19.5 Birds

Lesson Objectives

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


  • bird
    • bipedal, endothermic, tetrapod vertebrate that lays amniotic eggs and has wings and feathers (DUH!)
  • courtship
    • animal behavior that is intended to attract a mate
  • crop
    • sac-like structure in the digestive system of birds that stores and moistens food before it is digested
  • generalist
    • organism that can consume many different types of food
  • gizzard
    • food-grinding organ in the digestive system of birds and some other animals that may contain swallowed stones
  • incubation
    • period of bird reproduction when one or both parents sit on, or brood, the eggs in order to keep them warm until they hatch


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. Birds are the most numerous vertebrates on Earth.

Structure and Function in Birds

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 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 suited for flight. Flight is used by birds as a means of locomotion in order to find food and mates and to avoid predators.

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

BBC – Ostrich Attack

Wings and Feathers

Wings were obviously created for flight. 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 Created 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 created 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 Systems Created for Flight. The intricate passageways in a bird’s lung were created 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.

BBC: Birds of Paradise mating dance

Dancing Birds Compilation set to music – Check out the red capped Manakin!

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.

BBC – Feeding wild baby birds

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

Several flightless birds are shown in Figure below. They include the ostrich, kiwi, rhea, cassowary, and moa. All of these birds have long legs created for running. The penguins shown in the figure are also flightless birds, but they have a very different body shape which is designed for swimming rather than running.

BBC Emperor Penguins

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

Flying Birds

TED Ed: Bird Migration

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.



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 created 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 was created to be able to sip nectar from tube-shaped flowers.

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?

BBC Hummingbirds

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.

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 http://www.kqed.org/quest/television/cool-critters-the-golden-eagle.

KQED: The Great Horned Owl

Owls are amazing creatures. They have many created designs 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 http://www.kqed.org/quest/television/cool-critters-great-horned-owls-.

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 athttp://www.kqed.org/quest/television/cool-critters-turkey-vultures.

Lesson Summary

  • Birds are endothermic tetrapod vertebrates. They are bipedal and have wings and feathers. Their organ systems are created 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 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.  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 significance of the differences?

9. Why did God give the hummingbird pictured in the Hummingbird Sipping Nectar Figure above 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?

Previous: Reptiles

Next: Chapter 20: Mammals and Animal Behavior

4 thoughts on “19.5 Birds

  1. I think that “most of which are adaptations for flight” sort of references evolution? I just wanted to let you know. 🙂 Thank you for the amazing curriculum by the way!

    1. You are awesome!! You would make a terrific proofreader! I’m going to fix these things asap! We’re so glad you are enjoying the curriculum!! Thank you SO much for leaving all the comments and helping us with finding the errors and references to evolution!

  2. I noticed this part has it too: “Wings are an obvious adaptation for flight.” 🙂

  3. Organ Systems Created 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) This diagram and its description seem to have the same thing?

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