avian brains

8/7/2019 Avian Brains

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Avian BrainsAnton Reiner, University of Tennessee, Memphis, Tennessee, USA

The brain in birds is large, complex and unique in a number of ways, and it underlies the

sophisticated cognitive, social and motor behaviours that typify birds.

Introduction

As birds are capable of sophisticated cognitive, social andmotor behaviours, it is not surprising that they possesslarge and complex brains. Notably, the cerebrum (ortelencephalon) and cerebellumin birds are as large and welldeveloped as in many mammalian species (Figure 1). Whilethe structure of the brain in birds does possess manycommonalities with that of mammals, the brain in birdsmore closely reflects the evolutionary origin of birds fromreptiles. As a consequence, the elaborate brain structures

underlying the sophisticated behaviour of birds, such asvocal learning in songbirds, appear to have evolved inparallel with the brain structures subserving comparablefunctions in mammals. (see Aves (birds).)

Common Features of Bird and ReptileBrains

Birds evolved fromarchosaurian reptiles about 150 millionyears ago. Many investigators favour the view that this

origin was directly from small bipedal carnivorousdinosaurs (Chiappe, 1995). The reptilian origin of birds isevidenced by the number of brain traits shared by birds andreptiles that neither shares with mammals. The mostconspicuous of these is the presence of a so-called dorsalventricular ridge (DVR) in the telencephalon of both birdsand reptiles. (see Dinosaurs and the origin of birds.)

The DVR is a mass of neural tissue that protrudes fromthe dorsolateral wall of the telencephalon into the lateralventricle (Figure2). While its relatively uniformdistributionof neurons makes the DVR superficially resemble thestriatal part of the basal ganglia in mammals, neuroana-

tomical and functional studies during the latter half of thtwentieth century clearly show the DVR is akin to parts othe cerebral cortex of mammals in its connections witthalamus and in its role in information processing anlearning (Karten, 1969). Unlike the cerebral cortex, whicconsists of various functionally distinct areas that eacpossess a laminar architecture, the various regions of th

Article Contents

Secondary article

. Introduction

. Common Features of Bird and Reptile Brains

. Bird Brains: Neither Small Nor Simple

. The Visual System of Birds and the Optic Tectum

. Evolution of the Avian Telencephalon

. Neurotransmitters and the Avian Brain

. Neural Circuits for Song in Bird Brains

. Summary

Cerebellum

Spinal cordOlfactory bulb

Optic chiasm

Hindbrain

Optic lobe

Cerebrum

Figure 1 Side view of a pigeon brain showing the major brain

subdivisions. The diencephalon lies between and is hidden by thecerebrum and the optic lobe.

HippocampusOlfactorcortex

S

Hy

Striatum

Dorsalventricular ridge

Dorsalcortex

Pallidum

(a) Turtle

Hippocampus

Pallidum

Olfactor

cortex

Wulst

Dorsalventricular ridge

Striatum

S

Hy

(b) Pigeon

Figure 2 Frontal views through the cerebrum of a turtle (a) and a pigeo(b), with the major regions identified; areas that are homologous for

reptiles and birds are shown by the same colours. Hy, hypothalamus; S,septum.

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net



DVR are organized into groupings of uniformly distrib-uted neurons. The most prominent of the specializedregions of the DVR in birds and reptiles are a visual regionand an auditory region (Figure 3). These regions are themajor visual and auditory processing regions of thecerebrum in both birds and reptiles. The visual region,called the ectostriatum in birds, receives its input from a

distinct round thalamic nucleus called the nucleus rotun-dus, which itself receives visual input from the central greylayer of the visual part of the midbrain roof (or tectum).The auditory region of the DVR receives its input from amidline thalamic nucleus, which receives its auditory inputfrom the auditory midbrain roof (the inferior colliculus).The auditory region of the DVR in birds is called field L,and the auditory thalamic nucleusin birdsis ovoid inshapeand consequently called the nucleus ovoidalis. (see Basal

ganglia and the regulation of movement.)Noteworthy similarities between birds and reptiles are

also evident for the other major constituent of the cerebralhemispheres, the basal ganglia (Figure 3). As in mammals,

the basal ganglia in birds and reptiles are divided into aninput zone called the striatum and an output zone calledthe pallidum, with the striatum sending its major output tothe striatum (Reiner et al ., 1998). Unique to birds andreptiles, however, the pallidal part of the basal ganglia hasbut one subdivision rather thanthe twofound in mammals,and among the pallidal projection targets is a majorpretectal cell group, termed the lateral spiriform nucleus inbirds. As this pretectal nucleus projects prominently to thedeep output layers of the tectum, this circuit provides a

means by which the basal ganglia in birds and reptiles maexert an influence on movement via the visuomotocircuitry of thetectum. In light of the similar visual outputof the tectum to the thalamus in birds and reptiles, and thsimilar motor projections from the pretectum to thtectum, it should be clear that the tectum and pretectumshare structural and functional similarities in birds an

reptiles. Both structures are more elaborately developethan in mammals, and individual tectal layers anindividual pretectal cell groups can readily be homologizebetween birds and reptiles.

Major differences are, however, present between birdand reptiles in the olfactory system and the cerebellumBirds typically do not have a keen sense of smell, and stheir olfactory bulbs and the region of the cerebrum twhich the olfactory bulbs convey information (tholfactory cortex) are much smaller in birds than in reptileOn the other hand, birds are capable of much morcomplex motor behaviour than reptiles, and one neuracorrelate of this is the highlydeveloped cerebellumof bird

(see Olfaction.) (see Cerebellum: movementregulation and cognitivfunctions.)

Bird Brains: Neither Small Nor Simple

Birds are capable of sophisticated foraging strategieelaborate parental and social behaviour, impressive homingand migratory behaviour,complex vocal learning,sonproduction, and remarkable motor behaviours such aself-powered flight. In light of this, it is not surprising thabirds possess a large and complex brain. Brain size

bodyweight correlations, in fact, show that brain size fobirds rivals that for many mammals (Jerison, 1985Despite any similarities to mammals in brain size obehavioural output, however, the structure of the brain ibirds closely reflects the evolutionary origin of birds fromreptiles, as emphasized by the commonalities notepreviously. Contributing to the similarity in brain sizbetween birds and mammals are the cerebrum ancerebellum (see Figure 1), which in birds are as large anwell developed as in many mammalian species, and clearlbetter developed than in any extant reptilian species. ThDVR, for example, is much larger and possesses morsubdivisions in birds, with some of these being higher

order visual and auditory areas. Additionally, birds havelaborated a dorsal part of their cerebrum that in reptilesrepresented only by a thin sheet of cortical tissue overlyina lateral extension of the lateral ventricle of the cerebrum(Figure2).Inbirdsthisregionissothickenedthatitcreatesbulge at the dorsal surface of the cerebrum – hence itname, the Wulst, which means swelling in GermanIrrespective of its degree of development, this corticaregion in birds and reptiles receives somatosensory anvisual input from the thalamus and thereby participates i

Neostriatum

Pontinenucleus

Archistriatum

Tectum

HV

NN Ecto

BG

Wulst

Hy

Rt

EW

SN

Field L

Rt

Rt

Ov

IC

(b) Auditory pathway(a) Visual pathway

Hp

Figure 3 Schematic diagrams of frontal views through the cerebrum,diencephalon and midbrain of the pigeon showing the cell groups that

make up the major visual circuit (a) and the major auditory circuit (b) inbirds. BG, basal ganglia; Ecto, ectostriatum; EW, nucleus of Edinger-Westphal; Hp, hippocampus; HV, hyperstriatum ventrale; Hy,

hypothalamus; IC, inferior colliculus; N, neostriatum; Ov, nucleusovoidalis; Rt, nucleus rotundus; SN, substantia nigra.

Avian Brains

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net



information processing. Note that in some avian lineages,such as owls, the Wulst is even further hypertrophied,whereas in others such as parrots it is the DVR that isenlarged. The hippocampus is also well developed in birds(Figure 2), although not as complexly laminated as inmammals. Nonetheless, as in mammals, the hippocampusplays a role in the spatial learning that underpins homing

behaviour, foraging and food storage. (see Hippocampus.)Consistent with the Wulst and DVR enlargement, the

thalamic nuclei providing input to them are also enlargedcompared with their state in reptiles. Also indicative of thecomplexity of the avian brain is the tectum, which is morehighly developed (i.e. larger and more complexly lami-nated) than in any other vertebrate group, as are theassociated nuclei of the pretectum. Finally, the cerebellumof birds resembles that of mammals in receiving a mossyfibre input from the pontine nuclei and a climbing fibreinput from the inferior olivary nucleus in the hindbrain,and in being organized into lobes, which greatly increase itssurface area and processing power. The expansion and

lobulation of the avian cerebellum, however, occurredindependently of the superficially similar expansion andlobulation of the mammalian cerebellum, because thecerebellum is small and unlobulated in living reptiles.(see Thalamus.) (see Cerebellum: anatomy and organization.)

The Visual System of Birds and the OpticTectum

Birds typically possess high visual acuity. One major factorthat contributes to this is that birds possess large eyes and

many large visual areas in the brain. Notable among thesebrain areas is the visualmidbrainroof, thetectum(Figure3).Much of the retinal input to the avian brain ends in theupper layers of the tectum, with the projection beinglargely crossed and organized in a retinotopic fashion. Thetectum makes up a considerably greater fraction of thetotal brain volume in birds than in reptiles or mammals,apparently because of the large surface area required forthe retinotopic input from the large eyes. To provide roomfor this enlargement, the tectal lobes in birds are rotatedoutward and protrude laterally below the overlyingposterior pole of the cerebrum (Figures 1 and 3). Bycontrast, in reptiles and mammals the tectum (which is

called the superior colliculus in mammals) is situateddorsally, and its surface area is relatively more modest.Theavian optic tectum also shows a high degree of laminardifferentiation, with 15 cytologically distinct layers typi-cally being identified (Hunt and Brecha, 1984). The firstseven layers are within the retinorecipient zone of thetectum, and the remainder give rise to the ascendingsensory and descending motor projections of the tectum.As noted above, among the ascending projections of thetectum is an output to the nucleus rotundus of the

thalamus, which projects to the visual area of the DVRcalled the ectostriatum (Figure 3). This tecto-rotundoectostriatal circuit plays a major role in visual informatioprocessing in birds (Hodos, 1976). (see Visual system.) (see Ey

anatomy.)Other central retinal targets in birds, however, also pla

important roles. For example, the retina also project

contralaterally to a prominent visual area of the thalamubest referred to by the name of its mammalian homologuthe dorsal lateral geniculate nucleus. This thalamic cegroup has a bilateral projection to a visual region withithe Wulst and it plays a role instereopsis, especially in birdsuch as owls with frontally placed eyes (Pettigrew, 1979Note that, in mammals possessing stereopsis, it is bilateral projection of retina to the dorsal lateral geniculatnucleus that allows the overlap of input from the two eyewithin the brain which serves as the neural substrate fostereopsis. Thus, stereopsis has seemingly evolved independently in birds and mammals. Two final visual areas onote in birds are the area pretectalis of the media

pretectum, which mediates the pupillary light reflex bmeans of a projection to the nucleus of Edinger–Westphain the midbrain, and the nucleus of the basal optic root athe anterior base of the midbrain, which mediatecompensatory eye movements in response to slow movementsof thevisual fieldor of the head throughvisualspace(see Prey detection by bats and owls.) (see Oculomotor system.)

Evolution of the Avian Telencephalon

The avian telencephalon clearly evolved from the telencephalon of reptiles. Study of the telencephalon of livinreptiles provides clues as to the nature of the evolutionarchanges that occurred as birds diverged from their nowextinct reptilian ancestors. For the basal ganglia, theschanges were minimal and consisted largely of aexpansion in the volume of the structure relative to bodsize (Reiner et al ., 1998). This change would have allowebirds more refined motor control and motor learnincapabilities than possessed by reptiles. (see Relationships o

birds - molecules v morphology.) (see Reptilia (reptiles).)Greater changes occurred in the two telencephal

regions comparable to the cerebral cortex of mammal

(Reiner, 2000).In reptiles these regions are calledthe dorsacortex and DVR (Figure 2). In living primitive reptiles sucas the tuatara Sphenodon punctatum, the cortex and DVRappear to form one continuous structure, and the DVR not broken into separate cell groups. Even in Sphenodonhowever, both dorsal cortex and DVR receive thalamisensory input. The changes in the DVR from Sphenodon tthe grade observed in later reptiles are likely to havinvolved differentiation of the DVR into distinct cegroups, and the transition to the avian grade appears t

Avian Brains




have involved further differentiation and enlargement of both the dorsal cortex (into the Wulst) and the DVR.

In living birds, the Wulst and DVR consist of a series of stacked slab-like zones, eachof which possesses subregions(Figure 4). For the Wulst, the slab-like zones are termed thehyperstriatum accessorium, intercalatus superior anddorsale, whereas for the DVR they are termed the

hyperstriatum ventrale and the neostriatum. It is theneostriatum that contains the thalamorecipient sensoryzones of the DVR, while the hyperstriatum intercalatussuperior and dorsale receive sensory thalamic input in theWulst. The hyperstriatum accessorium and a posteriorbasal region of the DVRtermed the archistriatum (Figure3)are the source of the major descending motor projectionsof the Wulst and DVR. Note that the suffix ‘-striatum’appearing in the names of the parts of the Wulst and DVRdoes not reflect a derivation or relationship of these zonesto the striatal part of the basal ganglia. These structureshave ‘-striatum’ in their name because, at one time, most of the avian telencephalon was thought to be an expansion of

the striatal part of the basal ganglia. This notion is nowknown to be incorrect, but the names for these structuresbecame entrenched and have not yet been displaced bymore accurate names.

In addition to the evolutionary relationship of the avianand reptilian telencephala, another key question of interestis the evolutionary relationship of the avian telencephalonto the telencephalon in mammals. For the basal ganglia,there is little disagreement. Basal ganglia closely resem-bling those in birds and mammals must have been presentin the common amniote ancestor, given the manysimilarities in cell types, projections, functions andtransmitters that are present (but see above regarding

differences between birds and mammals in pallidalorganization and projections to the pretectum) (Reineret al ., 1998). There are, however, two major schools of

thought on the evolutionary relationship of avian DVRand Wulst to mammalian cerebral cortex (Reiner, 2000Both accept that stem amniotes possessed a structurresembling the dorsal cortex of extant reptiles,and thatthregion was the forerunner of the superior part omammalian cerebral cortex, the dorsal cortex in livinreptiles and the Wulst in birds, all of which possess a visua

area receiving input from the dorsal lateral geniculatnucleus of the thalamus and a primary somatosensorarea. (see Cerebral cortex.) (see Brain evolution and comparativ

neuroanatomy.)The two hypotheses diverge as to the relationship o

DVR to mammalian cerebral cortex. One viewpoinproposes that the DVR, which seems to resemble thtemporal sector of cerebral cortex in possessing thalamorecipient auditory and visual regions, is derived from subcortical pallial region in stem amniotes that in thmammalian lineage came to give rise to several smaamygdalar and periamygdalar cell groups. The evidencused to argue for this viewpoint involves the simila

positions of the DVRand these periamygdalar nuclei in thcerebrum and the embryology of these regions, especiallin terms of similar expression patterns for certain genethat regulate development called homeobox genes. Thalternative hypothesis proposes that a region in stemamniotes situated at the ventrolateral edge of dorsal cortegave rise to the temporal part of cerebral cortex in thmammalian lineage and to DVR in reptiles and birds. Thevidence favouring this view is the high similarity in thidentity, connections and locations of the visual anauditory areas of the DVR and temporal cerebral cortexRegardless of which viewpoint is correct, however, thevidence is clear that the DVR and Wulst have bot

evolved a high degree of sophistication in birds that allowbirds to perform complex behaviours similar to thoscontrolled by cerebral cortex in mammals. (see Cerebr

cortex development.) (see Mammalian embryo: Hox genes.)

Neurotransmitters and the Avian Brain

The organization of neurotransmitter-specific systems othe avian brain reflects both the evolutionary derivation obirds from reptiles, as well as the common evolutionarderivation of birds, reptiles and mammals from stem

amniotes (Reiner et al ., 1998). In the cerebrum, the laterawall of the telencephalon in birds, reptiles and mammals idivided into the same two fundamental ontogenetisubdivisions: the pallium and the subpallium. The vasmajority of the neurons of the pallium, which consists othe cerebral cortex in mammals and the Wulst/DVR ibirds, use the excitatory neurotransmitter glutamate, as dthe thalamic neurons that convey information to thpallium. As a consequence of this, the Wulst and DVR arrich in glutamate receptors. By contrast, the vast majorit

Olfactorycortex

Hyperstriatumaccessorium

Hyperstriatumventrale

Neostriatum

Basal ganglia

Hyperstriatumintercalatussuperior

Hyperstriatumdorsale

Figure 4 Frontal view through the cerebrum of a pigeon, with the major regions identified and the slab-like zones making up the Wulst(hyperstriatum accessorium, hyperstriatum intercalatus superior, and

hyperstriatum dorsale)and dorsalventricular ridge (hyperstriatumventraleand the neostriatum) indicated by different colours.

Avian Brains




of the neurons of the subpallium, which consists of thebasal ganglia in mammals and birds, use the inhibitoryneurotransmitter g-aminobutyric acid (GABA). As com-monly true in the nervous system, many neurons of thecerebrum also use one or more neuropeptides as neuro-modulators of their primary neurotransmitters. This isespecially evident for the basal ganglia, in which one

population of GABA-containing striatal neurons containsthe neuropeptides substance P and dynorphin, whileanother contains the enkephalin neuropeptides.(see Glutamate as a neurotransmitter.) (see GABA as a neurotrans-mitter.) (see Peptide neurotransmitters and hormones.)

These two types of striatal neuron each make up nearlyhalf of all striatal neurons and they play distinct roles inmovement control, with the former promoting plannedmovements and the latter inhibiting movements thatpotentially conflict with planned movements. The striatalpart of the basal ganglia receives prominent glutamatergicinputs from cortex and thalamus (which provide theinformation required for the striatum to play a role in

movement control) and a prominent dopaminergic inputfrom the midbrain substantia nigra (which plays animportant role in modulating the striatum according tomotivational state). Owing to these two inputs, thestriatum is rich in glutamate and dopamine receptors. Asthe neurons of the striatum and pallidum are GABAergic,the neurons in their projection targets possess GABAreceptors. Among the targets of striatal neurons areincluded other neurons of the striatum itself and theneurons of the pallidum. Pallidal targets include thalamicsites projecting to motor regions of the Wulst and thelateral spiriform nucleus of the pretectum. The lateralspiriform nucleus is itself noteworthy because it uses both

GABA and enkephalin, as does its homologue in reptiles.(see Dopamine.)

An additional major neurotransmitter system of thetelencephalon in birds is the basal forebrain cluster of cholinergic neurons. In mammals, this group of neuronshas extensive projections to the cerebral cortex and plays arole in learning and memory. It is likely that they play asimilar role in birds. Degeneration of these neurons inhumans has been implicated as a contributor to thelearning and memory defects of Alzheimer disease. Inaddition, a set of cholinergic neurons with ascendingprojections to midbrain and diencephalon is located in therostral hindbrain, and a series of catecholaminergic

neurons with diverse projections is distributed fromforebrain to hindbrain levels in birds, as is a series of serotonergic neurons (Reiner et al ., 1994). Similar choli-nergic, catecholaminergic and serotonergic neurons arepresent in mammals and reptiles, and they appear thus tobe fundamental constituents of amniote brain organiza-tion. Various neurons of the avian brain, such as some of the cholinergic neurons, also make the gaseous neuro-transmitter nitric oxide, as is also true in mammals.(see Acetylcholine.) (see Learning and memory.) (see Alzheimer

disease.) (see Amine neurotransmitters.) (see Nitric oxide as

neuronal messenger.)Finally, a diversity of neuropeptides is found in variou

neurons throughout the brain in birds, and these appear tserve as modulators of the primary neurotransmitters cocontained in these neurons. Details of the distribution othese neuropeptides is beyond the scope of the presen

overview, but in addition to those named above theinclude vasoactive intestinal polypeptide (VIP), neurotensin, somatostatin, neuropeptide Y, cholecystokinin, calctonin gene-related peptide and corticotrophin-releasinfactor. In many cases, the distribution of neurons containing these neuropeptides helps to identify homologouneuronal groups in birds and mammals, while in otherthey show the unique brain circuits that birds have evolvedAdditionally, hormones such as vasopressin and oxytocinand such hypophysiotropic factors as adrenocorticotropihormone (ACTH), somatostatin and gonadotropin-releasing hormone are found in hypothalamic neurons ibirds, and again help to define functional circuits, many o

which represent common features of amniote braiorganization. They also express steroid receptors, such aoestrogen receptor (ER) and glucocorticoid recepto(GR). (see Cellular neuromodulation.) (see Somatostatin

(see Peptides: biological activities of small peptides

(see Neurotransmitters.)

Neural Circuits for Song in Bird Brains

Passerine songbirds, such as zebra finches (which are thbest studied of songbirds), are born in a highly altricia

state and are dependent on their parents, for the first 30–40 days after hatching in the case of zebra finches (Bottje1997). Zebra finches fledge from the nest at around 20 dayof age, which is the time at which active vocal learning imales begins. Juvenile male zebra finches must heaconspecific song from approximately 20 to 40 days of agin order eventually to produce a song of their own, whichmodelled after the tutor song. Hearing conspecific song ithought to result inthe formation of a stable memory of thtutor song which then guides the sensorimotor learninneeded to produce a copy of that pattern. Male zebrfinches begin to produce their first song-related vocalizations at around 25days, and gradually refine the

utterances until they achieve a close match to the tutosong. By 80–90 days, male zebra finches are sexuallmature and produce a stereotyped song pattern that, undenormal circumstances, will be maintained without changthroughout adult life. In contrast, female zebra finches dnot demonstrate singing ability. In addition to the need thear an external song model during early developmentmales must also hear their own vocalizations during thperiod of song learning so that they canlearn to adjust thevocal output to match the tutor song. Once a stereotype

Avian Brains




song is produced, adult males rehearse their song and relyon the auditory feedback during the rehearsals to maintaina stable song pattern. Song learning in other songbirdsshows similar characteristics to those in zebra finches.(see Bird song: steroid hormones and plasticity.)

Two functionally distinct telencephalic circuits underlievocal learning and song production in passerine songbirds

(Figure 5): (1) a direct pathway from the cortical highervocal centre (HVC) to a cortical motor region termed therobust nucleus of the archistriatum (RA), and (2) amultisynaptic pathway connecting the HVC to the RA,whose components are: HVC–area X of the basal ganglia– the dorsolateral medial nucleus of the thalamus (DLM)– the lateral magnocellular anterior neostriatum (LMAN)– RA (Bottjer, 1997). These pathways differ in their roles,with the former important for adult song production andthe latter for song learning in juveniles and maintenance inadults. In keeping with the fact that female songbirds donot sing, the size of song control nuclei is several timesgreater in males than in females, and some (such as area X)

are not even evident in females.The HVC–RA circuit appears to be required for

ongoing production of stable song in adult male birds(Bottjer, 1997). HVC receives input from the auditorycortex (field L) in birds and is the source from which theother song controlnuclei derive their song-related auditoryinformation. Song-related auditory responses are weak inavian auditory cortex but are prominent in HVC in adultsongbirds. The activity of HVC neurons in awake, singing

birds correlates precisely with the specific song syllableheard, whereas the activity of RA neurons is more relateto the motor aspects of song production. Thus, the HVCRA pathway appears to be part of the circuitry foproducing learned song in adult birds, with RA directllinking the telencephalic song control circuitry witdescending motor circuitry that activates the voca

musculature (Figure 5). (see Auditory processing.)The multisynaptic pathway connecting HVC to RA

(HVC–X–DLM–LMAN–RA) is necessary for sonlearning. Neurons in HVC that are distinct from thosprojecting to RA project to area X in the basal gangliawhich relays through the thalamic nucleus DLM to thcortical region LMAN, which in turn projects on to thsame RA neurons that receive inputs from HVC. Lesionof the X– DLM–LMAN pathway have no major effect osong production in adult birds, but lesions in this pathwaduring song acquisition profoundly disrupt vocal production in juveniles and prevent song acquisition. Thus, thanterior forebrain song control circuit involving area X

plays a critical role in shaping song-related activity in RAduring song learning so that song output comes to matcthe tutor song. The area X circuit has also been shown to bactive during adult rehearsal of song and for the long-termmaintenance of song (Jarvis et al ., 1998). It is of interesthat similar vocal control circuitry to that in songbirds haevolved independently in the other two vocalizing orders obirds, parrots and hummingbirds.

Summary

During the early part of the twentieth century, birds werthought to possess an inflexible behavioural repertoire thaconsisted largely of innate behaviours. Concomitant witthis view, the forebrain anatomy of birds was thought to bdominated by a hypertrophied basal ganglia and relatecircuitry, which at that time were thought to mediatreflexive behaviour. It is now clear that birds possess complex and malleable behavioural repertoire mediated ba brain that rivals the brain of many mammals in size ansophistication. The cerebral hemispheres of birds arespecially enlarged, and the part of them devoted thigher-order processes such as learning and cognitionnamely the Wulst and DVR, are comparable to thcerebral cortex of mammals, which subserves similafunctions. (see Neural networks and behaviour.) (see Synapt

plasticity as a mechanism of learning.)

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Chiappe LM (1995) The first 85 million years of avian evolution.Natu

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HVC

RA

AudCTX

DLM

nXII

LMAN

Area XCb

Vocal organ

Figure 5 Side view of a songbird brain showing the song control cellgroups of the forebrain and their interconnections. The green arrow

indicates the serially connected structures forming the forebrain motor circuit for song control, while the red arrows show the connections of the

forebrain song learning circuit. Aud CTX, auditory cortex (also known asfieldL); Cb, cerebellum; DLM,dorsolateral medial nucleus of the thalamus;HVC, higher vocal centre; LMAN, lateral magnocellular anterior

neostriatum; nXII, hypoglossal nucleus; RA, robust nucleus of thearchistriatum.

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Hodos W (1976)Vision andthe visualsystem: a bird’seye view.Progress

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Further Reading

Benowitz LI (1980) Functional organization of the avian telencephalo

In: Ebbesson SOE (ed.) Comparative Neurology of the Telencephalo

pp. 389–421. New York: Plenum Press.

Butler A and Hodos W (1996) Comparative Vertebrate Neuroanatomy

Evolution and Adaptation. New York: Wiley-Liss.

Doupe AJ and Kuhl PK (1999) Birdsong and human speech: commo

themes and mechanisms. Annual Review of Neuroscience 22: 567–63Durand SE, Heaton JT, Amateau SK and Brauth SE (1997) Voc

control pathways through the anterior forebrain of a parr

(Melopsittacus undulatus). Journal of Comparative Neurology 37

179–206.

Jarvis ED, Ribeiro S, da Silva ML et al . (2000) Behaviorally driven gen

expression reveals song nuclei in hummingbird brain. Nature 40

628–632.

Karten HJ (1979) Visual lemniscal pathways in birds. In: Granda AM

and Maxwell JH (eds) Neural Mechanisms of Behavior in the Pigeo

pp. 409–430. New York: Plenum Press.

Medina L and Reiner A (2000) Do birds possess homologues

mammalian primary visual, somatosensory and motor cortice

Trends in Neurosciences 23: 1–12.

Pepperberg IM, Willner MR and Gravitz LB (1997) Development

Piagetian object permanence in a grey parrot (Psittacus erithacusJournal of Comparative Psychology 111: 63–75.

Sherry DF, Vaccarino AL, Buckenham and Herz RS (1989) T

hippocampal complex of food-storing birds. Brain, Behavior an

Evolution 34: 308–317.

Strasser R, Bingman VP, Ioale P, Casini G and Bagnoli P (1998) Th

homing pigeon hippocampus and the development of landma

navigation. Developmental Psychobiology 33: 305–315.

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