The Adélie penguin is a species of penguin common along the entire Antarctic coast, which is their only residence. They are among the most southerly distributed of all seabirds, as are the emperor penguin, the South Polar skua, the Wilson's storm petrel, the snow petrel, and the Antarctic petrel. They are named after Adélie Land, in turn named for the wife of French explorer Jules Dumont d'Urville who discovered these penguins in 1840. The Adélie penguin is one of three species in the genus Pygoscelis. Mitochondrial and nuclear DNA evidence suggests the genus split from other penguins around 38 million years ago, about 2 million years after the ancestors of the genus Aptenodytes. In turn, the Adélie penguins split off from the other members of the genus around 19 million years ago. Based on a 2014 analysis of fresh guano-discolored coastal areas, there are 3.79 million breeding pairs of Adélie penguins in 251 breeding colonies, a 53 percent increase over a census completed 20 years earlier. The colonies are distributed around the coastline of the Antarctic land and ocean. Colonies have declined on the Antarctic Peninsula, but those declines have been more than offset by increases in East Antarctica. During the breeding season, they congregate in large breeding colonies, some over a quarter of a million pairs. Individual colonies can vary dramatically in size, and some may be particularly vulnerable to climate fluctuations. The Adélie penguins breed from October to February on shores around the Antarctic continent. Adélies build rough nests of stones. Two eggs are laid, these are incubated for 32 to 34 days by the parents taking turns (shifts typically last for 12 days). The chicks remain in the nest for 22 days before joining crèches. The chicks moult into their juvenile plumage and go out to sea after 50 to 60 days. These penguins are mid-sized, being 46 to 75 cm (18 to 30 in) in height and 3.6 to 6 kg (7.9 to 13.2 lb) in weight. Distinctive marks are the white ring surrounding the eye and the feathers at the base of the bill. These long feathers hide most of the red bill. The tail is a little longer than other penguins' tails. The appearance looks somewhat like a tuxedo. They are a little smaller than other penguin species. Their appearance is closest to the stereotypical image of penguins as mostly black with a white belly. Adélie penguins can swim up to 45 miles per hour (72 km/h). Adélie penguins are preyed on by leopard seals, skua, and occasionally, orcas. Specifics of their behaviour were documented extensively by Apsley Cherry-Garrard (a survivor of Robert Falcon Scott’s fateful final journey to the South Pole) in his book The Worst Journey in the World. Cherry-Garrard noted: "They are extraordinarily like children, these little people of the Antarctic world, either like children or like old men, full of their own importance". Certain displays of their selfishness were commented upon by George Murray Levick, a Royal Navy surgeon-lieutenant and scientist who also accompanied Scott on his ill-fated British Antarctic Expedition of 1910, during his surveying of penguins in the Antarctic: "At the place where they most often went in, a long terrace of ice about six feet in height ran for some hundreds of yards along the edge of the water, and here, just as on the sea-ice, crowds would stand near the brink. When they had succeeded in pushing one of their number over, all would crane their necks over the edge, and when they saw the pioneer safe in the water, the rest followed". The Adélie penguin is known to feed mainly on Antarctic krill, ice krill, Antarctic silverfish, sea krill and glacial squid (diet varies depending on geographic location) during the chick-rearing season. The stable isotope record of fossil eggshell accumulated in colonies over the last 38,000 years reveals a sudden change from a fish-based diet to krill that started two hundred years ago. This is most likely due to the decline of the Antarctic fur seal since the late 18th century and baleen whales in the 20th century. The reduction of competition from these predators has resulted in a surplus of krill, which the penguins now exploit as an easier source of food. Adélie penguins arrive at their breeding grounds in October or November, at the end of winter and the start of spring. Their nests consist of stones piled together. In December, the warmest month in Antarctica (about −2 °C or 28 °F), the parents take turns incubating the egg; one goes to feed and the other stays to warm the egg. The parent who is incubating does not eat. In March, the adults and their young return to the sea. The Adélie penguin lives on sea ice but needs the ice-free land to breed. With a reduction in sea ice, populations of the Adélie penguin have dropped by 65% over the past 25 years. Young Adélie penguins who have no experience in social interaction may react to false cues when the penguins gather to breed.
