Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Several species are found in the temperate zone, and one species, the Galapagos Penguin, lives near the equator. The largest living species is the Emperor Penguin: adults average about 1.1 m tall and weigh 35 kg or more. The smallest penguin species is the Little Blue Penguin, also known as the Fairy Penguin, which stands around 40 cm tall and weighs 1 kg.Source
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They are a type of bird even though none of the species are able to fly. It is believed they are descendents of very early types of birds. Scientists believe that they had to adapt to living in the water rather than just on land in order to survive. As a result what was once feathers and wings for them to fly became flappers for them to swim with but they lost the ability to fly. Early Antarctic explorers actually thought penguins were fish and classified them accordingly. In fact, as birds, they are very well designed for their job, flying underwater with great skill. Their compact bodies have a breastbone that makes an excellent keel and they have massive paddle muscles to propel them at speeds up to 25 miles per hour. Their heads retract to create a perfect hydrodynamic shape. When traveling quickly, penguins will leap clear of the water every few feet, an action called 'porpoising.' This enables them to breathe, and decreases their chances of being taken by a predator. Source1 Source2
The Rockhopper penguins are small, aggressive, crested penguins , so named because of the way they hop from boulder to boulder when moving around their rocky colonies. Rockhoppers have distinctive crest feathers on their heads, bright orange-red bills and tiny blood red eyes. Males and females are very similar in appearance, though males are larger. Source


Penguins on Sealion Island, South Atlantic.

Gentoo Penguins at Cuuverville.

Emperor penguins.
