Females' choice of male mates is based on male behaviors. First, the male gentoos find the best spot for a potential nest. The prime areas are those that are flat with little to no snow or ice. Once a potential nest site is chosen, the males point their bills vertically in the air and bellow out calls. The calls announce to the females to come and investigate their nest site. If a female waddles by and "likes" the nest site, the male and female will mutually display by trumpeting or bowing.

Gentoo penguins are monogamous during a breeding season, with some pair bonds lasting a lifetime. "Divorces" (the breaking of pair-bonds) do occur between breeding seasons. In this case, females choose a new partner that has displayed greater reproductive success. As colonial breeders, direct observation of nesting success is possible. The divorce rate in gentoos is less than 20 percent, which is relatively low compared to other penguin species. The benefit to a monogamous relationship is that mates do not have to expend time and energy finding new mates each year.

Mating System: monogamous

Gentoo penguins can begin breeding at two years of age, although most don't until they are about three or four. Living in colonies, gentoo penguins can gather in groups of over 2,000 pairs at one breeding site. At the beginning of the breeding season, nests are built by the parents. Gentoo nests are spaced about a meter apart. The egg-laying season for P. papau begins from June to mid-August and usually finishes in late October to late November.

After the nest has been completed, the female will stay at the nest and lay her egg 5 days post-breeding. A second egg is laid three days later. The eggs are spherical and greenish-white. The weight of the first egg in relation to the second egg varies between nesting locations, but on average egg weight is 125 grams. There have been rare cases where one or three eggs were laid.

If the set of eggs is lost, gentoo penguins can lay a second set of eggs during the same breeding season. These eggs are laid near the end of the breeding season when the female regains sufficient energy. The downside to a late laying is reduced energy, causing a late molting period. Females do not have enough energy to begin their molt right after breeding and therefore delay molting. This, in turn, delays egg-laying the following year. Indeed, the female may not have enough energy to lay a clutch the next year.

The eggs are incubated for an average of 35 days before hatching. Although the eggs are laid days apart from each other, they typically hatch on the same day or one day apart. The chicks are frail and weigh about 96 grams. The chicks stay at the nest for the first 75 days until they are ready to fledge and visit the ocean for the first time. During this fledging period, gentoo chicks make an average of 5 trips to sea. The young reach independence 20 days post-fledging.

Breeding interval: Gentoo penguins breed once a year.

Breeding season: The breeding season for gentoo penguins starts in June to mid-August and ends in late October to late November.

Range eggs per season: 1 to 3.

Range time to hatching: 34 to 35 days.

Average time to hatching: 37 days.

Range birth mass: 85 to 107 g.

Range fledging age: 75 to 105 days.

Average fledging age: 80 days.

Range time to independence: 85 to 105 days.

Average time to independence: 100 days.

Range age at sexual or reproductive maturity (female): 2 to 3 years.

Average age at sexual or reproductive maturity (female): 2 years.

Range age at sexual or reproductive maturity (male): 2 to 3 years.

Average age at sexual or reproductive maturity (male): 2 years.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)

Both parents are involved in nest-building. The nest is bowl shaped with a wide edge and a hollow center. Nest size ranges between 10 to 20 cm in height and around 45 cm in diameter. Nests are made from small stones found around the nesting site, including stones stolen from other nests. Medium-sized nests can contain over 1,700 pebbles. Although pebbles are the main component of nests, sometimes molted feathers, twigs, and vegetation are used.

Members of both sexes defend their nests from other birds that come too close. P. papau will stick out its bill toward the invader and let out a low hiss. Competition for territory exists between two adjacent nests in which parents will turn their neck towards their neighbor and try to grab and twist their bill. Although rare, fighting with bills and flippers has been observed.

The female lays two eggs within 3 days of each other. The eggs are kept safely under the male or female for the 35 days of incubation. For the first three to four weeks, the chicks are guarded in the nest. The parents take turns getting food and regurgitating it for the chicks. Near the end of this stage, the chicks begin to move short distances away from the nest and form groups with other chicks (creches). These groups serve to protect against predators while both parents to forage for the growing young. The young fledge at 70 days old and will enter the sea for the first time. Both parents will still feed their chicks (although not as often) during the fledging period. Feedings have occasionally been recorded post-independence.

Parental Investment: altricial ; male parental care ; female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Male, Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female); pre-independence (Provisioning: Male, Female)