Over 150 million years ago, huge flying reptiles, called pterodactyls, sailed across the sky. One of the largest was the pteranodon, which had a wingspan of more than 27 feet (8 m). That’s about the size of a large mobile home.
These giant meat-eaters of the sky would have paid little attention to Archaeopteryx lithographica, a delicate flying reptile. It had the teeth and tail common to many reptiles, but its wings had a new kind of covering called feathers.
Modern times have seen the extinction of the giant reptiles of the sky, but Archaeopteryx evolved into an entirely new class of vertebrate—birds. Besides having feathers, birds differ from reptiles in a few other ways. Unlike their reptilian ancestors, birds have no teeth. They are also warm-blooded, which means they can regulate their body temperatures internally. Ye birds can relate to their ancestors in one way: they have scales—but only on their legs.
Today, there about 9,000 species of birds in the world, from the graceful Caspian tern you see here, to the tiny ruby-throated hummingbird, to the flightless ostriches of Africa—that can weigh as much as three average seventh graders put together. Obviously, birds are a highly evolved group of animals with amazing adaptations.
Most birds have hollow bones. Bone tissues are arranged as a network of interlaced bone strands. This allows bird bones to be as light as possible, yet still strong enough to provide support as birds glide. This is an adaptation for flight. Flying birds need to shift their center of gravity from their hind limbs when they walk to their wings when they lift off. Of course, birds that are adapted for running have thicker bones that provide greater support.
This skin covering is unique to birds. Birds usually have two kinds of feathers; contour feathers, which are large, sleek, and aid in flight, and down feathers, which are smaller, fluffy, and provide warmth.
Birds have evolved in special ways for flight. A bird’s breastbone juts outward and is connected to strong muscles that allow the bird’s wings to make the powerful downward thrust necessary for flight.
Because birds lack teeth, their bills have evolved to perform feeding activities. The variety of bills is astounding. This Caspian tern has a bill modified for catching small fish. But a pelican, which catches large fish, has a bill that is simply a huge scooper.
Birds generally show highly developed parental care of their young. All birds lay eggs, which are usually cared for in the nest. After hatching, some young can fly immediately, though most babies need extended parental care. Caspian terns lay eggs right on the beach, and thus must guard them from the many enemies seeking a meal.