12.21: Bird Structure and Function

Why is flight so important to birds?

One of the defining traits of many birds is the ability to fly. Obviously, flight is a major evolutionary advantage. But together with the ability to fly must come a number of structural modifications. What do you think these might be?

Structure and Function in Birds

Birds are endothermic tetrapod vertebrates. They are bipedal, which means they walk on two legs. Birds also lay amniotic eggs with hard, calcium carbonate shells. Although birds are the most recent class of vertebrates to evolve, they are now the most numerous vertebrates on Earth. Why have birds been so successful? What traits allowed them to increase and diversify so rapidly? Birds can vary considerably in size, as you can see from the world’s smallest and largest birds, pictured in Figure below. The tiny bee hummingbird is just 5 centimeters (2 inches) long, whereas the ostrich towers over people at a height of 2.7 meters (9 feet). All modern birds have wings, feathers, and beaks. They have a number of other unique traits as well, most of which are adaptations for flight. Flight is used by birds as a means of locomotion in order to find food and mates and to avoid predators. Although not all modern birds can fly, they all evolved from ancestors that could.

Range of Body Size in Birds. The bee hummingbird is the smallest bird. The ostrich is the largest.

Wings and Feathers

Wings are an obvious adaptation for flight. They are actually modified front legs. Birds move their wings using muscles in the chest. These muscles are quite large, making up as much as 35 percent of a bird’s body weight.

Feathers help birds fly and also provide insulation and serve other purposes. Birds actually have two basic types of feathers: flight feathers and down feathers. Both are shown in Figure below. Flight feathers are long, stiff and waterproof. They provide lift and air resistance without adding weight. Down feathers are short and fluffy. They trap air next to a bird’s skin for insulation.

Types of Bird Feathers. These two types of bird feathers have different uses. How is each feather’s structure related to its function?

Organ Systems Adapted for Flight

Birds need a light-weight body in order to stay aloft. Even so, flying is hard work, and flight muscles need a constant supply of oxygen- and nutrient-rich blood. The organ systems of birds are adapted to meet these needs.

• Birds have light-weight bones that are filled with air. They also lack a jaw, which in many vertebrates is a dense, heavy bone with many teeth. Instead, birds have a light-weight keratin beak without teeth.
• Birds have air sacs that store inhaled air and push it into the lungs like bellows. This keeps the lungs constantly filled with oxygenated air. The lungs also contain millions of tiny passages that create a very large surface area for gas exchange with the blood (see Figure below).
• Birds have a relatively large, four-chambered heart. The heart beats rapidly to keep oxygenated blood flowing to muscles and other tissues. Hummingbirds have the fastest heart rate at up to 1,200 beats per minute. That’s almost 20 times faster than the human resting heart rate!
• Birds have a sac-like structure called a crop to store and moisten food that is waiting to be digested. They also have an organ called a gizzard that contains swallowed stones. The stones make up for the lack of teeth by grinding food, which can then be digested more quickly. Both structures make it easier for the digestive system to produce a steady supply of nutrients from food.

Organ System Adaptations for Flight. The intricate passageways in a bird’s lung are adapted for efficient gas exchange. Find the crop and gizzard in the digestive tract diagram. What are their functions? Bird Lung (left), Bird Digestive Tract (right)

Nervous System and Sense Organs

Birds have a large brain relative to the size of their body. Not surprisingly, the part of the brain that controls flight is the most developed part. The large brain size of birds is also reflected by their high level of intelligence and complex behavior. In fact, birds such as crows and ravens may be more intelligent than many mammals. They are smart enough to use objects such as twigs for tools. They also demonstrate planning and cooperation. Most birds have a poor sense of smell, but they make up for it with their excellent sense of sight. Predatory birds have especially good eyesight. Hawks, for example, have vision that is eight times sharper than human vision.

Summary

• Birds are endothermic tetrapod vertebrates. They are bipedal and have wings and feathers.
• Bird organ systems are adapted for flight. For example, they have light-weight air-filled bones and a large four-chambered heart.
• Birds also have relatively large brains and a high level of intelligence.

Review

1. Why do birds fly?
2. List two functions of feathers in birds.
3. Describe the bird crop and gizzard. What are their functions?
4. How do birds keep their lungs filled with oxygenated air?
5. Give an example of bird behavior that shows their relatively great intelligence.