Like humans, birds have the senses of sight, hearing, smell, taste, and touch. They use these senses much as we do.
Sight
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.
Hearing
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, gamine, 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.
Touch
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.
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A considerable amount of training and experience are needed to enable one to sex chicks with a high degree of accuracy at a fast enough rate to make the undertaking an economic success at most hatcheries.
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