BIO 554/754

Nervous System: Brain & Senses

Photo of a Common Raven
See The Bird in Black

The Avian Nervous System consists of

The functions of the avian nervous system are to
Drawings showing general shape of spinal cords of a bird, snake, and human
Comparison of the spinal cords of a typical bird (left), snake (center), and human (right).
 In the bird spinal cord, the enlargement in the cervical region is due to the many axons needed to control the
muscles of the wings. The lumbar enlargement is due to the many axons that control the legs. The thoracic section
is rather thin because movement of the wings and limbs need not be coordinated (so fewer interneurons needed).

Accumulation of testosterone in the spinal cord of a bird with an elaborate courtship display -- Elaborate courtship displays are common features of the reproductive behavior of male birds. However, little is known about their neural and hormonal control. One bird that performs such a display is the Golden-collared Manakin (Manacus vitellinus) of Panamanian forests. Adult males, but not females, perform a display requiring substantial neuromuscular control of the wings and legs. Schultz and Schlinger (1999) tested the hypothesis that steroid sensitivity is a property of neurons in the manakin spinal cord. Males and females were captured from active courtship leks, treated with drugs to block steroidogenesis, injected with 3H-labeled testosterone, and the spinal cords were removed and processed for autoradiography. Most sex steroid-accumulating cells were found in the cervical and lumbosacral enlargements. Because motor neurons in these areas control muscles of the wings and legs, these cells may have multiple behavioral functions, perhaps innervating muscles controlling the elaborate dancing and wing-snapping of these birds. This evidence indicates that sex steroids may control diverse behaviors in male birds in part by acting directly on the spinal neural circuits.

Figure to the right. Distribution of sex steroid-accumulating cells in the manakin spinal cord. Each symbol represents three cells. The cord is illustrated at three levels: Ventral, the cord ventral to the bifurcation of the ventral horns; dorsal, the cord dorsal to the beginning of the bifurcation of the dorsal horns out to their tips; and middle, the remaining cord between the ventral and dorsal levels.

White-collared Manakin display
Photo of a Golden-collared Manakin

Drawings of manakin spinal cords showing where sex steroids accumulate

Due to common ancestry, the brains of reptiles & birds are similar. However, birds have relatively larger cerebral hemispheres & cerebella. In addition, birds have larger optic lobes & smaller olfactory bulbs (Husband and Shimizu 1999).

Drawings of the brains of a pterosaur, goose, and crocodile

The avian brain includes:
  • medulla - part of the brainstem; includes neurons that help control heart rate, respiration, & blood pressure
  • optic lobe - part of the midbrain; relatively large in birds compared to other vertebrates (reflecting the importance of vision for most birds)
  • cerebellum - involved in the coordination of skeletal muscle activity; relatively large (reflecting the need for precise coordination of muscle activity during flight)
  • cerebrum - consisting of 2 cerebral hemispheres plus olfactory lobes; the olfactory lobes are relatively small in most birds (suggesting a poor sense of smell, but some birds do have a well-developed sense of smell)

Drawings of dorsal and ventral views of an avian brain

Sharp-shinned Hawk skull and brain

Wood Stork skull and brain

Drawings of the cerebellar lobes of a long-eared owl and a green woodpecker
Woodpeckers, corvids, and parrots have longer, larger cerebellar lobes IV, VI, VII, VIII, and IX than many other birds.
These areas of the cerebellum help coordinate visual and beak-related movements, and woodpeckers, corvids, and parrots
are generally very adept when it comes to using their beaks and.or tongues to manipulate and explore external objects.
Surprisingly, birds that are excellent flyers, e.g., swifts and falcons, do not have unusually large cerebellums, suggesting that well-developed
motor skills do not require an increase in cerebellum size (Sultan 2005).

Photos of a songbird brain and a human brain
Side views of  Zebra Finch & human brains. Inset (next to human brain) is the
Zebra Finch brain to the same scale (From: Jarvis et al. 2005).

Bird brains (Nova - Science Now)

Avian brains and a new understanding of vertebrate brain evolution -- A consortium of neuroscientists (Jarvis et al. 2005) has proposed a renaming of the structures of the bird brain to correctly portray birds as more comparable to mammals in their cognitive ability. The scientists assert that the century-old traditional nomenclature is outdated and does not reflect new studies that reveal the brainpower of birds. "We believe that names have a powerful influence on the experiments we do and the way in which we think," wrote the consortium members. "For this reason, (we have) reconsidered the 100-year-old terminology used to describe the avian cerebrum. Our current understanding of the avian brain requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains." The old terminology -- which implied that the avian brain was more primitive than the mammalian brain -- has hindered scientific understanding. 

The  revised nomenclature for avian brains is aimed at replacing the system developed in the 19th century by Ludwig Edinger, the father of comparative neuroanatomy. Edinger used prefixes such as palaeo- ("oldest") and archi- ("archaic") to designate structures in the avian brain and neo- ("new") to designate supposedly new structures, particularly in the mammalian brain. "According to this theory, the avian cerebrum is almost entirely composed of basal ganglia, a structure involved only in instinctive behavior, and the malleable behavior thought to typify mammals exclusively requires the so-called neocortex," wrote the researchers. However, "We have to get rid of the idea that mammals -- and humans in particular -- are the pinnacle of evolution. We also have to understand that evolution is not linear, but an intricate branching process. So, we can't automatically expect to track a structure in the human brain back to other current vertebrate species."

Drawings showing difference between the classic and modern view of the brain organization of birds
Image credit: Zina Deretsky - NSF

Drawings of a bird brain and a mammal brain
Classic view of avian and mammalian brain relationships. Ac, accumbens; B, nucleus basalis; Cd, caudate 
nucleus; CDL, dorsal lateral corticoid area; E, ectostriatum; GP, globus pallidus (i, internal segment; 
e, external segment); HA, hyperstriatum accessorium; HD, hyperstriatum dorsale; HIS, hyperstriatum 
intercalatus superior; HV, hyperstriatum ventrale; L2, field L2; LPO, lobus parolfactorius; 
OB, olfactory bulb; Pt, putamen (From: Jarvis et al. 2005).

Jarvis et al. (2005) describe studies demonstrating that so-called "primitive" regions of avian brains are sophisticated processing regions homologous to those in mammals. These regions carry out sensory processing, motor control and sensorimotor learning just as the mammalian neocortex. Studies have also shown that the avian and mammalian brain regions are comparable in their genetic and biochemical machinery. The neocortex and related areas in the mammalian brain are derived from a region in the embryonic cerebrum called the pallium, which means mantle or covering. Edinger thought, however, that most of this region in the bird cerebrum was part of the basal ganglia. Accordingly, he gave them names that ended in the basal ganglia term “-striatum”, a practice he also employed in naming the parts of the mammalian basal ganglia. As a result of the recent studies, the consortium recommends such changes as renaming the avian brain region called the "archistriatum" as the "arcopallium," (arched pallium); and renaming the region that includes part of the true basal ganglia in birds, the "palaeostriatum primitivum" and the "ventral palaeostriatum" which sits below the pallium as the "pallidum" (pallidal or pale domain). 

Drawings of a bird brain and mammal brain
Modern consensus view of avian and mammalian brain relationships. Solid white lines are lamina (cell-sparse 
zones separating brain subdivisions). Dashed grey lines divide regions that differ by cell density or cell size; 
dashed white lines separate primary sensory neuron populations from adjacent regions. Abbreviations that 
differ from above diagram (classic view): E, entopallium; B, basorostralis; HA, hyperpallium apicale; 
HI, hyperpallium intercalatum; Hp, hippocampus; MD, mesopallium dorsale; MV, mesopallium 
ventrale (From: Jarvis et al. 2005).

Jarvis pointed out that "there were people in the field of avian neurobiology who knew the real structures behind these names and knew the names were wrong." For example, researchers not familiar with studies demonstrating the sophistication of the avian brain could not understand how birds could exhibit sophisticated cognitive abilities with brains that held only what the nomenclature designated as the equivalent of the human basal ganglia. This new nomenclature will help people understand that evolution has created more than one way to generate complex behavior -- the mammal way and the bird way. And they're comparable to one another. In fact, some birds have evolved cognitive abilities that are far more complex than in many mammals."

Lecture: Harvey Karten explores what goes on inside a bird's brain. Learn how brains of birds compare to those of humans and other mammals and find out what the study of bird's brains can teach us about the nature and origins of human brains.

The cerebral hemispheres of birds, like those of other vertebrates, consists of 2 regions:

All vertebrates have a cerebrum based on the same basic plan; major phylogenetic changes are due to loss, fusion, or enlargement of the various regions.

Drawing showing theories of brain evolution
Schematic representation of two theories of brain evolution. (A) The outdated ‘scala naturae’ theory, where evolution occurs in a linear, progressive fashion up a ladder in which ‘lower’ (simple) species evolve into ‘higher’ (complex) species; going from fish and amphibians at the bottom through reptiles and birds to primates and humans at the top. With respect to brain evolution, the increasing complexity resulting from climbing the ladder leads to the appearance of completely new areas which are then added onto old ones. Each colour represents a different hypothetical brain region, either old or new. (B) The modern theory, where evolution is tree-like and new species evolve from older ancestral forms. With respect to brain evolution, complexity is derived from refining neural structures which are already present in ancestral forms, such that brain regions increase in size. There are no truly new brain areas, only elaborations of established regions. The colors represent different brain regions, but rather than new areas being added, evolutionarily old areas are increased or decreased in size (or complexity) (From: Emery and Clayton 2005).

Drawing showing brain organization of a fish and other vertebrates

Drawings showing organizations of the sensory system of a bird brain and a mammal brain
Schematic showing the organization of a sensory system in the bird/reptilian telencephalon (A) and the location
of the homologous neurons in the mammalian cortex (B). The avian telencephalon has only a thin lateral cortex, and
a prominent protrusion into the lateral ventricle (V), the dorsal ventricular ridge (DVR). The dorsal ventricular ridge contains
several different neuronal populations, directly corresponding to those found in different layers of the mammalian neocortex,
including thalamic recipient neurons, interneurons, and descending projections that leave the cortex to contact brainstem
and spinal cord neurons. The dorsal ventricular ridge protrudes into the ventricle of birds and reptiles, and hence for
many years was erroneously thought to be homologous to the mammalian basal ganglia (BG). In mammals (B),
homologous populations of neurons are found within distinct layers of the cortex. The striking similarities in basic neuronal
properties and circuits in reptiles, birds, and mammals led to the proposal that the specific neurons evolved prior to the
evolutionary appearance of mammalian cortex. Hp, hippocampus (Karten 1997).

Drawing of bird brain with main parts labelled
From: Jarvis et al. (2005).

Increasing sophistication in sensory processes, motor control, and behavior in reptiles and, particularly, birds over evolutionary time may have been the selective force driving the development & increasing volume of the avian pallium.  The basic function of the pallium is to serve as a linkage between sensory inputs and motor outputs; an interface between sensory and perceptual processing and mechanisms which modulate behavior. This is also the basic function of the mammalian cortex.

Graphic illustrating neuronal circuitry in a bird brain and a mammal brain
Schematic summary comparing the neuronal circuitry of auditory pathways in the dorsal ventricular ridge of the avian telencephalon and the equivalent neocortical circuit of the mammalian auditory cortex. In the avian forebrain, the populations of neurons corresponding to the individual layers of mammalian cortex are organized as clusters, rather than layers. This circuit in the mammal is represented as a simplified three-layered cortex, consisting of a layer of thalamic recipient neurons (receiving sensory input; blue) forming layer IV, a group of interneurons (yellow) forming the more superficial layers, and a group of descending motor neurons (DTEs) (red), the output neurons of this cortical region, forming layers V and VI. The morphology of individual neurons, their transmitters, and physiological properties at each parallel step of the circuit are virtually identical in bird dorsal ventricular ridge and mammalian cortex (Karten 1997). 

As Autumn Approaches, New Nerve Cells Put Chickadee Foraging On Fast Track - Every autumn, chickadees gather seeds and store them in hundreds of hiding places in trees and on the ground. Over the winter that follows, chickadees faithfully re-visit their caches to feed. The chickadee's spatial memory is remarkable enough, says Colin Saldanha, assistant Drawing of a Zebra Finch brain in cross-sectionprofessor of biological sciences at Lehigh University. But it is what happens inside the tiny songbird's brain that Saldanha finds amazing. In the fall, as the chickadee is gathering and storing seeds, Saldanha says, its hippocampus, the part of the brain responsible for spatial organization and memory in many vertebrates, expands in volume by approximately 30% by adding new nerve cells. In the spring, when its feats of Photo of a Black-capped Chickadeememory are needed less, the chickadee's hippocampus shrinks back to its normal size, Saldanha says. Songbirds are the first vertebrates in which brain growth during adulthood has been found to occur, Saldanha says. By studying neurogenesis in the Black-capped Chickadee, Saldanha hopes to learn how hormones help guide the brain's development and reorganization. He is particularly interested in the role played by the hormone estrogen in the growth of the hippocampus. Songbirds (like most vertebrates) make estrogen in their ovaries; scientists have determined that their brains also express aromatase, the enzyme that makes estrogen. Perhaps not surprisingly, the area of the songbird brain with the highest estrogen-making capability is the hippocampus. "We know hormones affect the reorganization of the brain in ovo, in utero and during the early physical development of most vertebrates," Saldanha says. "We are trying to figure out whether the ability to make estrogen in the hippocampus is helping the dramatic reorganization of the [adult] brain." His goal is to determine whether estrogen is being made in the cellular body or in the synapse, and whether the location of this estrogen-making ability changes seasonally. "We're looking at the ability of nerve cells and connections to make estrogen in the brain and asking if this ability is involved in brain reorganization," he says. 

Among birds, the DVR is perhaps best developed in crows, parrots, & passerines (i.e., 'intelligent' birds), and is less developed in pigeons & doves, quail, & domestic chickens. As stated by David Attenborough, "The level of intelligence among birds may vary. Each generation of birds that leaves the protection of its parents to become independent has the inborn genetic information that will help it to survive in the outside world and the skills that it has learned from its parents. They would never have met the challenge of evolution without some degree of native cunning. It’s just that some have much more than others."  (see 'Tool Use in Birds')

In general, the cognitive abilities of birds are increasingly appreciated as more complex than presumed several years ago. For example, as listed by Jarvis et al. (2005):

So, many birds have cognitive abilities that are quite sophisticated, and some birds and mammals have cognitive abilities that clearly exceed those of all other birds and mammals.

Tool-making New Caledonian Crow  

Photo of a New Caledonian Crow
Click on the photo to see a short video clip from
"The Life of Birds" - New Caledonian Crows



Green Heron - using bait to capture fish  


Drawings that illustrate self-recognition by a magpieExamples of self-directed behaviors. (A) Attempt to reach the mark with the beak, (B) touching the mark area with the foot, (C) touching the breast region outside the marked area, and (D) touching other parts of the body.

Magpies exhibit self-recognition -- Comparative studies suggest that at least some bird species have evolved mental skills similar to those found in humans and apes. This is indicated by feats such as tool use, episodic-like memory, and the ability to use one's own experience in predicting the behavior of conspecifics. It is, however, not yet clear whether these skills are accompanied by an understanding of the self. In apes, self-directed behavior in response to a mirror has been taken as evidence of self-recognition. Prior et al. (2008) investigated mirror-induced behavior in European Magpies. Magpies are corvids (order Passeriformes), a phylogenetic group characterized by large brains relative to body weight. The relative brain size of passerines is similar to primates in allometric analyses and, among passerines, corvids stand out with particular high relative brain size. Thus, magpies belong to a group of animals with very high relative brain size

As in apes, some individuals behaved in front of the mirror as if they were testing behavioral contingencies. When provided with a mark (see photo below), magpies showed spontaneous mark-directed behavior. These findings provide the first evidence of mirror self-recognition in a non-mammalian species, and suggest that essential components of human self-recognition have evolved independently in different vertebrate classes with a separate evolutionary history and that a laminated (layered) cortex is not a prerequisite for self-recognition.

New Caledonian Crows can use tools to manipulate other tools.
This one is using a small stick to obtain a larger stick that, in turn, is used to obtain food. 

Metatool use by New Caledonian Crows -- A crucial stage in hominin evolution was the development of metatool use—the ability to use one tool on another. Although the great apes can solve metatool tasks, monkeys have been less successful. Taylor et al. (2007) provided experimental evidence that New Caledonian crows can spontaneously solve a demanding metatool task where a short tool is used to extract a longer tool that can then be used to obtain meat. Six of seven crows initially attempted to extract the long tool with the short tool. Four successfully obtained meat on the first trial. The experiments revealed that the crows did not solve the metatool task by trial-and-error learning during the task or through a previously learned rule. The sophisticated physical cognition shown appears to have been based on analogical reasoning. The ability to reason analogically may explain the exceptional tool-manufacturing skills of New Caledonian Crows (also check out these videos).

Drawings showing experimental design of a caching experiment
The position of caching trays is shown in Compartments A and C, and of the food bowl in Compartment B.
Dotted lines represent the compartmental divisions, although during caching no dividers were in place. In the second experiment,
the compartmental layout was the same except that two food bowls, equidistant from compartments A and C, were used.

Scrub-jays plan ahead -- In the evening, jays were kept in the middle section, and fed powdered pine nuts that they couldn't store. In the morning, they were kept either in the 'breakfast room', where they were given food, or went hungry in the 'no-breakfast room'. After getting used to this set-up, the jays were given whole pine nuts in the evening that they could bury in trays of sand. The jays put three times as many in the no-breakfast room than in the breakfast room, so that they wouldn't go hungry in the morning. In a second experiment, the jays got breakfast in both rooms. However, their breakfast comprised whole peanuts in one room, and dried dog food in the other. When given both foods in the evening, the birds stored each food in the room where it would be lacking the next morning.

Planning for the future by Western Scrub-Jays -- Knowledge of and planning for the future is a complex skill that is considered by many to be uniquely human. We are not born with it; children develop a sense of the future at around the age of two and some planning ability by only the age of four to five. Raby et al. (2007) performed experiments to test whether Western Scrub-Jays (Aphelocoma californica) plan for the future. They showed that the jays plan for future needs, both by preferentially caching food in a place in which they have learned that they will be hungry the following morning and by differentially storing a particular food in a place in which that type of food will not be available the next morning. Although some primates and corvids take actions now that are based on their future consequences, these have not been shown to be selected with reference to future motivational states, or without extensive reinforcement of the anticipatory act. The results described here suggest that the jays can spontaneously plan for tomorrow without reference to their current motivational state, thereby challenging the idea that this is a uniquely human ability.

Bird IQ Index -- Corvids and falcons top the list, followed by hawks, woodpeckers and herons. The world's first bird IQ index was compiled by Louis Lefebvre based on 2000 published observations of unusual feeding behavior. Observation reveal that birds do not deserve their reputation for being featherweight thinkers. Among the examples of bird intelligence was a story of war zone vultures which waited by a minefield for stray animals to be blown up. There have also been many sightings of fly-fishing herons, with heronsl catching  insects and laying them on the water's surface to attract fish. Gulls frequently drop shells onto rocks to break them  and tits in England learned to open milk bottles left on people's doorsteps. Birds also use tools. The most skilled tool-users were New Caledonian Crows, which fashioned specially shaped implements out of leaves to catch insects. At the bottom of the IQ index, in no particular order, were the Emu, theOostrich, New World quails, and the night-jar. Parrots were disappointingly uninventive despite having the biggest brains of any bird. The exceptions were Australian parrots, which were "extremely opportunistic". "I can remember seeing them at the train station where they were loading sacks of grain, and the parrots were ripping up the bags," said Lefebvre. 
Graphic illustrating differences among different species of birds in levels of intelligence

Intelligent crows  

Bird Brains Like Human Brains: Episodic Memory Processes Similar - Birds can remember not only where, but when, they hid food items & even dig up less perishable food if too much time has passed & their favorite worms have probably rotted (Clayton and Photo of a Scrub Jay Dickinson 1998). The study of Scrub Jays (Aphelocoma californica) marks the first demonstration of episodic, or event-based, memory in animals other than humans. This type of memory is referred to as “mental time travel” because it involves mental images of past events. To remember where you put your car keys, you might “see” yourself walking into the house the night before & placing the keys on a table. Previous work had shown that birds can remember what kind of food they had stored & where they had hidden it, even without sensory clues like smell & appearance. But making decisions based on the timing of past events is crucial to episodic memory. In the study, N. S. Clayton  (Univ. of California-Davis) & Anthony Dickinson (Cambridge Univ.) allowed Scrub Jays to store their favorite food (wax worms) on one side of a sand-filled tray & peanuts on the other side. Jays retrieved the wax worms if less than four hours old, but birds that had learned that wax worms decompose avoided older worms in favor of peanuts. 

Scrub Jay episodic memory

Half-asleep birds choose which half dozes - Birds that are literally half-asleep - with one brain hemisphere alert & the other snoozing - control which side of the brain remains awake. The brain hemispheres take turns sinking into the sleep stage characterized by slow brain waves. The eye controlled by the sleeping hemisphere shuts, while the wakeful Photo of a sleeping Mallardhemisphere's eye stays open and vigilant. Birds also can sleep with both hemispheres resting at once. To check whether birds can control half-brain sleeping, Rattenborg et al. (1999) rows of Mallards napping. Decades of studies of bird flocks led researchers to predict extra vigilance in the more vulnerable, end-of-the-row sleepers. Sure enough, the end birds tended to keep peeled the eye on the side away from their buddies. Mallards snuggled into the inner spots showed no preference for gaze direction. Also, birds dozing at the end of the line resorted to single-hemisphere sleep, rather than total relaxation, more often than inner ducks did. Rotating 16 birds through the positions in a four-duck row, the researchers found outer birds half-asleep during some 32% of snoozing time versus about 12% for birds in internal spots. "We believe this is the first evidence for an animal behaviorally controlling sleep and wakefulness simultaneously in different regions of the brain," the researchers said. The results provide the best evidence yet for a long-standing conjecture that single- hemisphere sleep evolved as creatures scanned for predators. The preference for opening an eye on the lookout side could be widespread. Useful as half-sleeping might be, it's only been found in birds and such aquatic mammals as dolphins, whales, seals, and manatees. Presumably, keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning, explains Rattenborg. 


Graphic showing differences in time spent sleeping between a bird that is not migrating and one that is migrating
Proportion of time in each behavioral state for nonmigrating (top) and migrating (bottom) birds. The proportion of every 
10-min period spent in each sleep/wakefulness state was averaged for all birds: wakefulness (black), drowsiness 
(gray), slow-wave sleep (blue), and REM sleep (red). Note that overall sleep propensity in migrating birds is greatly diminished between about 22:30 and 06:00. Note also the increased propensity for REM sleep from 18:00 to 20:00 as compared to the same 
time period when not migrating.

No rest for the weary? -- Every spring and fall, billions of songbirds fly thousands of miles between their summer breeding grounds and their wintering grounds in Mexico, and Central and South America. While some birds fly during the day, most fly at night. The migratory pace of most birds—as well as the increased activity required to sustain migrations—suggests little time for sleep. Yet field observations indicate that presumably sleep-deprived fliers appear no worse for wear, foraging, navigating, and avoiding predators with aplomb. How do songbirds cope with so little sleep? 
      Rattenborg et al. (2004) characterized the activity levels of White-crowned Sparrows with motion-detection measurements and video recordings, and placed sensors on their brains to monitor their seasonal sleep patterns. Brain recordings showed a seasonal difference: migrating birds spent about two-thirds less time sleeping than nonmigratory birds and fell into REM sleep (the dream stage of sleep, marked by rapid eye movements) sooner. Cognitive tests - birds performed a task involving pecking a key for  seed - revealed that birds in the nonmigrating state suffered cognitive deficits when sleep-deprived, but displayed an “unprecedented” ability to maintain cognitive function in the face of ongoing sleep loss in the migratory state.
       These results suggest that birds sleep much less during migration, though it's impossible to know for sure without recording the birds in action. And it is unclear what molecular mechanisms jumpstart the migratory mindset. Whatever the mechanism, the unprecedented imperviousness of migrating songbirds to sleep deprivation, clearly warrants further testing. Source: (2004) No Rest for the Weary: Migrating Songbirds Keep Their Wits without Sleep. PLoS Biol 2(7): e221.

Sense organs

Tactile organs - touch receptors (Herbst corpuscles, which are similar to Pacinian corpuscles) are abundant in the bills of some birds, such as waterfowl & shorebirds, & in the tongues of other birds, such as woodpeckers. Additional touch/pressure receptors (Merkel cells) are found in the dermis (skin) of birds.

Photo showing sensory pits in the bill tips of sandpipers
Sensory pits in the upper and lower bill of a female Western Sandpiper (A, lower bill and B, upper bill),
male Western Sandpiper (C, lower bill), female Dunlin (D, lower bill), male Dunlin (E, upper bill), and male
Least Sandpiper (F, upper bill). The scale indicates 1 mm. The skin has been removed and bills were soaked in
bleach to dissolve the remaining soft tissue and expose the pits. Herbst corpuscles are found in high densities in
these sensory pits (Nebel et al. 2005).

Micrograph of a Herbst corpuscle
Herbst corpuscle from a duck's bill.
OC, outer capsule; IB, inner bulb; IS, inner space; dark line = neuron. Magnified 600x (Saxon 1996).
Micrograph of a Grandry corpuscle
Grandry corpuscle from duck's bill. NF, nerve fiber. Two Grandry cells, one on each side of the neuron. Magnified 1000x (Saxon 1996).

Cross-section through the beak skin of a quail
Cross-section through quail beak skin. In the dermis, below the epidermis (E), numerous Merkel cells (M) with nerve terminals are visible (From: Halata et al. 2003)..
Micrograph showing Merkel cells
Cross-section showing two twin-groups of Merkel nerve endings from the tarsometatarsal skin of a quail. K, keratinocyte; M, Merkel cells with cytoplasmic protrusions (arrows), T, nerve terminals; S, terminal Schwann cells (From: Halata et al. 2003).

Birds "Feel" Their Prey Under the Sand - Red Knots (Calidris canutus) can locate their favorite food (shellfish) in wet sand by inserting their beak half a centimeter into the sand for a few seconds (Piersma et al. 1998) This ability was demonstrated in experiments in which researchers hid small stones in the sand. Because stones do not send out any signals, the ability of Knots to detect them must be based on the sensitivity of their beaks to differences in currents in the water in wet sand between the individual grains, stones, or shells. Knots used in the experiments were unable to find hidden stones in dry sand. At the end of their beak, knots have clusters of 10 to 20 Herbst corpuscles that are sensitive to differences in pressure. When the bird sticks its sensitive beak into the sand at low tide, it produces a pressure wave because of the inertia of the water in the interstices between the particles. The pattern thus created betrays the presence of objects larger than the grains of sand. The rapid up-and-down movements of the bird's beak loosen the grains of  sand, which then become packed together more tightly, displace the interstitial water, & cause the residual pressure around the object to increase. The nature of their localizing ability means that knots cannot distinguish between stones & shellfish in the sand, which is why they rarely look for food in areas where the sand contains stones, no matter how much shellfish could be found there. 
Photo of a Red Knot

Shorebirds and other birds foraging on a barrier island  

Drawings of rictal bristles of five species of New Zealand birds
Facial bristle locations in five New Zealand bird species. (A) New Zealand Robin, Petroica australis; (B) Hihi, Notiomystis cincta; (C) New Zealand Fantail, Rhipidura fuliginosa; (D) Morepork, Ninox novaezealandae; (E) Brown Kiwi, Apteryx mantelli. Scale bars = 5 mm.
Micrographs showing bristle follicles and Herbst corpuscles
Facial bristle follicles and associated Herbst corpuscles from four avian species: (A) New Zealand Robin; (B) New Zealand Fantail; (C) Morepork; (D) Brown Kiwi. Black arrows indicate examples of Herbst corpuscles; black stars indicate nerve bundles, white stars indicate muscles.

Morphology, histology, and possible functions of facial bristles -- Knowledge of structure in biology may help inform hypotheses about function. Little is known about the histological structure or the function of avian facial bristle feathers. Cunningham et al. (2011) examined the morphology and histology, with inferences about function, of bristles in five predominantly insectivorous birds from New Zealand, including Brown Kiwi, Morepork, Hihi, New Zealand Robin, and New Zealand Fantail. Average bristle length corrected for body size was similar across species. Bristles occurred in distinct groups on different parts of the head and upper rictal bristles were generally longest. The lower rictal bristles of the fantail were the longest possessed by that species and were long compared to bristles of other species. Kiwis were the only species with forehead bristles, similar in length to the upper rictal bristles of other species, and the lower rictal bristles of fantails. Herbst corpuscles (vibration and pressure sensitive mechanoreceptors) were found in association with bristle follicles in all species. Nocturnal and hole-nesting birds had more heavily encapsulated corpuscles than diurnal open-nesting species. These results suggest that avian facial bristles generally have a tactile function in both nocturnal and diurnal species, perhaps playing a role in prey handling, gathering information during flight, navigating in nest cavities and on the ground at night, and possibly in prey-detection.

Smell (olfaction):

Drawing of the nasal cavity of a Mallard
Nasal cavity of a Mallard (scale bar = 1 cm). nar = naris, ros co = rostral concha, mid co = middle concha,
caud co = caudal concha, ch - choana (opening between nasal cavity and naspharynx), a o sin ost - ostium (openng into sinus cavity),
vest = nasal vestibule (anterior portion of nasal cavity), vom = vomer, Ram med nas = branch of ophthalmic nerve,
n l du ost = opening into lacrimal duct (Figure from Witmer 1995). Turbinate bones (like those shown in the diagrams below) are
found in the caudal (olfactory) concha.

Anatomical features of procellariiforms indicate that they should have a well-developed sense of smell. Within the complex turbinate
structure of a Snow Petrel's nose (above), large amounts of surface area are lined with olfactory epithelium, containing cells that
detect odor (light green). A puffin's nose is smaller and has less olfactory epithelium. Procellariiforms are often called "tubenose"
seabirds because of the prominence of these structures, seen clearly in a White-chinned Petrel’s profile (right). They also have
enlarged brain structures that process scents (Nevitt 1999)

Photos of the nasal cavities of a crow
Nasal cavity of the Japanese Jungle Crow (Corvus macrorhynchos).

Head of the jungle crow (the cranial bone and eyeball have been removed). Bar = 10 mm. (B) The left and right olfactory nerve bundles independently project to the brain. L, left olfactory bundle; R, right olfactory bundle. Bar = 10 mm. (C) Sagittal and coronal 3-dimensional structural computed tomography (CT) images of the nasal cavity. The small white arrows (a–c) in the sagittal image indicate the positions through which the planes of the transverse CT images pass. Bar = 10 mm. (D and E) Transverse sections of the nasal cavity through the rostral end (D) and middle portion (E) of the maxillary conchae (MC). The bold black arrows in (C and D) indicate the rostral end of the MC. Bar in (D) = 1 mm. Ce, cerebrum; st, olfactory septum (From: Yokosuka et al. 2009).

Drawing of olfactory sensory neurons
Olfactory sensory neurons in the olfactory epithelium detect odors and conduct impulses to the olfactory bulb (mitral cells).
From the olfactory bulb, impulses travel other areas of the brain (olfactory tubercle) where odor is actually perceived (Figure
from: Firestein [2001]).

Drawing showing how productive areas of ocean release dimethyl sulfide gas
DMS and seabird foraging -- The function of DMS (dimethyl sulfide) and other volatile infochemicals in aquatic trophic interactions. (a) A suspension-feeding copepod (mesozooplankton) produces a feeding current that deforms the boundary layer at the surface of microzooplankton. This layer is enriched with infochemicals such as DMS (produced by bacteria metabolizing dead plankton). (b) The interaction between phyto- and microzooplankton produces more DMS and is represented by the circular shaded area around the microzooplankton. Such infochemicals are exploited by higher-order mesozooplankton when searching for microzooplankton prey. Dotted lines indicate the mode of the trophic interaction with negative effects on the microzooplankton and indirect positive effects for the phytoplankton. (c,d) The accumulation of infochemicals from microzooplankton prey (d) provides chemical cues to migrating mesozooplankton [individual dots in (c)]. (e) Local upwelling generates biological activity (dotted area) that increases the sea–air flux of DMS and other volatile infochemicals. The resulting chemical gradient serves as a directional cue to seabirds (Pohnert et al. 2007).


This video shows the movements of adult Galapagos Albatrosses making foraging trips to the west coast of South America during the breeding season in 2008. Along with the birds' movements, this animation also shows actual patterns of wind (arrows) and ocean chlorophyll (light green) during the same time. By comparing the animals' movements with environmental data like chlorophyll (which represents food availability), you can see why adultss fly so far to feed their young.

Photo of a Kakapo
The estimated number of OR genes ranged from 107-667 in the nine species studied. The higher values (600-667; Red Jungle Fowl, Brown Kiwi, and the Kakapo pictured above) resemble those of mammalian genomes (Image source: Wikipedia).

A well-developed sense of smell in birds? -- Among vertebrates, the sense of smell is mediated by olfactory receptors (ORs) expressed in sensory neurons within the olfactory epithelium. Comparative genomic studies suggest that the olfactory acuity of mammalian species correlates positively with both the total number and the proportion of functional OR genes encoded in their genomes. In contrast to mammals, avian olfaction is poorly understood, with birds widely regarded as relying primarily on visual and auditory inputs. Steiger et al. (2008) found that in nine bird species from seven orders (Blue Tit, Cyanistes caeruleus; Black Coucal, Centropus grillii; Brown Kiwi, Apteryx australis; Canary, Serinus canaria; Galah, Eolophus roseicapillus; Red Jungle Fowl, Gallus gallus; Kakapo, Strigops habroptilus; Mallard, Anas platyrhynchos; and Snow Petrel, Pagodroma nivea), most amplified OR sequences are predicted to be from potentially functional genes. This finding is somewhat surprising as one previous report suggested that most OR genes in a bird (Red Jungle Fowl) genomic sequence were non-functional pseudogenes. Steiger et al. (2008) also show that it is not the estimated proportion of potentially functional OR genes, but rather the estimated total number of OR genes that correlates positively with relative olfactory bulb size, an anatomical correlate of olfactory capability. We further demonstrate that all the nine bird genomes examined encode OR genes belonging to a large gene clade, the expansion of which appears to be a shared characteristic of class Aves. In summary, these findings suggest that olfaction in birds may be a more important sense than generally believed.

a. Emu b. Kiwi c. Barn Owl d. Rock Pigeon

Photos of the brains of four species of birds
Visual processing areas of the brains of four species of birds. Ventral and dorsal views of the brains of (a) Emu (diurnal, flightless);
(b) Kiwi
(nocturnal, flightless); (c) Barn Owl (nocturnal, flying), and (d) Rock Pigeon (diurnal, flying). OT: optic tectum; ON: optic nerve;
OB; olfactory bulb (which actually consists of a cortical-like sheet in the adult kiwi; V: vallecula. Note the reduced diameter of the
optic nerve in Kiwi compared with that in the three other species (see text for actual measurements). In the dorsal view of Kiwi, note the caudal extension
of the large telencephalic hemispheres that completely hide the underlying midbrain. Note also in Kiwi that there is no obvious bulge on the
dorsum of the hemisphere that identifies the Wulst in species such as Barn Owl and Emu.
Scale bars: Emu, 1 cm; Kiwi, Barn Owl and Rock Pigeon: 0.5 cm (From: Martin et al. 2007).

Kiwi (Apterygidae) show minimal reliance on vision as indicated by eye structure and brain structures, and increased reliance upon tactile and olfactory information.This lack of reliance on vision and increased reliance upon tactile and olfactory information in Kiwi is markedly similar to the situation in nocturnal mammals that exploit the forest floor. That Kiwi and mammals evolved to exploit these habitats quite independently provides evidence for convergent evolution in their sensory capacities that are tuned to a common set of perceptual challenges found in forest floor habitats at night and which cannot be met by the vertebrate visual system. Martin et al. (2007) proposed that the Kiwi visual system has undergone adaptive regressive evolution driven by the trade-off between the relatively low rate of gain of visual information that is possible at low light levels, and the metabolic costs of extracting that information.

Photos showing sensory pits in the bill tip of a Kiwi
Nostrils and sensory pits at the bill tip of Kiwi. a, Lateral view of the bill tip of a museum skin specimen with the rhamphotheca intact and showing the position of the nostril (arrow). b–e, bones of the bill tip. b latero-ventral view of the maxilla showing the complex blunt shape of the bill tip whose surface is covered with closely packed sensory pits; the approximate position of the nostril is indicated (arrow). c, dorsal view of the maxilla with the approximate positions of the nostrils indicated (arrows), d, dorsal view of the mandible showing that sensory pits are found at the bill tips within the mouth. All scale bars 5 mm. (From: Martin et al. 2007).

Kiwis are unique among birds in having the opening of their nostrils close to the tip of the maxilla. In all other birds, the nostrils open externally close to the base of the bill, or internally in the roof of the mouth. Martin et al. (2007) provide evidence that Kiwi bill tips are the focus of both olfactory and tactile information. Inspection of prepared skulls shows that clustered around the tips of both the maxilla and mandible, on both internal and external surfaces, is a high concentration of sensory pits. Such pits house clusters of mechanoreceptors (Herbst and Grandry corpuscles) protected by a soft rhamphotheca. These sensory pits function in foraging to detect objects touching or close to the bill tips. In Kiwi, the sensory pits cover the entire surface of the tip of the maxilla and almost encircle the nostrils that open laterally ca. 3 mm behind the bill tip, suggesting that the bill tip is a focus for gaining both tactile and olfactory information for guiding the bill when foraging.


Evolution of olfaction in non-avian theropod dinosaurs and birds -- Little is known about the olfactory capabilities of extinct basal (non-neornithine) birds or the evolutionary changes in olfaction that occurred from non-avian theropods through modern birds. Although modern birds are known to have diverse olfactory capabilities, olfaction is generally considered to have declined during avian evolution as visual and vestibular sensory enhancements occurred in association with flight. To test the hypothesis that olfaction diminished through avian evolution, Zelenitsky et al. (2011) assessed relative olfactory bulb size, here used as a neuroanatomical proxy for olfactory capabilities, in 157 species of non-avian theropods, fossil birds and living birds. Relative olfactory bulb size increased during non-avian maniraptoriform evolution, remained stable across the non-avian theropod/bird transition, and increased during basal bird and early neornithine evolution. From early neornithines through a major part of neornithine evolution, the relative size of the olfactory bulbs remained stable before decreasing in derived neoavian clades. These results show that, rather than decreasing, the importance of olfaction actually increased during early bird evolution, representing a previously unrecognized sensory enhancement. The relatively larger olfactory bulbs of earliest neornithines, compared with those of basal birds, may have endowed neornithines with improved olfaction for more effective foraging or navigation skills.

Olfactory bulbs - Bambiraptor (Bambiraptor image), Lithornis, Presbyornis (Presbyornis reconstruction),
and present-day pigeon


Drawings showing location of taste buds on the tongue, palate, and floor of the oral cavity of a chicken
Diagram illustrating the distribution of taste buds (~250 total) in the oral cavity of a domestic chicken. Each dot = one taste bud.
(a) palate, (b) tongue and floor of the oral cavity, and (c) tip of tongue moved to the side to show taste buds on floor of oral cavity
(From: Kudo et al. 2008).

Micrograph of a Kiwi taste bud
Taste bud from the tongue of an Emu. Arrows demarcate the taste bud and the * indicates the taste bud pore (Source: Crole and Soley 2009).
Drawing of a taste bud
Structure of a typical vertebrate taste bud
(Source: Northcutt 2004).


Foraging Dunlin

Long-billed Curlew

Bar-tailed Godwits

Sharp-tailed Sandpipers

Nervous System: Brain & Special Senses II

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Useful links:

Hearing and the Bird Ear

The Life of Birds: Bird Brains

Back to Avian Biology

Back to BIO 554/754 Syllabus