swimming bird

 Penguins are highly social, often breeding in large colonies. Some species forage cooperatively and may dive synchronously while foraging in small or large groups. Species that breed in large colonies often have elaborate visual and vocal displays. Vocalizations are characterized as loud, short brays, grunts or whistles, the type depending upon species, for instance, banded penguins (Spheniscus) are also known as 'jackass penguins' because of their braying, while the giant penguins of the genus Aptenodytes most often whistle. In colonial species in which chicks group together in a nursery (or crèche) recognition of mates and offspring seems based on individually distinguishable calls. Penguins have a number of characteristic physical features, such as their erect posture, stiff wings, and counter-shaded body coloring (dark backs and white fronts). Penguins are medium to large birds. The smallest (the Little Penguin) is typically 40 cm (16 inches) tall and a body mass of one kilogram, while the Emperor Penguin can reach 1.15 meters (43 inches) and attain a body mass of 22-37 kg.Depending upon the terrain, penguins on land will waddle along shorelines and ice, or hop from rock to rock, some species being more adept at one form of locomotion on land than another. When on ice or snow, penguins can move swiftly by tobogganing (sliding along on their belly and propelled by their wings and feet).  Underwater, penguins use their modified wings (or 'flippers') to propel themselves forward, and it is this underwater 'flying', combined with the solidity of their bones, that enables them to reach great depths. The Emperor penguin is an exceptional diver and can stay submerged for 18 minutes and dive as deep as 500 meters (1,640 ft). Swimming speeds average two to three knots (3.7km/hr), but may reach 15-20 knots (27.8-37km/hr) for short distances. Swimming often includes porpoising (repeatedly breaking the water's surface with enough momentum to lift the bird into the air for about one meter), which may be carried out for reasons of speed, escaping predators or even purely to aid breathing (and possibly a combination of these factors).