What Makes a Bird a Bird?

All birds belong to the group of animals called vertebrates, meaning animals having a backbone. Birds make up a special group of vertebrates called Aves.

Examples of other groups of vertebrates are - fish, amphibians, mammals, and reptiles.

Reptilian Relatives

Birds have a lot in common with Reptiles, such as turtles, crocodiles, and lizards. Scientists theorize that birds and reptiles are old relatives, and have many shared traits.

For example, both birds and reptiles:

• lay eggs
• have similar eyes and brain
• have similar skull and ear bones
• have partially hollow bones
• have similar blood proteins
• have scales covering parts of their body

Scientists also believe that birds have some very different traits from their relatives, such as feathers instead of scales (though most birds have reptile-like scales on their legs and feet), pointed beaks, and wings. People who study birds are called ornithologists.

All birds have the same basic parts and functions, but are unique in their own ways. All birds are warm-blooded, which means they can control and maintain a constant body temperature even if the temperature around them changes. Cold-blooded animals can only control their body temperatures by moving into warmer or cooler areas.

Feathers

All birds grow feathers, making them different from all other animals. The different types of feathers help a bird survive. Feathers not only help a bird to fly or swim, they also:

• protect its sensitive skin
• help attract mates
• serve as insulators to trap body heat
• serve as camouflage

Feathers are held together and in place by a hollow tube called the shaft or rachis. The shaft is made of a very hard material called keratin. Birds have three basic types of feathers.

Wings

All birds have wings even flightless birds such as ostriches and penguins. Birds' wings are attached to chest muscles called pectoral or flight muscles. In birds that actually fly those muscles are very powerful.

Wings are streamlined similar to an airplane's wings to move easily through the air. The wings are curved on top (convex) and are flat or slightly curved (concave) on the bottom. This special shape gives a bird the lift needed to get off the ground.

Once a bird is in the air, the outer ends of the wings ( flight feathers) act as propellers and rudders, helping the bird move up, down, and forward in the air - usually they don't go backwards - well except for hummingbirds!

The kind of flying a bird does depends on the size and shape of its wings, and even the type of prey that is pursued. The types of wings a bird may have include:

• Big wings that are broad and long. These wings allow large birds to soar and glide for long distances without flapping their wings, conserving energy. Birds of this group often have longer and stronger tails, which help them in landing and turning quickly.

• Short wings that are stubby and somewhat broad. These wings provide a lot of lift and some speed, and allow mid-sized birds, typically with short tails, to fly in and out of thick vegetation.

• Small wings that are thin, narrow, and have long pointed tips. These wings are built for speed and allow birds to spend a lot of time flying. The narrow wings provide less lift, and require birds to flap more often, compared to birds with big broad wings.

Even for flightless birds, wings are important. Penguins use their wings as flippers to help them swim underwater and Ostriches use their wings for balance as they run from place to place

Bones

A bird's skeleton is light and very strong. The bones are fused or joined together, giving the skeleton extra strength. The bones are also hollow or partially hollow, and some even have thin braces for support. Having a strong yet lightweight physique allows a bird to get off the ground and stay in the air, while giving it the strength needed to support flight muscles and protect internal organs.

Eyes

Birds often see much better than other animals, including people. They have very large eyes that focus keenly on near and faraway objects. Unlike many mammals, birds can see color.
Birds use their keen eyesight to: find food; spot mates; keep an eye on enemies and find a place to live.

A bird's eyes are usually located toward the front and/or sides of its head, and may point almost directly forward or in opposite directions, giving it one of two types of vision or a combination of both.

A bird that can focus its eyes independently, meaning that it can see two different objects at the same time (one with each eye) has monocular vision. Owls have monocular vision.

A bird that can only focus both of its eyes straight ahead on an object (sees an object with both eyes) has binocular vision, just like humans.

Examples of birds with binocular vision include eagles, falcons and hawks.

Illustration reproduced with permission from:
© Owls: Whoo Are They by K. Jarvis and D. Holt, 1996.

Although birds may have monocular or binocular vision, or a combination of both, they vary in their capability to move their eyes in their sockets. Many birds can see all around without moving their head.

Some birds have eyes that are relatively fixed in their sockets. These birds cannot roll their eyes around the way humans can. Instead, they have long, flexible necks that enable them to turn their heads to see in different directions. This is the reason you see some birds twisting and titling their heads.

Some birds can twist their heads from a half-circle (180°) to more than three-quarters of a circle (270°). Many birds have eye movement capability somewhere between the two extremes.

Ears

Birds have a keen sense of hearing. Their ears are located on each side of the head, just below and in back of the eyes. In most birds the ear openings are positioned symmetrically on either side of the head, but in some birds the openings are asymmetrical in size and position (owls).

Illustration reproduced with permission from:
© Owls: Whoo Are They by K. Jarvis and D. Holt, 1996.

Beak

Since birds have no teeth, their beak shapes vary depending on the type of food they eat. Birds use their beaks to:

• gather food
• feed their young
• tear food into pieces
• drink water
• touch their mates
• signal aggression by clacking them loudly
• preen their feathers
• collect-nesting materials
• attack/kill enemies
• scratch their bodies

Check out the
Curvy Beaks Activity Sheet

Nose

A bird's nose is on its beak. Birds seem to have a poor sense of smell and rely heavily on their sense of sight and hearing to find food and avoid predators. However, one exception is the vulture. The olfactory part of a vulture's brain is well developed, and they rely heavily on their keen sense of smell to locate food.

Tongue

Bird tongues come in many shapes and sizes, and are used in many different ways. Birds use their tongues to taste, spear, tear and hold their food.

Legs and Feet

Birds' legs and feet come in many different shapes and sizes, and reflect the different ways they make their living. Most birds have three or four toes, while the ostrich just has two.
Birds that rarely land, like swifts, have extremely weak legs and find walking very difficult. The pointed ends of birds' toes are called talons, which vary in shape, size, and sharpness depending on how they are used.

See Sharp Feet Activity Sheet

Breathing

Birds have an efficient breathing system, with two lungs that have special balloon-like air sacs. These air sacs spread into different parts of a bird's body, including the hollow parts of the larger bones. The air sacs allow a bird to:

• store up more air
• push more air through the lungs
• help cool down if too hot
• bring more oxygen to the cells
• help some swimming birds stay afloat

Birds need a lot of oxygen in order to turn the food into extra energy needed for flying and maintaining body temperature.

Birds do not have sweat glands, and cannot sweat the way humans and other mammals do to cool off. Instead, birds pant, breathing in and out very quickly in the same manner as a dog.
Panting cools a bird by evaporating water from the lungs, throat, mouth, and other parts of the body. Birds can also cool off by taking a bath or sitting in shade.

Illustration reproduced with permission from:
© The Cambridge Encyclopedia of Ornithology by M. Brooke and T. Birkhead, 1991. Cambridge University Press, New York, NY.

Crop for Storage

When a bird swallows food, muscles in the esophagus push the food down the throat. In many birds, the bottom of the esophagus forms a large sac called the crop.

The crop simply stores undigested food before it enters the stomach. Birds with a crop can gorge (eat more than needed) when food is found, store it in the crop, and then slowly digest it later.

Illustration reproduced with permission from:
© The Cambridge Encyclopedia of Ornithology by M. Brooke and T. Birkhead, 1991. Cambridge University Press, New York, NY.

Teeth in the Stomach

Once food has passed from the crop into the stomach it is attacked by strong acids to help digest the food chemically. The partially digested food then passes into the gizzard, a specialized muscular portion of the stomach. A bird will use its gizzard in the manner that other animals use their teeth, to grind and crush hard nuts, seeds, grain, and other foods. Birds do this because they don't have teeth.

Some birds may swallow small stones and grit that can help the gizzard grind and crush. Things (feathers, fur, stones, bones, etc.) swallowed by birds that the stomach cannot break down, are stored in the gizzard and regurgitated later as pellets.

Preen Gland

Many birds have what is called a preen gland located just above the base of the tail. This special gland secretes oil that the bird rubs over its feathers with its beak. This is called preening.

This oil . . .

• helps condition and clean the feathers
• helps make the feathers water-repellent
• may contain special vitamins absorbed into the skin, helping to keep the bird health