Penguins

In one sense, it could be stated that penguins do fly. It's just that they fly underwater, rather than in the air.

There are a number of reasons why penguins do not fly in the air.

Penguins eat fish. The fish they eat swim fairly deep under water, so flying is not an advantage in getting the fish, but swimming is. Therefore, penguins swim rather than fly. Quite simply, they don't fly because they don't need to.

Penguins cannot fly in air because their wings are too small for their body weight. Power required to stay aloft to counteract gravity is proportional to wingspan. Penguins have very small wings, yet are very heavy. The heaviest penguins are heavier than the heaviest flying birds. The heavier bodies of penguins are the result of their special bone structure and the blubber that they carry. The blubber protects penguins from the cold. Cold water can remove heat from the body much faster than air, so a good insulator is required. Colin Pennycuick calculates in his book "Modelling the Flying Bird" that a typical gentoo penguin would need to flap its wings 35 times a second (about equivalent to a hummingbird) to remain aloft. The penguin is simply unable to create enough energy to take off.

If penguins can't fly because they have such small wings, then why do they have small wings in the first place? The answer is that small wings are much more useful underwater because they have lower drag and greater agility. Their wing bones are fused straight, rather than angled like a flying bird's, and this has the effect of making the wing rigid and powerful, like a flipper. The small wings and a streamlined body shape are ideal for diving in water. Larger wings would create too much drag, while the small penguin wings are excellent for high speed swimming.

Unlike the majority of other birds, penguins do not have hollow bones, so are much heavier and harder to support with their small wings. The solid, as opposed to hollow, bones act as ballast to help them dive. Also, being solid, they are less prone to breakage from the stresses of swimming. Some (but not all) flying birds have hollow bones to be lighter.

Penguins also have higher levels of myoglobin and feathers optimized for the aquatic environment. Myoglobin is the main way penguins store oxygen during their long dives. The muscles of flying birds are filled with mitochondria and enzymes to power flight, and there is no space left-over for myoglobin. So, flying birds cannot spend us much time underwater as penguins because they have less myoglobin (less oxygen per body mass) and lower body mass (less over oxygen). Another benefit is that penguins are able to optimize their feathers for the aquatic environment. Penguin feathers are short and tight to keep water away from the skin and to create a smooth surface to lower drag. Flying birds have very different feathers that are fluffy to trap air for insulation.

Penguins do not live in the northern hemisphere; nor do they live inland any distance from the ocean. They are not found in freshwater creeks and rivers. They are not found in Europe, North America or Asia, or in Arctic waters.

It is perhaps more pertinent to mention where they dolive.

Four of the 17 species of penguins live along the coast of Antarctica and the Antarctic Peninsula. None are able to venture far inland on the Antarctic, as there is nothing on the Antarctic continent to support life. They depend on the ocean for their survival, whether it be the Antarctic waters or the more temperate waters of the coasts of South America and Australia.

Others are found elsewhere within the Antarctic Circle to temperate regions further north, including New Zealand, southern Australia, the Falkland Islands, the Galápagos Islands (which is a tropical habitat) and the coastlines of South America and southern Africa.