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Structure and function of birds
Birds are warm-blooded tetrapod vertebrates. They are bipedal, which means that they walk on two legs. Birds also lay amniotic eggs with hard carbonated calcium carcarons. Despite the fact that birds are the most recent class of vertebrates to evolve; they are, today, the most numerous vertebrates on Earth. Why have birds been so successful in evolution? What characteristics allow you to grow and diversify your class so quickly? Birds can vary considerably in size, as you can see in the Image below. where the smallest and largest bird in the world is shown. The tiny hummingbird is only 2 inches (5 centimeters) long, while the ostrich surpasses the human at a height of 9 feet (2.7 meters). All modern birds have wings, feathers, and beaks. They possess numerous unique characteristics, most adaptations for flying. Birds use flight as transportation to find food, to find their mates, and to escape predators. Although not all modern birds can fly, they all evolved from ancestors who did have the ability.

Image 13.30
Range of Body Size in Birds. The zunzuncito is the smallest bird. The ostrich is the largest bird.
Wings and Feathers
The wings are an obvious adaptation for flight. They are actually modified front ends. Birds move their wings using the chest muscles. These muscles are large and comprise 35 percent of the bird's body weight.
Feathers help birds to fly, provide thermal insulation, and serve other purposes as well. Birds actually have two basic types of feathers: flight feathers and down. Both types of feathers are shown in Figure below. Flight feathers are long, stiff and waterproof. They provide lift and air resistance without adding weight. Down are short, fluffy feathers. They trap air that is close to the bird's skin to provide thermal insulation.

Image 13.31
Types of feathers of birds. These two types of bird feathers have different uses. How is the structure of each pen in relation to its function?
Adaptation of the organ system for flight
Birds need a light body to stay in the air. Even so, flying is difficult and the flight muscles need a constant supply of blood rich in nutrients and oxygen. Organ systems in birds are adapted to meet these needs.
Birds possess light bones that are filled with air. They do not have a jaw, which in many vertebrates is a hard, heavy bone with many teeth. Instead, birds possess a toothless, light keratin beak.
Birds have air sacs that store inhaled air and carry it to the lungs. This keeps the lungs constantly filled with oxygenated air. The lungs also contain millions of tiny passages that create a large surface area for the exchange of gases and blood (see Figure below).
Birds have a relatively large four-chamber heart. The heart beats fast to keep oxygenated blood flowing to the muscles and other tissues. Hummingbirds have the fastest beating heart, 1,200 beats per minute. This is almost 20 times faster than the human heartbeat!
Birds have a structure similar to a sac called a crop to store and moisten the food that is waiting to be digested. They also have an organ called the gizzard that contains the stones that are swallowed. The stones fulfill the function of the teeth, they grind the food so that it can be digested in less time. Both structures facilitate the work of the digestive system to produce a constant supply of nutrients from food.

Image 13.32
Adaptations in the Organ System for Flight. The complex passageways in the lungs of birds are adapted for more efficient gas exchange. Find the crop and gizzard in the picture of the digestive tract. Its duties? Bird Lung (right), Bird Digestive Tract (left)
Nervous system and sensory organs
Birds have large brains in relation to their body size. Not surprisingly, the part of the brain that controls flight is the most developed. The large brain size in birds is also reflected in their high level of intelligence and complex behavior. In fact, birds like crows can be smarter than many mammals. They are smart enough to use branches as tools. They have also shown planning and cooperation. Most birds have a low sense of smell, but they make up for it with their excellent sense of sight. Predatory birds have very good eyesight. Hawks, for example, have vision eight times sharper than human vision
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