Senses, Brains & Intelligences

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.

 

Vision

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.

 

Eye Anatomy

Bird eyes are similar to human eyes, but with some marked differences.

  • Like human eyes, bird eyes are divided into two compartments, separated by the iris and lens.  However, bird eyes also contain 12 to 15 small bones, called [sclerotic] ossicles, which make up a scleral ring.  As with human eyes, each compartment is filled with a gel-like fluid which helps maintain its shape.  The forward compartment contains the aqueous humor; the larger rear compartment contains the vitreoushumor.
  • Two sets of muscles attach to the scleral ring and act to focus the eye.
    • Crampton's muscle changes the shape of the cornea to aid in focusing.  (The cornea is the clear outer layer that covers the front of the eye and allows light to enter.)  Changing the cornea does not aid focusing underwater (since its refractive index is close to that of water), so diving birds tend to have weak Crampton's muscles.  Mammals, like us, do not adjust the cornea for focusing at all.
    • Brucke's muscle changes the shape of the lens, to focus the light passing through it.  Lens shape varies widely among different species of birds.
  • The pupil is the hole through which light passes into the rear chamber of the eye.  Bird pupils are round, like ours, except for those of skimmers - which are similar to cat pupils.  (Closing to a vertical slit helps filter out polarized light reflecting off of water.)  The size of the pupil is controlled by the iris, which varies in color from deep brown to bright red or yellow to white to green to pale blue.  (This may aid species recognition.)
  • The retina is the inner lining of the eye, which contains the light receptor cells: rods and cones.  Bird have a nuch higher density of cones that humans do - giving them much sharper vision.  At best, human cones cells are packed as densely as 200,000 per mm2.  House sparrows have 400,000 cones per mm2, and Common Buzzards (a close relative of the Red-Tailed Hawk) can have 1 million per mm2.
  • A fovea (pl. foveae) is a depression in the retina that contains the highest concentration of cones.  Humans, and most birds, have one fovea in the center of the retina (opposite the pupil).  Fast-flying birds (hawks, eagles, hummingbirds, terns, swallows, and kingfishers) also have a second fovea to help them judge speed and distance accurately.  (These birds also tend to have forward-looking eyes, which gives binocular vision and, thus, depth perception.)
    • Some birds also have a horizontal ribbon of high-cone density across their retinas.  This seems to help them perceive the horizon, and aids in sensing body position.
    • Birds can resolve both very fast movements and very slow movements.  We see a fluorescent light as steady, but a bird would see it flickering (60 Hz is the flicker-fusion frequency for humans).  They also seem able to see stars moving across the sky at night.  Flicker-fusion article here.
  • The pecten is a large, black structure attached to the retina near the optic nerve.  It is pleated (usually 20 pleats, fewer in nocturnal birds) and vascularized (contains blood vessels).  There have been at least 30 different theories as to its purpose, but recent evidence suggests that it is to supply nutrients and oxygen for the retina.  (Unlike mammal retinas, bird retinas do not contain a network of blood vessels.  This interferes less with vision.)

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...

  • Common Starlings feed nestlings with UV-reflective skin, in preference over those that do not.
  • The european Redwing thrush prefers eating Viburnum berries that reflect UV to those that do not.
  • Common Kestrels use UV light to hunt voles - which line their trails with urine and feces that shine in UV light.

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.

  1. Birds' ophthalmic nerves contain some particles of magnetite-like substance.  (Magnetite is a magnetic crystal, which is attracted by even weak magnets.)  Nerve tissues or pressure sensors around these particles could detect their movement in response to magnetic fields.  These receptors are sensitive to very small changes in local magnetic field, and give the bird a sense of its location - especially when migrating.
  2. Birds' retinas also respond to magnetic fields.  The photopigment rhodopsin (found in rod cells) is thought to be able to convert magetic fields - as well as light - into nerve impulses.  These receptors are sensitive to "poleward" and "equatorward" directions of the Earth's magnetic field, and give the bird a sense of direction.

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.

 

Hearing

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

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...

  • the tendency of birds to feed more actively before a winter storm to build up their energy reserves. 
  • the ability of birds to choose the best altitudes for migration.


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...

  • select and reject food items (goslings),
  • select nest-building materials (starlings),
  • locate food (many species),
  • select mates (many species),
  • locate their nest in dark conditions,
  • locate and identify mates and young.


Bird Brains

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:

  1. Striatal elements of the brain control instinctive behaviors and reflexes.
  2. Pallial elements of the brain allow advanced learning and cognitive (thinking) abilities.

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.

 

Spatial Memory

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...

  • One titmouse may cache 50,000 seeds (each in a separate location) each autumn - and still find its cache up to 28 days later.
  • A Clarke's Nutcracker may hide 22,000 to 33,000 pine seeds, spread across 2,000 different cache sites, and relocate them as much as nine months later.


Neurogenesis

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.

  • Atlantic Canaries grow new neurons in the spring when they learn new songs.  These neurons die out in the fall when the birds stop singing.
  • A Chickadee's hippocampus may expand by 30% in the fall, when they need to remember where they cached all those seeds.  It then shrinks in the spring when insects become available as a food source.

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

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...

  1. Slow-Wave (SWF) Sleep only uses one side of the brain at a time.  A bird experiencing SWF (or unihemispheric) sleep only closes one eye - they other, which is controlled by the "awake" side of the brain, watches for predators. 
    • Mallards on the edge of a flock use SWF sleep more than those in the middle, so they can act as sentries for the flock.
  2. Intermediate Sleep - possibly a transition between SWF and REM sleep?
  3. Rapid Eye Movement (REM) Sleep uses the entire brain, so both eyes get closed.  Birds tend to have short, frequent REM sleeps, alternating with SWF sleeps.

Studies have shown that birds dream.  Zebra Finches were found, in one study, to practice new song patterns in their dreams.

 

Intelligence

Cognition - taking in and processing information from the environment.  Cognition includes...

  • perception
  • learning
  • memory
  • decision making

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...

  • complex social interaction
  • creative foraging behavior
  • problem solving

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.

Crow Mystery Solved

Researchers for the Massachusetts Turnpike Authority found over 200 dead crows near greater Boston recently, and there was concern that they may have died from Avian Flu.  A Bird Pathologist examined the remains of all the crows, and, to everyone's relief, confirmed the problem was definitely NOT Avian Flu.  The cause of death appeared to be vehicular impacts.  However, during the detailed analysis it was noted that varying colors of paints appeared on the bird's beaks and claws.  By analyzing these paint residues it was determined that 98% of the crows had been killed by impact with trucks, while only 2% were killed by an impact with a car.  MTA then hired an Ornithological Behaviorist to determine if there was a cause for the disproportionate percentages of truck kills versus car kills.

The Ornithological Behaviorist very quickly concluded the cause: 
when crows eat road kill, they always have a look-out crow in a nearby tree to warn of impending danger.

The conclusion was that while all the lookout crows could say "Cah", none could say "Truck."

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...

  • recognize up to 725 different visual patterns
  • distinguish synthetic (human-made) objects from natural ones
  • distinguish different styles of painting,
  • communicate with visual symbols


Innovative Foraging/Feeding

Examples of innovative foraging...

  • English Titmice learned to rip the cardboard tops off of milk bottles, so they could drink the cream.  (This evolved from normal bark-tearing behavior.)  As the skill spread through the whole population of titmice in England, milk companies had to switch to using aluminum caps.  The titmice learned to open these, too.
  • In the Galapagos Islands, Small Ground Finches and Large Cactus Finches learned to move rocks with their feet by bracing their heads against a larger rock.  One 27g bird was observed moving a 378g rock.

Birds select food based on the energy received from the food compared to the amount of energy expended to get it.

  • White Wagtails eat medium-size flies, even though larger flies have more energy.  The larger flies cost too much energy to catch and eat.
  • Northwestern Crows drop whelks (type of snail) onto rocks - sometimes up to 20 times per whelk to crack it open.  They select whelks that supply more energy (8.5kJ) than it takes to drop them (2.3kJ).
  • American Crows drop walnuts onto hard surfaces to open them.  The height from which they drop them depends on...
    • the hardness of the surface,
    • how many times they've already dropped it,
    • the possibility that another crow could swoop in and steal it.
  • Carrion Crows in Japan wait for a red traffic light, then put their walnuts in the crosswalk in front of the stopped cars.  They return on the next red light to pick up the meat from the crushed walnuts.

Some birds use time as a tool in how they forage for food.

  • Hummingbirds return to the same flowers to harvest nectar over and over.  Some hummers will wait longer between visits, so that they can collect more nectar.  However, this raises the risk that another hummer will collect from their flowers.  When this happens, the hummingbird will harvest frequently to keep the flower empty - this makes the flower less attractive to competing hummingbirds.
  • Western Scrub Jays remember where and when they hid food - and whether it is perishable (insects) or nonperishable (nuts).  The insects are recovered quickly, before they spoil; but if too much time passes, the jays will switch to nuts without bothering to check the "freshness" of their insect caches.
    • If a competitor sees where a jay has hidden its food, the jay will relocate its cache as soon as it gets a private moment.

Tool Use

Several types of birds have been known to use bait to attract prey...

  • Herons use bits of paper or bread to attract fish
  • Burrowing Owls use mammal dung to attract dung beetles (which it eats)

Some birds use tools to get food...

  • Woodpecker Finches (Galapagos Islands) use sticks or cactus spines to get grubs out of crevices.
  • New Caledonian Crows craft tools (some with hooked ends, some with serrated edges) to get insect prey out of crevices.
    • Betty, a NC Crow, crafted hooked tools in a lab to get food out of a tube.  She used wire (an unfamiliar material), and selected the diameter and length of wire she needed out of a large assortment.
    • The concepts behind this kind of tool-making (standardization, deliberate construction of specific tool types, and the creation of hooks) have not been observed in any other nonhuman organism.
    • Over time, the design of these tools has diversified over the island.  this is similar to the way that human ancestors evolved their tools.

Crows and jays demonstrate intelligence and cognition on a level similar to chimpanzees and other great apes.

 

Resources

Subpages (1): Files for Intelligence