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Penguins are flightless seabirds that live almost exclusively below the equator. Some island-dwellers can be found in warmer climates, but most—including emperor, adélie, chinstrap, and gentoo penguins—reside in and around icy Antarctica. A thick layer of blubber and tightly-packed, oily feathers are ideal for colder temperatures.
The type of food utilized varies with the species, the geographic region, and the time of year. Most of the smaller southern penguins feed primarily upon krill, which attain high densities in the rich, well-oxygenated Antarctic waters. Cephalopods (squid and cuttlefish) and small fishes may form substantial fractions of the food, and in a few, such as the African penguin, fish is the basic element of the diet. The total weight of food consumed by a large penguin colony is prodigious, often exceeding several tons per day.
The penguins are highly specialized for their flightless aquatic existence. The feet are located much farther back than those of other birds, with the result that the bird carries itself mostly upright; its walk can thus be described as plantigrade (i.e., on the soles). The sole comprises the whole foot instead of just the toes, as in other birds. The most notable characteristic of the group is the transformation of the forelimb into a paddle. This is accompanied by a body morphology particularly adapted to movement in a liquid medium. The thoracic (rib) cage is well developed, and the sternum bears a pronounced keel for the attachment of the pectoral muscles, which move the flippers. The flipper has the same skeletal base as the wing of flying birds but with its elements shortened and flattened, producing a relatively rigid limb covered with very short feathers—an ideal organ for rapid propulsion. The body plumage likewise consists of very short feathers, which minimize friction and turbulence. The density of the plumage and the layer of air that it retains provide almost complete insulation of the body.
Most adults stand 40 to 50 cm (about 15–20 inches) tall, and some individuals reach up to 60 cm (about 24 inches) in length. The average weight of the adult ranges from 3 to 4 kg (about 7 to 9 pounds), and males are slightly larger than females. Adult members of both sexes have a black head, throat, and back. They also possess a white underside and red eyes. The bill is separated from the black plumage on the face by a narrow strip of pink- or white-coloured skin. Juveniles are somewhat smaller than adults, possessing shorter, paler crests and a paler chin, whereas chicks are brown with a dull white underside.
The diet of Snares penguins is not well documented, but ornithologists believe that it is mostly made up of krill; however, squid and fish are also eaten. They are relatively fast swimmers, reaching speeds of perhaps up to 24 km (15 miles) per hour in pursuit of their prey. At sea, adults are sometimes preyed upon by Hooker’s sea lions (Phocarctos hookeri). On land, however, eggs and chicks are more vulnerable, falling prey to skuas (Catharacta) and giant fulmars (Macronectes giganteus).
Between May and August, Snares penguins travel widely throughout their range to feed. In August, males return to the breeding grounds to excavate bowl-shaped nests into the ground, which are later lined with grasses and twigs. The nests are built beneath forests of tall shrublike tree daisies (Olearia lyallii and Brachyglottis stewartiae) or on rocky slopes. Because the penguins often nest in dense colonies of up to 1,500 breeding pairs, their excavation activities coupled with their droppings take a physical and chemical toll on the forest, and the colonies are forced to move to a new part of the forest each year. Copulation occurs shortly after the arrival of females in early September. By late September and early October the female deposits two eggs in the nest: a smaller egg followed by a larger egg about five days later. Both parents take turns incubating the eggs in shifts lasting 10 days or more. For the first few weeks after hatching, the chicks are fed by their mother and guarded by their father. It is during this period that the chick hatching from the smaller egg typically dies, often succumbing to hypothermia brought on by frequent rain. About three weeks after the young have hatched, both parents forage in the ocean during the day, and surviving offspring join a “crèche” (group) with other members of the cohort for protection. The fledging period, in which the young are prepared for adulthood, concludes when the young are about 11 weeks old, and they leave the nest to live on their own. Snares penguins become sexually mature by age six, and they may live as long as 20–22 years.
In 2008 ecologists estimated that the Snares penguin population totaled 26,000 breeding pairs, with most of the birds living on North East Island, the largest island in the group. Compared with other species in the genus, the population is thought to be relatively stable. (It fluctuated between 23,000 and 30,000 breeding pairs between 1985 and 2008.) This relative stability has been attributed to the remoteness of the islands coupled with New Zealand’s strict rules regarding who can visit them. (Access to the islands is given only to scientific expeditions with special permits.) As a result, the Snares Islands are free from invasive mammals—such as rats, rabbits, foxes, and feral dogs and cats—that plague the habitats of several other penguin species. Since 1994, however, the species has been listed as vulnerable on the International Union for Conservation of Nature’s Red List of Threatened Species because of its limited breeding range.
Penguins are adapted for rapid locomotion in water, in which the wings, or flippers, are used for propulsion; the birds “fly” underwater. When moving at high speed, they frequently leave the water in leaps that may carry them a metre or more through the air; it is during this time that they breathe. On land, penguins are much more awkward, even amusing, as they rock from side to side as they walk. Despite their short legs, however, penguins can run with surprising speed. Some, such as the northern rockhopper (Eudyptes moseleyi), the southern rockhopper (E. chrysocome), and Adélie penguins, move among rocks with agility, using the flippers for balance. On snow or ice, many penguins “toboggan,” sliding on the belly as they propel themselves with the feet and flippers. The flippers, along with the beak, are the prime weapons in defense and attack. Scientists have long wondered how penguins are able to find their way back to their colonies from far out at sea, where currents may have carried them great distances. Also perplexing is how they are able to direct themselves correctly on land in the absence of clear-cut landmarks. Studies of penguins transported to the interior of Antarctica have found that they are able to find their way back to the ocean by using the sun as a directional aid. It is probable that the same means of orientation is used at sea. Upon approaching the coast they are able to recognize features of the shoreline and ocean bottom.