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The avian brain is the central organ of the nervous system in birds. Birds possess large, complex , which process, integrate, and coordinate information received from the environment and Decision-making on how to respond with Bird anatomy Like in all , the avian brain is contained within the skull bones of the head.
The bird brain is divided into a number of sections, each with a different function. The cerebrum or telencephalon is divided into two hemispheres, and controls higher functions. The telencephalon is dominated by a large Avian pallium, which corresponds to the cerebral cortex and is responsible for the Cognitive skill of birds. The pallium is made up of several major structures: the hyperpallium, a dorsal bulge of the pallium found only in birds, as well as the nidopallium, mesopallium, and archipallium. The bird telencephalon nuclear structure, wherein neurons are distributed in three-dimensionally arranged clusters, with no large-scale separation of white matter and grey matter, though there exist layer-like and column-like connections. Structures in the pallium are associated with perception, learning, and cognition. Beneath the pallium are the two components of the subpallium, the striatum and Globus pallidus. The subpallium connects different parts of the telencephalon and plays major roles in a number of critical behaviours. To the rear of the telencephalon are the thalamus, midbrain, and cerebellum. The hindbrain connects the rest of the brain to the spinal cord.
The size and structure of the avian brain enables prominent behaviours of birds such as Bird flight and vocalization. Dedicated structures and pathways integrate the Hearing and Bird vision senses, strong in most species of birds, as well as the typically weaker olfactory and tactile senses. Social behavior, widespread among birds, depends on the organisation and functions of the brain. Some birds exhibit strong abilities of cognition, enabled by the unique structure and physiology of the avian brain.
Within the pallium, the nidopallium caudolaterale is thought to be a centre of goal-directed action. It is the part of the pallium that has the most Nerve sensitive to dopamine, and has a structure suggesting strong dopamine control. Neural activity in the nidopallium caudolaterale is correlated with rewards, rules, categories, and information held in the memory. The nidopallium caudolaterale is also proportionally much larger in birds with stronger cognitive abilities. Showing diversity in bird brains and potentially the role of the nidopallium caudolaterale is that structure's separation into multiple structures in songbirds.
Research through the 1960s demonstrated that the basal ganglia of birds occupied only the ventromedial telencephalon and not the entire forebrain, a historically held belief. The basal ganglia include the dorsal somatomotor basal ganglia (DSBG), which possesses both a pallidal and a striatal component, and is made up of the medial striatum, lateral striatum, globus pallidus, and intrapeduncular nucleus. In , it also contains Area X, which is responsible for aspects of vocal function in songbirds. The DSBG has important functions in voluntary motor control. Pathways including the DSBG allow birds to effect movement when desired and to reject movement when undesired. The basal ganglia also include the ventral viscerolimbic basal ganglia (VVBG). The VVBG, like the DSBG, occupies portion of both the striatum and pallidum. It contains the ventral portion of the medial striatum, the nucleus accumbens, the olfactory tubercle, and the ventral pallidum. The VVBG functions as a "Reward center" as well as a facilitator of "action selection". In these roles, the VVBG is important in supporting Reward system-seeking behaviour and discouraging behaviour leading to negative stimulus. (2025). 9780323853514, Academic Press. ISBN 9780323853514
Area X, unique to songbirds, is critical to learning song, which is a reproduction-critical behaviour in songbirds. It has been described by scientists as analogous to the mammalian striatum, due to a similar composition to that part of the brain. However, it also contains brain cells from the pallidum. Area X is activated most strongly when songbirds are learning new songs, and decline thereafter.
The extended amygdala is composed of the central extended amygdala and medial extended amygdala. The central extended amygdala is connected to behaviours involving ingestion, as well as stress, anxiety, and fear. The role and structure of the medial extended amygdala are debated by scientists. The medial extended amygdala receives inputs from the olfactory bulb, as well as the rest of the olfactory system, including the olfactory organs in the front of bird heads. It is related to sexual and social behaviours. Certain structures in the median extended amygdala have been demonstrated in birds to be sexually dimorphic. In chickens ( Chicken), research shows that part of the median extended amygdala plays a part in male sexual behaviour.
The basal corticopetal system in birds is made up of three nuclei: the basal magnocellular nucleus, and the horizontal and vertical limbs of the nuclei of the diagonal band. The role of the basal corticopetal system is poorly known in birds, although that system is known to be correlated to memory in mammals. However, it has been shown that damage to the basal corticopetal system impairs the memory of chicks ( Gallus gallus).
Based on studies of a variety of passerine birds, the avian septum is divided into four main parts: the lateral septum, medial septum, septoHippocampus septum, and caudocentral septum. The septum has functions related to stress, as well as responses to stimuli Photoreception. The preoptic area has functions related to sexual behaviour.
There is substantial variation in the foliation (folding) in the cerebella of birds, although all birds have at least some folding of the cerebellum. The folding of the cerebellum is also not strictly associated with the structure and size of the rest of the brain: penguins, seabirds, parrots, and crows, exhibit a similar degree of folding, despite very different brain characteristics. Likewise, owls, galliformes, and pigeons exhibit similar folding patterns. In general, the folding patterns of the cerebellum in birds reflect differences of behaviour, as well as variations in skull shape constraining cerebellar development and sensory and sensorimotor requirements of animals living disparate lifestyles.
The nidopallium caudolaterale of the avian brain, responsible for goal-directed action, has been found in Crocodilia, the closest living Archosaur, separated evolutionarily by 245 million years. Though this could represent an example of convergent evolution between crocodylians and birds, scientists believe it is more likely that the last common ancestor of birds and crocodiles possessed a nidopallium caudolaterale.
The Enantiornithes, which dominated the Cretaceous avifauna and were the first large radiation of birds, exhibited brains anatomically intermediate between those of Archaeopteryx and modern birds. As seen in brain endocasts of Navaornis, the telencephalon is expanded mediolaterally compared to in Archaeopteryx and non-avialan pennaraptorans, but not to the same degree as in modern birds, and the cerebellum is relatively small. However, it also shared traits like large optic lobes situated beneath the telencephalon and an inner ear canal modern in form with modern birds.
The Wulst, the physically projecting hyperpallium, is of interest in bird brain evolution because it is not present in any other living reptiles other than birds. Moreover, it is thought to be analogous to the neocortex in mammals, with an important role in higher cognition. No non-avialan dinosaurs possess a Wulst, and indeed neither does Archaeopteryx, a primitive bird. However, the Ornithurae Ichthyornis, despite having a brain shape resembling primitive , possesses a Wulst, showing that the structure likely originated earlier in bird evolution and exists outside of modern birds.
In general, the brain-to-body ratio of dinosaurs doubled from basal Theropoda to Coelurosauria and again doubled from Coelurosauria to Maniraptoriformes. From Maniraptoriformes, the general form of the brain took upon a form that would be retained at least in Ichthyornis, close to modern birds. Three major grade shifts in brain-to-body ratio are inferred by scientists to have taken place in the evolution of birds from basal Paraves to the base of crown Neoaves.
The tremendous diversity of modern birds leads to a diverse range of patterns for the brain. In the clade Neoaves, comprising all birds save fowl and Palaeognathae, the brain-to-body ratio increases, but this is driven primarily by a decrease in average body size. This pattern is observed in swifts, , , , as well as in the line leading up to Telluraves, the "higher landbirds". However, many groups of Water bird, collectively known as the Aequornithes, do not follow this trend, rather tending to increase body size and brain size at an equal rate. The basalmost Telluraves are two branches of large, predatory birds. One, in Afroaves, is made up of the , which have large brains evolved for visual acuity, and the Accipitriformes, including and . The other, in Australaves, includes the , , and the extinct Phorusrhacidae. Within Afroaves, there are successive shifts towards higher brain-to-body ratios. The and rollers in Coraciimorphae have greater brain-to-body ratios than Afroaves, and the woodpeckers nested within Coraciimorphae have yet larger ratios still.
Some of the largest brain-to-body ratios in birds, especially of the telencephalon, that part of the brain responsible for cognition, are found in the Psittaciformes (parrots) and Corvidae (crows, ravens, jays, magpies, and allies), both members of the Australaves. Indeed, the parrots and corvids are unique among birds for their large brain sizes. Moreover, scientists believe that their increased brain-body ratios evolved the most rapidly of any brain-body ratio shift in birds.
In contrast to what is currently thought to be relatively few grade changes in the brain-to-body ratio of birds in the Mesozoic, researchers have found that nine such shifts took place in the Paleocene, possibly as a response to the K-Pg extinction event. Moreover, they find that the largest brain sizes in birds evolved only recently, with the Neogene radiation of crown corvids and crown parrots. Moreover, in these lineages, the density of neurons in the brain also increases, contributing to significant cognitive complexity.
For years, too, scientists assumed that birds were not capable of advanced thought, as their brains were perceived to be devoid of complex pallial structures. Moreover, they lacked striated structures such as those found in the mammalian cerebral cortex, which were thought to be responsible for complex cognition. In fact, neurologists believed that birds were creatures that acted on instinct rather than on any sort of thought, and that they were highly unintelligent. From this conception arose the term colloquial term bird-brain, used to denigrate persons as unintelligent.
Research into bird cognition, behaviour, and anatomy, as well as into the brain, specifically, it became apparent that the traditional accretionary view of vertebrate telencephalic evolution was incorrect. It was also becoming clear that what were then referred to as striatal parts of the brain were really pallial in origin. In the late 1990s and early 2000s, a movement began, culminating in 2002, resulting in a new nomenclature for the avian brain that has since become standard. New research into bird resulted in a better understanding of the structure and organisation of the avian brain.
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