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19.5: Birds

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Outline structure and function in birds.
  • Describe how birds reproduce and care for their young.
  • Identify several common orders of modern birds.
  • Give an overview of the evolution of birds.
  • Summarize the diversity of bird habitats and food sources.


  • bird
  • courtship
  • crop
  • generalist
  • gizzard
  • incubation


Birds are endothermic tetrapod vertebrates. They are bipedal, which means they walk on two legs. Birds also lay amniotic eggs, and the eggs have 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?

Structure and Function in Birds

Birds can vary considerably in size, as you can see from the Figure below. The world’s smallest bird, the tiny bee hummingbird, is just 5 centimeter (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 times 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.

Bird Reproduction

Reproduction in birds may be quite complicated and lengthy. Birds reproduce sexually and have separate sexes and internal fertilization, so males and females must mate for fertilization to occur. Mating is generally preceded by courtship. In most species, parents also take care of their eggs and hatchlings.

Courtship and Mating

Courtship is behavior that is intended to attract a mate. It may involve singing specific courtship songs or putting on some type of visual display. For example, a bird may spread out and display its tail feathers or do a ritualized mating “dance.” Typically, males perform the courtship behavior, and females choose a mate from among competing males.

During mating, a male bird presses his cloaca against his mate’s cloaca and passes sperm from his cloaca to hers. After fertilization, eggs pass out of the female’s body, exiting through the opening in the cloaca.

Nesting and Incubation

Eggs are usually laid in a nest. The nest may be little more than a small depression in the ground, or it may be very elaborate, like the weaver bird nest in Figure below. Eggs that are laid on the ground may be camouflaged to look like their surroundings (also shown in Figure below). Otherwise, eggs are usually white or pastel colors such as pale blue or pink.

Variation in Bird Nests. A weaver bird uses grasses to weave an elaborate nest (left). The eggs of a ground-nesting gull are camouflaged to blend in with the nesting materials (right).

After birds lay their eggs, they generally keep the eggs warm with their body heat while the embryos inside continue to develop. This is called incubation, or brooding. In most species, parents stay together for at least the length of the breeding season. In some species, they stay together for life. By staying together, the males as well as females can incubate the eggs and later care for the hatchlings. Birds are the only nonhuman vertebrates with this level of male parental involvement.


Ground-nesting birds, such as ducks and chickens, have hatchlings that are able to run around and feed themselves almost as soon as they break through the eggshell. Being on the ground makes them vulnerable to predators, so they need to be relatively mature when they hatch in order to escape. In contrast, birds that nest off the ground—in trees, bushes, or buildings—have hatchlings that are naked and helpless. The parents must protect and feed the immature offspring for weeks or even months. However, this gives the offspring more time to learn from the parents before they leave the nest and go out on their own.

Classification of Birds

There are about 10,000 living species of birds. Almost all of them can fly, but there are several exceptions.

Flightless Birds

Some birds have lost the ability to fly during the course of their evolution. Several flightless birds are shown in Figure below. They include the ostrich, kiwi, rhea, cassowary, and moa. All of these birds have long legs and are adapted for running. The penguins shown in the figure are also flightless birds, but they have a very different body shape. That’s because they are adapted for swimming rather than running.

Flightless Birds. Flightless birds that are adapted for running include the ostrich, kiwi, rhea, cassowary, and moa. Penguins are flightless birds adapted for swimming.

Flying Birds

Birds that are able to fly are divided into 29 orders that differ in their physical traits and behaviors. Table below describes seven of the most common orders. As shown in the table, the majority of flying birds are perching birds, like the honeyeater described in the last row of the table. The order of perching birds has more species than all the other bird orders combined. In fact, this order of birds is the largest single order of land vertebrates.

Order Description Example
Landfowl: turkeys, chickens, pheasants They are large in size; they spend most of their time on the ground; they usually have a thick neck and short, rounded wings; their flight tends to be brief and close to the ground.


Waterfowl: ducks, geese, swans They are large in size; they spend most of their time on the water surface; they have webbed feet and are good swimmers; most are strong flyers.


Shorebirds: puffins, gulls, plovers They range from small to large; most live near the water, and some are sea birds; they have webbed feet and are good swimmers; most are strong flyers.


Diurnal Raptors: hawks, falcons, eagles They range from small to large; they are active during the day and sleep during the night; they have a sharp, hooked beak and strong legs with clawed feet; they hunt by sight and have excellent vision.


Nocturnal Raptors: burrowing owls, barn owls, horned owls They range from small to large; they are active during the night and sleep during the day; they have a sharp, hooked beak and strong legs with clawed feet; they have large, forward-facing eyes; they have excellent hearing and can hunt with their sense of hearing alone.

burrowing owl

Parrots: cockatoos, parrots, parakeets They range from small to large; they are found in tropical regions; they have a strong, curved bill; they stand upright on strong legs with clawed feet; many are brightly colored; they are very intelligent.


Perching Birds: honeyeaters, sparrows, crows They are small in size; they perch above the ground in trees and on buildings and wires; they have four toes for grasping a perch; many are songbirds.


Evolution of Birds

Birds are thought to have evolved from a group of bipedal dinosaurs called theropods. The ancestor of birds was probably similar to the theropod called Deinonychus, which is represented by the sketch in Figure below. Fossils of Deinonychus were first identified in the 1960s. This was an extremely important discovery. It finally convinced most scientists that birds had descended from dinosaurs, which had been debated for almost a century.

Extinct Bird Relative: Deinonychus. Deinonychus shared many traits with birds. What similarities with birds to you see?

What was Deinonychus?

Deinonychus is the genus name of an extinct dinosaur that is considered to be one of the closest non-bird relatives of modern birds. It lived about 110 million years ago in what is now North America. Deinonychus was a predatory carnivore with many bird-like features. For example, it had feathers and wings. It also had strong legs with clawed feet, similar to modern raptors. Its respiratory, circulatory, and digestive systems were similar to those of birds as well. The location of fossilized eggs near Deinonychus fossils suggests that it may have brooded its eggs. This would mean that it was endothermic. (Can you explain why?) On the other hand, Deinonychus retained a number of reptile-like traits, such as jaws with teeth and hands with claws at the tips of its wings.

Evolution of Flight

Scientists have long speculated about the evolution of flight in birds. They wonder how and why birds evolved wings from a pair of front limbs. Several hypotheses have been suggested. Here are just two:

  1. Wings evolved in a bird ancestor that leapt into the air to avoid predators or to capture prey. Therefore, wings are modified arms that helped the animal leap higher.
  2. Wings evolved in a bird ancestor that lived in trees. Thus, wings are modified arms that helped the animal glide from branch to branch.

Scientists still don’t know how or why wings and flight evolved, but they continue to search for answers. In addition to fossils, they are studying living vertebrates such as bats that also evolved adaptations for flight.

Ecology of Birds

Birds live and breed in most terrestrial habitats on all seven continents, from the Arctic to Antarctica. Because they are endothermic, birds can live in a wider range of climates than reptiles or amphibians, although the greatest diversity of birds occurs in tropical regions. Birds are important members of every ecosystem in which they live, occupying a wide range of ecological positions.

Bird Diets

Some birds are generalists. A generalist is an organism that can eat many different types of food. Other birds are highly specialized in their food needs and can eat just one type of food.

Raptors such as hawks and owls are carnivores. They hunt and eat mammals and other birds. Vultures are scavengers. They eat the remains of dead animals, such as roadkill. Aquatic birds generally eat fish or water plants. Perching birds may eat insects, fruit, honey, or nectar. Many fruit-eating birds play a key role in seed dispersal, and some nectar-feeding birds are important pollinators.

Bird beaks are generally adapted for the food they eat. For example, the sharp, hooked beak of a raptor is well suited for killing and tearing apart prey. The long beak of the hummingbird in Figure below co-evolved with the tube-shaped flowers from which it sips nectar.

Hummingbird Sipping Nectar. A hummingbird gets nectar from flowers and pollinates the flowers in return. What type of relationship exists between the bird and the flowering plant?

Birds at Risk

Hundreds of species of birds have gone extinct as a result of human actions. A well-known example is the passenger pigeon. It was once the most common bird in North America, but over-hunting and habitat destruction led to its extinction in the 1800s. Habitat destruction and use of the pesticide DDT explain the recent extinction of the dusky seaside sparrow. This native Florida bird was declared extinct in 1990.

Today, some 1,200 species of birds are threatened with extinction by human actions. Humans need to take steps to protect this precious and important natural resource. What can you do to help?

KQED: The Golden Eagle

Although not as famous as its bald cousin, Golden Eagles are much easier to find in Northern California - one of the largest breeding populations for Golden Eagles. The largest of the raptors, Golden Eagles weigh typically between 8 and 12 pounds, and their wing span is around 6 to 7 feet. These eagles dive towards earth to catch prey, and can reach speeds of up to 200 mph! Meet one of the largest birds of prey at video below. 

KQED: The Great Horned Owl

Owls are amazing creatures. They have many adaptations that allow them to thrive in their environments. Their claws are enormous and powerful, they have excellent hearing, and fantastic vision in low light. And the Great Horned Owl can fly almost silently due to "fringes" on their feathers that help to break up the sound of air passing over their wings. Learn more of the Great Horned Owl at the video below. 

KQED: The Turkey Vulture

Ever wonder why a vulture's head is bald? Turkey Vultures are very interesting birds. The Turkey Vulture has no vocal organs – they can only grunt or hiss, although they usually stay silent. They do not build nests – they lay their eggs directly on the ground in caves, crevices, burrows, hollow logs, under fallen trees, or even in abandoned buildings. While these vultures have few natural predators, their main form of defense is vomiting. The foul smelling substance deters most creatures, and will also sting if the offending animal is close enough to get vomit on them. Learn more about Turkey Vultures at the video below. Also see http://www.wildlife-museum.org/ for the Lindsay Wildlife Museum.

Lesson Summary

  • Birds are endothermic tetrapod vertebrates. They are bipedal and have wings and feathers. Their 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.
  • Birds reproduce sexually and have internal fertilization. Mating is generally preceded by courtship. Their amniotic eggs have hard shells and are laid in a nest. The eggs are usually incubated until they hatch. Most species have a relatively long period of parental care.
  • There are about 10,000 living species of birds, almost all of which can fly. Flying birds are divided into 29 orders. The most common orders include landfowl, waterfowl, shorebirds, diurnal and nocturnal raptors, parrots, and perching birds.
  • Birds are thought to have evolved from theropod dinosaurs around 150 million years ago. Their ancestor may have been similar to the extinct theropod Deinonychus, whose fossils convinced most scientists that birds evolved from dinosaurs. Scientist still don’t know how or why wings and flight evolved, but they continue to search for answers.
  • Birds live and breed in most terrestrial habitats on all seven continents. They occupy a wide range of ecological positions. Raptors are carnivores; aquatic birds eat fish or water plants; and perching birds may eat insects, fruit, honey, or nectar. Some birds are pollinators that co-evolved with plants. Human actions have caused the extinction of hundreds of species of birds, and some 1,200 species are threatened with extinction today.

Lesson Review Questions


1. List two functions of feathers in birds.

2. Describe the bird crop and gizzard. What are their functions?

3. How do birds keep their lungs filled with oxygenated air?

4. Give an example of bird behavior that shows their relatively great intelligence.

5. What is courtship? What is its purpose?

Apply Concepts

6. Draw a sketch of a hypothetical bird that preys on small mammals. The bird must exhibit traits that suit it for its predatory role.

Think Critically

7. Relate two unique traits of birds to flight.

8. Contrast hatchling maturity in birds that are ground-nesting and those that nest off the ground. What is the adaptive significance of the differences?

9. Why did the hummingbird pictured in the Hummingbird Sipping Nectar Figure above evolve such a long, pointed beak?

Points to Consider

Birds share a number of important traits with mammals, including a four-chambered heart and endothermy. The next chapter describes mammals in detail.

  • What are some examples of mammals?
  • What other traits do you think mammals might have? What traits do you think set mammals apart from all other vertebrates, including birds?

For Table above, from top to bottom,

For Table above, from top to bottom,

For Table above, from top to bottom,

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