The dazzling diversity of the world's birds is one of nature's wonders. Birds seem to come in almost every conceivable size, form and color -- not to mention behavior and lifestyle. There are more than 9,000 species of birds alive today, occupying all of the world's continents and habitats. Yet despite their amazing variety and diversity, all birds have one unique characteristic: feathers.
Feathers do a number of things. They protect a bird's skin from wet, cold and injury; their colors and shape play a large role in the mating game; and of course, feathers allow a bird to fly.
Feathers are made up of the protein keratin (the same substance found in our hair, teeth and nails). They are lightweight enough to allow the bird to fly, yet are also strong and waterproof.
Most birds fly (and those that don't, like penguins, descended from ancestors who flew). This amazing behavior is the product of numerous body adaptations. Birds' front limbs have evolved into wings. Their bones are "hollow" (honeycombed with cells of air) to make them light enough for flight. Birds have also evolved a keel-shaped breastbone, which provides an anchor for the wing muscles that power their flight.
The ability to fly has helped make birds one of the most successful animal groups on the planet. Once they took to the skies, they could escape ground predators, travel farther for food, colonize more habitats, and live where few other creatures could go.