(Bonus Material) 7.1.2 Bird Basics (ADVANCED/MORE DETAILED INFO)

Lesson Objectives

  • List and describe general traits of birds.
  • Explain how birds are adapted for flight.
  • List different breeding systems in birds and describe nesting, incubation and parental care.
  • Illustrate the diversity of birds with examples of some of the varied groups.
  • Explain how birds are important, both economically and ecologically.

Check Your Understanding

  • Birds and reptiles have some traits in common. For example, birds are egg-layers and most reptiles are also oviparous. What do the eggs of both groups have in common?
  • What traits do birds have as a result of their being warm-blooded?


  • aerofoil
  • altricial
  • monogamous
  • polygamous
  • precocial

Characteristics of Birds

How many different types of birds can you think of? Robins, ostriches, hummingbirds, chickens. All of these are birds, but they are also all very different from each other. There is an amazingly wide diversity of birds. Like amphibians, reptiles, mammals, and fish, they are vertebrates. What does that mean? It means they have a backbone. Birds have forelimbs modified as wings, but not all birds can fly.

Birds are in the class Aves. All birds have the following key features: they are endothermic (warm-blooded), have two legs, and lay eggs.

Birds range in size from the tiny 2-inch bee hummingbird to the 9-foot ostrich (Figure below). With approximately 10,000 living species, birds are the most numerous vertebrates with four limbs. They live in diverse habitats around the globe, from the Arctic to the Antarctic.

The ostrich can reach a height of 9 feet! Pictured here are ostriches with young in Namibia, Africa.[Figure1]

The digestive system of birds is unique, with a gizzard that contains swallowed stones for grinding food. Birds do not have teeth. What do you think the stones do? They help them digest their food. Defining characteristics of modern birds also include:

  • Feathers.
  • High metabolism.
  • A four-chambered heart.
  • A beak with no teeth.
  • A lightweight but strong skeleton.
  • Production of hard-shelled eggs.

Which of the above traits do you think might be of importance to flight?

Adaptations for Flight

In comparing birds with other vertebrates, what do you think distinguishes them the most? In most birds, flight is the most obvious difference. Birds have adapted their body plan for flight:

  • Their skeleton is especially lightweight, with large air-filled spaces connecting to their respiratory system.
  • Their neck bones are flexible. Birds that fly have a bony ridge along the breastbone that the flight muscles attach to (Figure below). This allows them to remain stable in the air as they fly.
  • Birds also have wings that function as an aerofoil. The surface of the aerofoil is curved to help the bird control and use the air currents to fly. Aerofoils are also found on planes.

A bony ridge along the breastbone (blue) allows birds to remain stable as they fly. [Figure2]

What other traits do you think might be important for flight? Feathers help because they more lightweight than scales or fur. A bird’s wing shape and size will determine how a species flies. For example, many birds have powered, flapping flight at certain times, while at other times they soar, using up less energy (Figure below).

One bird

One bird [Figure3]

About 60 living bird species are flightless, such as penguins, as were many extinct birds. Flightlessness often evolves when birds live on isolated islands, probably due to limited resources and the absence of land predators. For example, the flightless cormorant can no longer fly, but its wings are now adapted to swim in the sea (Figure below).

A flightless cormorant can no longer fly, but uses its wings for swimming. [Figure4]

Reproduction in Birds

How do birds reproduce? We know that chickens lay eggs. But how do they do that?

It all starts with behavior aimed at attracting a mate. In birds, this will involve a type of display, usually performed by the male. If successful, it will lead to breeding. Most male birds sing a type of song to attract females. Some displays are very elaborate and may include dancing, aerial flights, or wing or tail drumming.

Listen here http://www.youtube.com/watch?v=rL4Z9d9oObY&feature=related for singing birds (4:53).

Birds reproduce by internal fertilization. Like reptiles, birds have cloaca, or a single exit and entrance for sperm, eggs, and waste. The male brings his sperm to the female cloaca. The sperm fertilizes the egg. The hard-shelled eggs have a fluid-filled amnion, a thin membrane forming a closed sac around the embryo. Eggs are usually laid in a nest.

How Do Birds Protect Their Offspring?

Why do you think eggs come in so many different colors? If an egg is hidden in a hole or burrow, away from predators, then the eggs are most often pale or white.

Birds that make nests in the open have camouflaged eggs (Figure below). This gives the eggs protection against predation. Some species, like ground-nesting nightjars, have pale eggs, but the birds camouflage the eggs with their feathers. To protect their young, different species of birds make different nests. Many are elaborate, shaped like cups, domes, plates, mounds or burrows. The albatross, however, makes a nest that is simply a scrape on the ground. Still others, like the common guillemot, do not use nests. Instead, they lay their eggs on bare cliffs. Male emperor penguins do not have a nest at all: they sit on eggs to keep them warm before they hatch, a process called incubation.

How else might a bird help protect its young from predators? Most species locate their nests in areas that are hidden, in order to avoid predators. Large birds, or those that nest in groups, may build nests in the open, since they are more capable of defending their young.

Nest and eggs of the common moorhen, showing camouflaged eggs. [Figure5]

Young Birds and Parental Care

In birds, 95% of species are monogamous, meaning the male and female remain together for breeding for a few years or until one mate dies. Usually, the parents take turns incubating the eggs. Birds usually incubate their eggs after the last one has been laid. In polygamous species, where there is more than one mate, one parent does all of the incubating.

The length and type of parental care varies widely amongst different species of birds. At one extreme, in a group of birds called the magapodes, parental care ends at hatching. In this case, the newly-hatched chick digs itself out of the nest mound without parental help and can take care of itself right away. These birds are called precocial. At the other extreme, many seabirds care for their young for extended periods of time, the longest being that of the great frigatebird. Their chicks receive parental care for six months, or until they are ready to fly, and then take an additional 14 months of being fed by the parents (Figure below). These birds are the opposite of precocial birds, and are called altricial.

In most animals, male parental care is rare. But it is very common in birds. Often tasks are shared between parents, like defense of territory and nest site, incubation, and the feeding of chicks. Since birds often take great care of their young, some birds have evolved a behavior called “brood parasitism.” This happens when a bird leaves her eggs in another bird’s nest. The host bird often accepts and raises the parasite bird’s eggs.

Some chicks, like those of the ancient murrelet, follow their parents out to sea the night after they hatch, in order to avoid land predators. In most species, however, the young do not leave the nest until they can fly.

Great frigatebird adults are known to care for their young for up to 20 months after hatching, the longest in a bird species. Here, a young bird is begging for food. [Figure6]

Diversity of Birds

About 10,000 bird species belong to 29 different orders within the class Aves. They live and breed on all seven continents. The tropics are home to the greatest biodiversity of birds.

Birds that live in different environments will encounter different foods and different predators. The feeding habits of birds are related to the beak shape and size, as well as the shape of the feet. Birds can be carnivores, insectivores, or generalists, feeding on a variety of foods. Some, such as hummingbirds, feed on nectar. Can you think of some examples of beak shape and size that are adapted to the type of food a bird eats?


As mentioned above, the size and shape of the beak is related to the food the bird eats, and can vary greatly among different birds. Parrots have down-curved, hooked bills, which are well-adapted for cracking seeds and nuts (Figure below). Hummingbirds, on the other hand, have long, thin, pointed bills, which are adapted for getting the nectar out of flowers (Figure below).

The down-curved, hooked bill of a scarlet macaw, a large colorful parrot. [Figure7]

A long, thin and pointed bill of the Swallow-tailed Hummingbird. [Figure8]


Bird feet can also vary greatly among different birds. Some birds, like gulls and terns, have webbed feet used for swimming or floating (Figure below). Other birds, such as herons, gallinules, and rails, have four long spreading toes, which are adapted for walking delicately in the wetlands (Figure below). Now you can predict how the beaks and feet of birds will look depending on where they live and the type of food they eat.

The webbed feet of a great black-backed gull. [Figure9]

The long spreading toes of an American purple gallinule. [Figure10]

Why Birds are Important

We are probably most familiar with birds as food. Around the world, people consume chicken, turkey, and even more exotic birds, like ostriches. Can you think of other ways that birds are important?

  1. In agriculture, humans harvest bird droppings for use as fertilizer.
  2. Chickens are also used as an early warning system of human diseases, such as West Nile virus. Mosquitoes carry the West Nile virus, bite young chickens and other birds, and infect them with the virus. When chickens or other birds become infected, this warns humans that they may also become infected in the near future.
  3. Nectar-feeding birds are important pollinators, meaning they move the pollen from flower to flower to help fertilize the sex cells and create new plants. Many fruit-eating birds help disperse seeds.
  4. Birds are often important to island ecology. In New Zealand, the Kereru and Kokako are important browsers, or animals that eat or nibble on leaves, tender young shoots, or other vegetation (Figure below andFigure below). Seabirds add nutrients to soil and to water with their production of guano.
  5. Birds have important cultural relationships with humans. Birds are common pets in the Western world. Sometimes, people act cooperatively with birds. For example, the Borana people in Africa use birds to guide them to honey that they use in food.
  6. Birds also play prominent and diverse roles in folklore, religion, and popular culture, and have been featured in art since prehistoric times, as in early cave paintings.

The kereru is an important browser species in New Zealand. [Figure11]

The kokako, another important browser species of New Zealand. [Figure12]

Lesson Summary

  • Birds are warm-blooded.
  • Adaptations for flight involve features that are lightweight, flexible, strong and that take advantage of air currents.
  • Reproduction usually involves a courtship display, nest production, egg laying, incubation and parental care.
  • With 10,000 bird species, there is a lot of diversity. Specialized structures are adapted for specific habitats or living requirements.
  • Birds are important economically, ecologically, and in human culture.

Review Questions


1. List three traits which are important for flight.

2. Give an example of how a bird’s breeding system is adapted to avoid predators.

Apply Concepts

3. Explain how the absence of land predators on islands results in flightlessness in birds.

4. A large bird that nests with other birds has pale eggs even though the environment is brown and the eggs stand out to predators. Why have these birds not evolved camouflaged eggs?

Critical Thinking

5. You detect the presence of antibodies to the West Nile Virus in young chickens. How did the chickens get the virus? What does this mean about human West Nile infections?

Further Reading / Supplemental Links

Points to Consider

Mammals are next.

  • Birds and mammals are the only warm-blooded vertebrates. As in birds, mammals exhibit wide diversity and live in varied habitats. Based on what you know about adaptations in birds, how do you think mammalian limbs are adapted for movement in different habitats?
  • As in birds, mammals have different foods they eat depending on their environment. Instead of beaks, mammals have different kinds of teeth. In what way(s) do you think teeth in mammals are adapted for different kinds of diets?

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0
  5. [5]^ License: CC BY-NC 3.0
  6. [6]^ License: CC BY-NC 3.0
  7. [7]^ License: CC BY-NC 3.0
  8. [8]^ License: CC BY-NC 3.0
  9. [9]^ License: CC BY-NC 3.0
  10. [10]^ License: CC BY-NC 3.0
  11. [11]^ License: CC BY-NC 3.0
  12. [12]^ License: CC BY-NC 3.0

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