Birds do not see the world the way humans do. They can see in ultraviolet and can navigate using magnetism, polarized light, and celestial cues. They are sensitive to changes in gravity and air pressure.
Bird eyes are large and prominent structures. In starlings, they account for 15% of the mass of the head. Eagle and owl eyes are as large as human eyes. Bird eyes can vary in shape from round to flat to tubular, and are capable of only limited movement.
Bird eyes are usually set on the sides of the head, giving good vision to the sides but with little binocular vision. Depth perception is aided by bobbing the ehad quickly, to see an object from two different directions.
Birds have three eyelids - upper and lower (like us) and a nictitating membrane that is typically thin, transparent, and cleans and moistens the cornea with each blink. Diving birds use this membrane as goggles, which owl membranes are opaque and used to protect the eyes.
Bird eyes are similar to human eyes, but with some marked differences.
Bird color vision is based on visual pigments in the cone cells, which convert light into nerve impulses. Birds have four types of cones: red, blue, green, and ultraviolet. (Humans have only red, blue, and green.)
Bird cone cells also contain colored oil droplets - which range from yellow to red. These protect the eye against damaging UV light. (Mammals have yellowish lenses, which do the same.) These also increase the contrast of objects seen against different backgrounds - yellow for blue background (sky), red for green background (foliage).
Ultraviolet plumage influences choice of mates, dominance, and reporductive success. The ability to see UV plays other roles, as well...
Nocturnal birds, such as owls, tend to have far fewer cones in their retinas - which contain mostly rods. Rods are simple receptors that see in black and white.
Magnetic Field Detection
Birds have been shown, in experiments, to be sensitive to even very small changes in magnetic fields. There are two main systems that they use to detect and use the Earth's magnetism to orient themselves.
Birds' "internal compasses" are so sensitive that they can be affected by natural factors that affect the Earth's magnetic fields, such as sunspots or hills of iron ore. These have a greater affect on birds migrating at night because birds use the sun as a guide during the day.
Like humans, birds have an outer, middle and inner ear.
The outer ear of birds lacks the pinna of mammals. Instead, birds have a set of specialized auricular feathers that protect the ears from air turbulence during flight, while funnelling sound waves into the ear canal (like the pinna does for mammals). Diving birds have strong protective auriculars, and can close their outer ears when diving. Nocturnal owls have efficient auricular funnels, and have ear flaps that can change the size of the outer ear opening - which enhances acoustics more than five times.
The middle ear consists of the eardrum (tympanic membrane) and one ear bone - the columella (or stapes). The ear bone connects the ear drum to the inner ear, and carries sound vibrations from one to the other.
The inner ear contains the cochlea which, like in human ears, converts sound vibrations into nerve impulses. This is done by hair cells in the cochlea. This is similar to humans, except that bird hair cells can regenerate after being damaged to restore full hearing - human hair cells cannot.
Bird ears have a simpler structure than mammal ears, but work just as efficiently.
Experiments show that most birds do not hear quite as well as humans. Mammals in general tend to have a wider range of hearing than birds, and humans can hear fainter sounds than most birds at most frequencies. The exception to this is owls. Great horned Owls are more sensitive to low-frequency sounds than humans; Barn Owls are more sensitive to high frequency sounds. However, birds do not hear ultrasonic sounds (above the range of human hearing).
Birds can detect small changes in frequency (pitch) and intensity (volume) of sounds - but humans can do it better. Birds can also detect changes in the length of notes, gaps in song, and the speed of change in song - as can other vertebrates. This directly relates to birds' abilities to recognize songs.
Owls locate prey by sound in complete darkness. Barn Owls, for example, are able to do this because the position of their ears is asymetrical. The ruff of feathers around the face enhances this - the left ruff faces slightly downward (more sensitive to sounds below the horizontal), the right ruff faces slightly upward (more sensitive to sounds above the horizontal). The owl tilts its head to equalize the sound input in both ears - this points it to the mouse.
A few species use echolocation (swiftlets of SE Asia, Oilbird of S. America), but at normal frequencies - not ultrasonics, like bats. This means that bird echolocation is only about 1/10th as effective as that of bats.
Mechanoreception refers to the mechanical stimulation of the senses. This can be through touch, balance (equilibrium), and the detection of air pressure.
Tactile sense (touch) is monitored by the tactile corpuscles. These have a layered sheath that surrounds a central nerve fiber. The layers of the sheath detect differences in pressure. Tactile corpuscles in the bill or tongue aid in finding small prey in the mud or in crevasses (sandpipers and woodpeckers). In the wing joints, they help monitor wing positions during flight. They are also present in the base of filoplumes and bristles.
Equilibrium is detected by the semicircular canals of the inner ear - just like in humans. The three canals (per ear) are filled with fluid. When the bird moves its head, the fluid moves through the canal. Small crystals of CaCO2 are suspended in the fluid, and push against nerve hair cells in the base of each canal. This gives the bird a sense of the direction of gravity and acceleration.
Sensitivity to air pressure is thought to be housed in a small structure in the middle ear. Evidence that birds detect changes in air pressure includes...
Taste & Smell (Chemical Senses)
Bird taste buds are similar to ours, but far fewer in number. Humans have about 10,000 taste buds on our tongues. Birds have them on the back of the tongue and the floor of the pharynx: 24 in chickens, 37 in pigeons, and 62 in Japanese Quail. Most likely, birds can taste sweet, sour, salty, and bitter (like us), but with less sensitivity.
On average, birds' sense of smell is comparable to that of mammals. Birds use their sense of smell to...
Birds have large brains for their body size, comparable to mammals. Parrots, owls, crows, woodpeckers, and hornbills have larger than average brains.
As with mammals, the forebrains and midbrains of birds are larger and more developed than reptiles. However, birds have a larger optic center and cerebellum (which controls balance and coordinates muscles - especially during flight).
When discussing the brain, it helps to consider two types of elements:
For over 100 years, bird brains were thought to be mainly composed of striatal elements. Recently, it has been discovered that they are largely made up of pallial domains.
Bird brains also show functional lateralization. This means that each side of the brain controls slightly different tasks, and that one side is generally dominant. As with humans, the left hemisphere of a bird's brain is usually dominant - which makes the bird "right-handed". If the left hemisphere is damaged, the right side can take control of its functions. Before it was discovered in birds, functional lateralization was thought to be exclusive to humans.
Memory is controlled, in birds and mammals, by a part of the brain called the hippocampus. The structure and function of the hippocampus is equivalent in both birds and mammals.
Spatial memory is seen in birds as the ability to find and revisit the same location reliably. These locations could be nest sites, feeding places or breeding/wintering grounds. Seed-caching birds have extraordinary spatial memory - and an enlarged hippocampus. Three specific bird families - Corvidae (crows, jays, nutcrackers), Sittidae (nuthatches), and Paridae (chickadees and titmice) - cache thousands of seeds annually. For example...
Bird brains have the ability to grow new neurons. In fact, bird brains can gain and lose neurons seasonally as they need more or less memory.
One advantage to this may be that it allows birds to continue learning new skills or information throughout their lives by refreshing parts of the brain. Long-term memory is stored in other neurons that are kept over the bird's life.
Sleep has evolved as a way to maintain the neural circuitry of the brain. It does this by stabilizing the synapses and allowing neurons to function properly. Without sleep, neurons do not function as effectively. This can lead to short-term memory loss and reduced coordination, among other things.
Birds go through three stages of sleep...
Studies have shown that birds dream. Zebra Finches were found, in one study, to practice new song patterns in their dreams.
Cognition - taking in and processing information from the environment. Cognition includes...
It is basically all the ways in which animals take in information through the sense, process this information, retain it in memory, and decide to act on it.
Evidence of intelligence in birds is found in examples of...
Intelligence comes at a cost: larger brains require greater resources and longer incubation times to mature. Parental care, foraging skills, life span, and play behavior all have an effect, as well.
Birds often out-perform many mammal species in lab experiments that require them to solve problems. One such experiment is the Krushinsky Experiment - in which the animal looks through a slit at two food dishes (one empty, one full). The food dishes move out of sight in opposite directions, and the animal has to decide which way to go to get the food. Crows and dogs score well on this test; cats, rabbits and chickens do poorly.
Counting is another area where birds excel. Monkeys may need thousands of trials to learn this skill (rats never do), but ravens and parakeets learn quickly and can identify a box of food by the number of small objects in front of it.
Insight learning is learning by watching and imitating others. This is common among many species of birds.
Pigeons can be taught to communicate using symbols. They have been found to have the ability to...
Examples of innovative foraging...
Birds select food based on the energy received from the food compared to the amount of energy expended to get it.
Some birds use time as a tool in how they forage for food.
Several types of birds have been known to use bait to attract prey...
Some birds use tools to get food...
Crows and jays demonstrate intelligence and cognition on a level similar to chimpanzees and other great apes.