about the same time.