Prior to hatching, the chicks call to their parents from inside the egg. The chicks use a small point on the tip of the bill, called the egg tooth, to break through the egg shell. Hatching can often be a prolonged process, lasting a couple of days.
When the chicks reach about two weeks of age, the original protoptile plumage, which is thin and readily transmits warmth from the parent bird, is replaced by a thicker mesoptile plumage. This provides good insulation, and in association with metabolic changes, it allows the chick to maintain its own body temperature. This allows both parents to go to sea in search of food, in order to meet the growing demand for food from the larger chicks. In most surface-nesting species, chicks whose parents are at sea form into creches, and this provides them with a certain degree of protection from cold weather and predators.
Returning adults identify their chicks by recognition of their distinctive calls. Chicks must beg for food in order to initiate a feeding response from the parent, and this is usually done by constant pecking around the parents bill. Penguins, unlike most other birds, do not have crops and regurgitate partially digested food directly from the stomach. Generally it is the adults who must be convinced that they are receiving feeding demands from their own chick, since hungry chicks will happily beg from any passing adult, or even other chicks.
Mortality amongst chicks is generally quite high, and varies from species to species according to different breeding strategies. Some species lay only one egg, or lay two eggs of different size, concentrating all their efforts into raising just one healthy chick. Such species are generally longer-lived, do not begin breeding until several years of age, and use a strategy of slow reproduction but lower adult mortality. Species adopting such a strategy often show lower annual fluctuations in breeding success and population size. Nevertheless, lower reproductive rates mean that they are slower to recover from population crashes or human exploitation.
Other species lay two equally sized eggs, and put equal effort into rearing both. This allows them to achieve very high reproductive rates during seasons of high food abundance, but they may also suffer from low reproductive rates when food is scarce. These species tend to be shorter lived, begin breeding at an early age, and use a strategy of rapid reproduction but variable adult mortality. Such species tend to show high annual fluctuations in both breeding success and population size. Because they can achieve high reproductive rates, they are perhaps more able to recover from natural disasters and direct exploitation, but would still be vulnerable to a long-term reduction in food abundance.
When chicks are ready to leave the nest site and take to the sea, they shed their mesoptile plumage and develop their adult waterproof plumage, allowing them to enter the water for the first time. The term "fledging" normally applies to the stage when young birds take their first flight from the nest, but in penguins the term refers to chicks changing into adult plumage. Some parental responsibility may still remain after fledging, but before long most adult penguins return to the sea in order to build up their body fat reserves in preparation for their annual moult.
These foraging trips usually last up to about four weeks, and allow the build up of thicker layers of sub-cuticular fat, which will provide better heat insulation during the forthcoming moult. This is particularly important, since adults are unable to feed during their 2 - 4 week moult period, and must sustain heat loss by burning up body fat. If insufficient body fat exists, adults may starve to death prior to completion of their moult. In practice this very rarely happens, but it has been observed following seasons of extreme food shortage, such as following the effects of El Niño Southern Oscillations (ENSO).
Healthy adult penguins have few natural predators on land, although on occasions Sea Lions have been known to come ashore to take adult penguins. At sea however, penguins are often killed by Leopard Seals, Sea Lions and Killer Whales. Skuas and gulls are regular predators of eggs and small chicks during the breeding season, but are unable to over-power healthy adults.
Penguins are the major avian top-predators in the southern oceans. The entire world population of all penguins consume around 20 - 25 million tons of fish, squid and crustaceans every year. By way of comparison, the world's commercial fisheries remove around 70 million tons per year. However, because penguins breed in very large numbers at particular sites, and generally forage within a range of 40km, there is considerable local competition for food. Breeding colonies therefore rely on highly productive feeding areas within their daily foraging range, in order to sustain chick production. Any significant reduction in food abundance within this foraging zone is likely to have adverse affects on chick-rearing ability.