Even flightless birds, like penguins share most major bird characteristics.
Some birds sing while others screech. Some run, while others fly. Some are diurnal and some are nocturnal. Some are as small as a quarter, while others are taller than a man. The list of differences goes on, but despite their diversity, all birds have a number of characteristics in common, which have caused scientists to group them together in the class Aves. These distinctions are important to remember when other common characteristics, such as flight, do not apply.
Feathers
All birds and only birds have feathers. These unique structures are made of keratin, like your fingernails, but are soft and flexible, which makes them useful for insulation, sensation and flight. The shaft, which runs down the middle of each feather, is called the rachis. On either side of the rachus are flexible branches called barbs, which are connected to one another with hooks called barbules. There are five types of feathers found on most birds, including contour feathers, down feathers, semiplumes, fibroplumes and powder feathers. Contour feathers allow the bird to fly, while down feathers and semiplumes provide insulation and shape. Fibroplumes are sensory feathers, which give the bird a sense of touch, and powder feathers, disintegrate into fine grains that helps keep the bird clean. A sixth type of feather, called a bristle, grows near some birds' eyes and beak to protect them from pests and irritants.
Wings
Although not all birds can fly, every species has wings. The shape of the wing dictates what kind of flying the bird can do. For example, birds that have short, broad wings can take off quickly and dodge obstacles in their path, but cannot fly at high speeds like birds with long, tapered wings can. Birds with proportionally large bodies and tiny wings, such as the ostrich, cannot fly at all, but use their wings for balance.
Porous Skeleton
Most birds have lightweight, porous skeletons, which lower their body weight and allow them to fly. However, there are some notable exceptions. Penguins and kiwis have dense bones filled with marrow. This extra weight keeps the birds grounded, but helps them survive in other ways, such as allowing penguins to dive deeper in search of food. Ostriches have some porous bones, but others are solid, such as those in their lower legs. This gives them additional stability and allows them to defend themselves with a strong kick.
Beaks
Birds use beaks instead of teeth to grasp and tear their food. Thick cone-shaped bills, such as those on finches, allow the birds to crack seeds. Carnivores, such as raptors, have strong hooked beaks for tearing through flesh. Birds with other feeding habits have beaks appropriate to their primary food source, such as a woodpecker's strong thin beak, which allows it to drill through trees in search of insects and the duck's fringed beak, which acts as a strainer to filter food from the water.
Eggs
All birds lay eggs, and most carefully tend to them until they hatch. Many birds sit on their eggs to keep them warm -- a habit called brooding -- but a few bury them in the earth, such as the Australian jungle fowl. Bird eggs contain an embryonic chick fed by a yolk sack and protected by a white albumen. Unlike insect and reptile eggs, bird eggs are covered in a hard shell made primarily of calcium.
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