Merck Manual

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Pet Owner Version

Description and Physical Characteristics of Birds


Manuals Staff

Reviewed/Revised Jul 2011 | Modified Oct 2022
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There are between 8,700 and 9,600 living species of birds today. These range in size from tiny (such as hummingbirds) to huge (such as ostriches and condors). Bird species are divided into 2 superfamilies, the Paleognathae or “old jaws,” and the Neognathae or “new jaws.”

The Paleognathae family includes 5 orders, the Tinamiformes, the Rheiformes, the Casuariiformes, the Apterygiformes, and the Struthioniformes. The Tinamiformes order is comprised of some 45 species, most in South and Central America. Members of the other orders are flightless and very large. Collectively, they are often known as ratites and include the ostriches, the emus and cassowaries, the kiwis, and the rheas.

The superfamily Neognathae is huge and includes most of the bird species that are alive today. While over the millennia, individuals from a large number of these species have been kept as pets at one time or another and some have been domesticated, only a relatively small number of species have been widely kept as companion animals. With some exceptions (such as swans and peacocks) these pet species have been birds that have adapted to caged life and have some specific appeal such as very colorful feathers, a pleasing range of calls, the ability to mimic human speech, or engaging behaviors.

Up until the mid-twentieth century, canaries were one of the most popular pet birds, prized for their beautiful songs and often colorful feathers. Parrots have long been another popular companion bird. For many bird owners, the attraction of these birds has been the often colorful feathers and the ability to thrive under human control. Today, members of the order Psittaciformes (the parrot order) include the cockatoo family (Cacatuidae) and parrots (Psittacidae). Other popular companion birds in this same order include budgerigars, lovebirds, and lorikeets ( see Table: Popular Companion Birds Popular Companion Birds Popular Companion Birds ).


Although birds are very different from human beings, they also share certain characteristics with people. For example, birds have all of the same senses (sight, hearing, touch, taste, and smell) that we have. While their front limbs are wings and their feet, beaks, and tongues have taken on many tasks for which we use our hands, birds are still creatures with appendages that they use to interact with their environment. A bird uses its beak as an additional appendage to assist with communication, mobility, and social interaction, such as grooming.

Some birds, such as canaries, have melodious voices while others may vocalize in screeches at volumes loud enough to cause ear pain in their owners. Individuals desiring a pet bird may select from hundreds of possible birds and should match their interests and lives to a suitable bird.

Body Size

Birds kept as pets come in sizes from tiny (many finches are only 4 inches [10 centimeters] from the beak to the end of their tail feathers) to large (wing spans of parrots can easily reach 40 inches [1 meter]) and in colors from dull grays and browns to brilliant reds, yellows, greens, and blues. Because they are specially adapted for flight, most birds weigh very little. Even the largest captive parrots (some types of Macaws) rarely exceed about 2.5 pounds (1,200 grams), and medium to large parrots may weigh from a half pound to 2 pounds (250 to 900 grams). Their bones are particularly light, and some are filled with air.


Birds have a much more rapid metabolism than people. Pet birds’ normal body temperatures usually range from 101 to 107°F (38.3 to 41.7°C), depending on the species. The more active a bird is, the more food it must consume relative to its body weight. Highly active birds, such as hummingbirds, may consume their body weight in food each day. Birds have very efficient digestive systems that allow them to eat enough to provide their bodies with needed energy while minimizing their body weight to allow flight.

While there are wide differences between types of birds, most birds’ heart rates will be faster than that of a healthy human. For example, a normal human heart beats about 70 times per minute, while a small songbird’s heart may beat 500 times in a minute and a hummingbird’s heart will beat about 1,000 times in a minute. Birds’ heart rates vary much more than people’s do, increasing dramatically following stress or exercise. An Umbrella Cockatoo, for example, may have a resting heart rate of 120 beats per minute, but that will increase to over 300 if it is nervous or startled.

There are differences in the lungs as well. Human lungs operate rather like bellows; we breathe in and out. Birds, however, have lungs that are filled continuously and possess air sacs to take in fresh air and expel used air including carbon dioxide. The air sacs also divert warm air into bones to help water birds and other species remain buoyant in water, maintain warmth in cold weather, and dissipate some of the heat generated during flight.

Temperature Regulation

Birds do not sweat, but they have developed other strategies to stay cool in very warm conditions. Most birds will hold their wings out to cool off. If water is available, they will bathe and cool as the water evaporates. Another cooling technique in birds is panting. In many cases involving pet birds, panting is a sign that the bird is seriously overheated. Panting involves a more rapid breathing rate (more breaths per minute) and, in many species, a rapid fluttering of the throat. Fluttering causes heat loss from the mucus membranes of the throat and from heavily filled blood vessels.

Overheated birds will often have hot feet, red nares (nasal openings), and hot beaks. The bird’s breath will feel obviously hot against your skin. This is an emergency, and you should contact your veterinarian immediately for advice on cooling the bird. While holding the wings out, panting, and hot feet and beaks may be signs of overheating, they can also signal other problems such as fever or respiratory distress. You should quickly evaluate your bird’s environment to determine whether overheating is the likely cause of distress.

If it is clear that the bird has been overheated, your veterinarian may recommend that you place the bird in a shallow pool of tepid water (not cold water, as this could bring on shock in the bird). Be careful that the bird’s beak and face are not covered with water. The bird must not inhale any water. Allow the bird to stay in the water until the nares return to a normal color and the bird’s breath is a normal temperature. Then, return the bird to a cage placed in an area that is not too warm.

Feathers provide good insulation for any bird and, within limits for each species, provide protection from low temperatures. Birds will often sit with their feathers covering their feet to reduce heat loss when it is cold. Fluffing their feathers also helps protect against the cold. Sitting with their darkest colored feathers toward the sun is a technique used by some birds to absorb additional heat. While tropical birds, especially canaries, are less susceptible to chilling due to drafts than has often been reported, abnormally cool temperatures are not good for your bird’s health.

If your bird is fluffing its feathers for extended periods or sitting on its feet, it may be chilled. Check the air temperature close to the cage and along its sides. Your bird may be exposed to cold drafts or air that is colder than appropriate. Cages placed close to windows are often colder than other areas of the room. Relocating the cage to a warmer spot can help your bird feel more comfortable and stay healthy. Fluffed feathers may also be a sign that your bird is seriously ill. If your bird is fluffing its feathers for an extended period, the bird should be checked for health problems.

The Senses

Like humans, birds have the senses of sight, hearing, smell, taste, and touch. They use these senses much as we do.


Birds depend heavily on the ability to see. Not only is good eyesight needed for flight, it is also critical to finding food and water, finding a mate, and avoiding predators. The degree to which a bird depends on sight is evident in the size of a bird’s eye in relationship to its body. In humans, the eyes take up only about 1% of the weight of the head. In birds, the eye makes up a much larger portion. For example, European Starlings have eyes that make up about 15% of the head. In many birds, the weight of the 2 eyes is greater than the weight of the brain. And, in comparison with human brains, the optic lobe in a bird’s brain is larger and better developed.

The keen eyesight of birds is also due to the position of the eyes in the head, the shape of the eyeball, the ability to focus rapidly, light regulation, and special variations in the retina of the eye. A bird’s eye adjusts to the level of light about twice as fast as a 20-year-old human. The lens in the eyes of many birds is very flexible, allowing them to rapidly change their focus from near to far. This is an advantage for birds that must spot their dinner from above and then accurately swoop down to catch it. Color vision is not universal in birds, but it does occur in many. Some birds can even see colors outside the range of humans. For example, several species of birds are known to be able to see ultraviolet light. It is believed that birds with ultraviolet light perception use this ability to help them choose mates.

These abilities, critical as they are, would be of little help to birds if they did not have a way to protect their eyes from drying out during flight. This task is accomplished by a third eyelid, known as the nictitating membrane. The nictitating membrane is clear, allowing birds to see while using the membrane to cover and protect the eye during flight. It also helps them “blink” and keeps their eyes moist and clear of debris. The easily visible eyelids in birds are normally closed only when the bird sleeps.


Most birds have ears located somewhat behind and just below the level of the eye. The opening of the ear is concealed by specialized feathers known as ear coverts. These feathers have a texture that differs from other feathers on the head. If you very gently move these feathers aside, you can see the opening of your bird’s outer ear. The exact shape and size of the ear opening varies between species. Sound captured by the outer ear is passed to the middle ear and then on to the inner ear. The inner ear passes these signals to the brain, which interprets the sounds.

Some birds may suffer an injury to, or infection of, the outer ear that can affect hearing. For example, macaws that started life as stunted chicks may have outer ear problems due to early infection or scarring. Lovebirds appear to be more prone to external ear infections than other species of birds, but all species of birds can experience outer ear infections. A regular check of your bird’s outer ears for signs of inflammation or accumulations of debris can help you catch such infections early when they are more easily treated. If you find reddened outer ear tissue or debris in an outer ear of your bird, you should take your bird to the veterinarian for a physical examination.

In birds, as in humans, the middle ear not only processes sounds, it also provides the sensory information the animal needs for balance and equilibrium.

Many birds have excellent hearing, although the frequency range over which they hear is somewhat narrower than the human hearing range. What is lost due to narrower frequency range is often more than compensated for by the ability to distinguish small changes in pitch and beat. Owls, for example, can sense rapid fluctuations in pitch and intensity 10 times more accurately than humans. Some owls use this ability to find their prey, even in complete darkness.

In their natural environments, birds use their voices and their hearing to help them find mates, avoid danger, stake out their territory, and communicate with other members of their flock. These same abilities are used when the birds are companions. For example, many parrots will listen carefully to the sounds they hear and can accurately imitate the words their owners use frequently. Many will exchange vocalizations with their owners, and it is quite common for birds to react to the sounds that are a part of their daily life. For example, many pet birds have learned that opening a refrigerator or cabinet door precedes the arrival of food. When they hear the sound of the refrigerator or cabinet being opened, they will often vocalize or start their own particular prefeeding behavior.

Smell and Taste

It was long believed that smell is a poorly developed sense in most birds. However, research over the past 20 years has shown that, while birds’ sense of smell may be more limited than some animals, birds do rely on this sense for feeding and navigation. Many birds, including vultures and sea birds, depend on their sense of smell to find food. The sense of smell also helps pet birds pick out their favorite foods. Many bird owners know that, even when they hide a preferred food, their pets will use smell to locate their favorites. In other cases, smell may be a critical sense in navigation. For example, homing pigeons were tested for the use of smell in finding their home nests; birds with experimentally plugged nostrils took longer to find their way home.

Taste is another sense which is believed to be similar to that of humans. In birds, the taste buds are located on the back part of the tongue and the bottom of the throat. Birds have fewer taste buds than humans, but as any bird owner will agree, birds do have taste preferences and have favorite foods based, at least partly, on taste. Parrots, hummingbirds, and other fruit and nectar feeders are known to have a fine sense for the differences in sweet and sour foods, while seed eaters seem to have no preferences for sweet or sour foods. Most birds can sense salt levels in foods. Bitter tastes are also sensed by birds and many will reject highly bitter foods. Research published in 2004 reported that caged cockatiels were able to detect and reject water with very small amounts of quinine, gramine, hydrolysable tannin, and condensed tannin. This research suggests that cockatiels—and probably other birds—use taste to detect and possibly avoid consumption of potentially toxic chemicals. This same sense of taste, along with texture, temperature, and color can be used to select preferred or favorite foods.


A bird’s skin possesses nerve endings that relay information to the brain about its environment and its condition. Birds are aware of any injury and sense pain, just as people do. Some birds have sensory bristle feathers located around the eyes, nostrils, and mouth. There are also concentrated nerve endings called Herbst corpuscles located in the bills or beaks and the tongues of some birds. Similar sensory cells are also found at the base of flight feathers and it is likely that these special sensory areas play an important role in flight.


Over many thousands of years, birds’ front limbs have become specially adapted for flight. The wings, together with the feathers, are what allow a bird to fly. They provide the lift that is needed to get off the ground and move in the air. The bones of the wing are particularly strong and light, and they serve as an anchor for the feathers and for the powerful muscles and tendons needed for flight.

Each bird has a wing shape that is appropriate for its life pattern. For example, long narrow soaring wings are typical of birds (such as the albatross) that spend much of their life soaring along thermal winds. High-speed wings are useful for birds that travel at fast speeds, such as swallows. Eagles, falcons, and other raptors have high-lift wings that allow them to get off the ground even with a heavy weight in their talons. Elliptical wings are shaped like a half of an oval. This shape increases maneuverability in the air and allows birds to rapidly change direction in flight—a useful skill for songbirds, sparrows, and others that must avoid becoming another bird’s dinner.

The speed at which a bird can fly varies greatly based on species and breed. As a general rule, the flight speed of birds varies from about 15 miles (24 kilometers) per hour to about 50 miles (81 kilometers) per hour. Most birds will have 2 flight speeds, one for ordinary flight and a second accelerated speed they use for escaping predators and chasing other birds. The accelerated speed may be up to twice their normal flight speed. Small songbirds and perching birds will, in the wild, have a normal flight speed close to 15 miles (24 kilometers) per hour. Larger birds in their original habitat will often reach 25 miles (40 kilometers) per hour. These flight speeds are for birds that have developed their flying skills over almost their entire lives and have strong flight muscles. In general, pet bird flight speeds are somewhat lower than those of the same species that have lived and flown in the wild. One exception to this general guideline is racing pigeons. Racing pigeons are raised and trained for speedy flight. These birds, also known as homing pigeons, can often cover 30 miles (48 kilometers) per hour over extended distances.

Beaks and Feet

Over time, both the beaks and feet of birds have adapted to the many different environments in which birds live. As Charles Darwin first noted during his famous visit to the Galapagos Islands, beaks have developed to help a particular bird eat its preferred diet. For example, hummingbirds have long, narrow beaks suited for eating nectar from flowers; hawks have sharp beaks useful for tearing apart their prey; and birds on islands where cacti are plentiful have beaks that allow them to efficiently find and eat cactus fruit.

Parrots have beaks that are strong and shaped rather like hooks. With their flexible necks, parrots can use their beaks not only to crack tough outer shells on nuts and fruit, but also to preen and groom and to defend themselves. In addition, their strong beaks can be used to help them climb and swing and to help build nests and feed their chicks.

By way of comparison, canaries have straight, short beaks. These beaks are ideally shaped for finding and consuming the seeds and grubs that comprise their diet in their native environment on the Canary Islands and Madeira. However, the short beaks are not as useful for climbing or defending themselves.

Birds do not have teeth; their jaws, therefore, are light in weight. This is just one of many adaptations in birds to reduce body weight and make flight easier. Birds use a special body organ, known as the crop, to store their food before it is passed on to the stomach. Many, though not all, birds will eat and store sand and small stones in their ventriculus (gizzard) to assist in grinding food.

Birds’ feet have also adapted to their particular habitat. Waterbirds, including ducks and geese, have webbed feet. As any scuba diver can confirm, webbed feet are very helpful in moving in the water. Owls, hawks, falcons, eagles, and other raptors have feet with sharp talons that help catch their prey. Perching birds (passerines and psittacines) have feet with specialized tendons for grasping tree branches. And smaller birds, such as sparrows, have feet that adapt to flat surfaces as well as branches.

Bird beaks

Bird beaks

Parrots, many of which are native to jungles, have feet that have evolved to function very efficiently. Parrots use their feet not only to help them perch securely on branches, but also to hold and move objects and foods in their environment. Unlike most other birds, parrots frequently use their feet in ways that people would use their hands.

Skin and Feathers

Birds have a sensitive skin that allows them to feel and sense pressure, heat, and cold. The feathers of a bird grow from the inner layer of skin (the dermis). Feathers provide assistance in flight, insulation from cold, waterproofing, and—in some cases—camouflage. In some species, the feathers can also indicate sex and mating status.

There are 8 main types of feathers, including 3 types of contour feathers, semiplumes, down feathers, natal down, bristles, and filoplumes. Contour feathers cover most of a bird’s body and are used for flight. Each contour feather has a central hollow quill that starts at the base of the feather and connects to a central shaft known as the rachis. Branching out from the rachis are vanes which, in turn, support the barbs. Each barb has many smaller, hooked segments known as barbules. The barbules hook together, making the feather strong and uniform. There are 3 types of contour feathers: body feathers, flight feathers (wing feathers called remiges and tail feathers called retices), and ear coverts.

Down feathers are small and soft. They grow between the contour feathers and keep the bird warm. Adult down feathers are structurally different than the down feathers found on chicks (natal down feathers). Some newly hatched chicks have only natal down feathers. Anyone who has ever slept under a down comforter knows how soft and warm these feathers are.

Sensory bristles are located around a bird’s eyes, nostrils, and mouth. They play a role in the sense of touch.

Filoplume feathers are long and shaped like hairs. Their function is not fully understood, although it is probable that some birds (for example, psittacines) use these feathers to sense vibrations and changes in pressure—abilities that would be very helpful during flight. Not all species of birds have filoplume feathers.

Preening is how birds maintain their feathers. They use their beaks or bills to clean their feathers and keep them aligned correctly. At the same time, some birds waterproof their feathers using an oil produced by a gland (called the uropygial or preen gland) located near the tail. Other birds use a powder formed by powder down feathers to waterproof their feathers.

Flight and normal rubbing against objects in their environment can damage and wear feathers. Molting is the process by which birds renew their feathers. During the molt, an old feather is pushed out of the skin by the growth of new feather cells at the base of the old feather. The weight of a feather has nothing to do with its loss; it is the growth of a new feather that dislodges the old one. In most birds, feathers are discarded on both sides of the bird simultaneously to avoid problems with flying. Most birds replace all their feathers at least once a year, most often after the breeding season. Some birds molt more frequently. No matter how frequently a bird molts, their bodies naturally time the loss of feathers in such a way that the bird always has sufficient feathers for warmth and flight.

Mouth and Digestive System

Birds do not have teeth. They use their beaks to break up the food they eat. Also, birds lack a soft palate at the back of their mouths. The food is pushed along by the muscles in the esophagus (the tube that connects the mouth to the stomach) until it reaches the crop, where it is stored and is sometimes partially digested before passing on to the stomach.

Unlike humans, birds have a 2-part stomach consisting of the proventriculus and the ventriculus or gizzard. The proventriculus produces digestive fluids that help break down the food. Once the digestive fluids have soaked into the food, it moves on to the gizzard where it is ground up. In some birds, the work of the gizzard is enhanced by sand and small stones that the bird has swallowed for this purpose. This grinding material is called grit. Not all birds need it, but some do. Your veterinarian can advise you on the amount and type of grit that is most appropriate for your particular bird.

From the gizzard, the bird’s food passes into the intestines, where intestinal enzymes as well as secretions from the liver and pancreas help finish digestion. Food then passes into the large intestine, colon, and finally into the cloaca. Here both liquid and solid waste (urine and feces) are stored prior to exiting the body through the vent. The vent is located near the base of the tail feathers.

Digestive system of a bird

Digestive system of a bird
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