Nervous System

• Central Nervous System (CNS) = brain and spinal cord (responsible for integration and memory).

• Peripheral Nervous System (PNS) = cranial and spinal nerves, autonomic nervous system, sense organs (both sensory and motor components).

Central Nervous System• Brain = same basic plan as in reptiles and

mammals, 3 Divisions:– Forebrain (Cerebrum) = integration, instinctive

behavior, intelligence– Midbrain = vision, muscular coordination, physiological

control (homoeostasis)– Hindbrain (medulla) = links brain with spinal cord and

peripheral nervous system• Birds and mammals both have enlarged cerebral

and cerebellar hemispheres; the brain in both Classes makes up 2-9% of total body weight.

Avian Forebrain• Pallial domains are responsible for learning

and intelligence in vertebrate brains. • In mammals, cerebral cortex is dominant

portion • Serves as the seat of higher intelligence.• Provides a great capacity for learning.• Bird cortex thin and relatively undeveloped

(thought to be the seat of conditioned behavior).

Avian Forebrain• Pallial domains are dominant part of the avian cerebrum

and are cellular homologs in birds and mammals. • Pallial domains = seat of learning, intelligence, complex

instinctual behaviors. • In general, pallial domains (= cortex of mammalian brain)

specialized for learning, corpus striatum for stereotypic behaviors.

• Former model: corpus striatum dominant in birds, so lower intelligence than mammals.

• Recent evidence: some birds are highly intelligent; outperform mammals in some advanced learning tasks (counting, spatial cognition, pattern recognition).

Evolution of Bird and Mammal Brains• Ancestral Stem-Amniote condition led to

layered cortex in mammals and pallium differentiated into several regions in birds

• Two Hypotheses: based on neuron connectivity patterns

• Nuclear-to-Layered Hypothesis = Ancestor with nuclear pallium (neurons grouped into clumps)– Evolved into layered arrangement in mammals, but

maintains ancestral connectivity patterns– Evolved into three semi-layered sets of neurons in birds

Evolution of Bird and Mammal Brains• Nuclear-to-Claustrum/Amygdala Hypothesis =

Connectivity patterns shared by neurons in layered mammal cortex and bird pallial divisions evolved independently

• Pallial divisions in birds (outside of hyperpallium) represent elaboration of parts of brain homologous to claustrum and amygdala regions in mammals– Both with nuclear, rather than layered, arrangement of

neurons

• Not currently known which hypothesis is correct and both may be partially correct

Avian Midbrain, Spinal Cord, PNS• Birds with large, well-developed cerebellum

(largest among the vertebrates), associated with very high degree of muscular coordination necessary for flight.

• Very large optic lobes are present, associated with the importance of vision in birds.

• Spinal Cord - similar in structure to other tetrapods, cervical and lumbar enlargements associated with appendages

• PNS similar to that in other vertebrates.

Senses• Sense of Smell - olfactory lobes are generally

small and birds formerly thought to have a generally poor sense of smell.

• New evidence suggests that birds have better developed sense of smell than previously believed. – Detect certain odors with similar abilities to mammals– Number of functional olfactory receptor genes in most

birds is roughly similar to that in humans– Many birds use smell for daily routines (feeding,

navigation, etc.)

Sense of Smell (cont.)• Birds with low olfactory receptor gene

numbers and small olfactory bulb region don’t necessarily have a poor sense of smell

• Examples:– Starlings can detect and discriminate volatile

plant compounds in nest materials– Blue Tits apparently use olfaction to maintain

an aromatic nest environment (for nestlings) and to detect predators

Senses

• Taste - all birds are capable of tasting• Birds are equally or less sensitive to certain

ingredients than mammals. • Birds have fewer taste buds than do

mammals.

Senses (cont.)• Mechanoreception

– Touch - possess typical touch, pressure, temperature, and pain receptors.

– Birds are also sensitive to barometric pressure.

• Many birds can sense oncoming storms and modify foraging behavior accordingly.

• Pigeons and thrushes can select proper altitude for migratory flights, presumably this is true for other birds as well.

Magnetism Detection

• Birds can use information from the earth's magnetic field for navigation.

• Magnetite Crystals are present near olfactory nerves (between eyes) of pigeon, and these may serve as the basis for the magnetism-detection system.

Hearing• Ear is divided into same 3 regions as in mammals:

External, Middle, and Internal.– Middle Ear is only one bone (columella) = transmits

sound vibrations from tympanum to inner ear.– Inner Ear serves both hearing and equilibrium

functions.• Optimal Hearing Range = 1 - 5 KHz, Limit = 10 KHz;

Owls to lower frequencies and up to 12 KHz. Overall, the range of optimal hearing in birds is narrower than that in mammals

P. 193, Gill

Hearing in Owls• Owls with specializations allowing them to detect

and capture prey by hearing alone.– Detect low frequency sounds effectively– Have high numbers of auditory neurons– Facial discs act as sound collectors and aid in focusing

sound to ear (see p. 194, Gill)– Asymmetry of external ears - allows binaural

comparison of intensity and frequency, which enables precise vertical distinction in addition to horizontal distinction similar to ours.

– Because of this they can capture prey by sound alone.

Echolocation in Birds

• A few birds are capable of echolocation for navigation (Cave Swiftlet, Oilbird).

• Use low frequency clicks. • This differs from the high frequency

ultrasound used by bats and is not nearly as effective.

Avian Vision• Vision is the most important sensory input for

birds, as they are visual animals.• Birds have large eyes relative to other vertebrates

(starling eye makes up 15% of head mass, humans = 1%)

• Shapes of avian eyes vary.– Globular = diurnal birds with high resolution over great

distances (hawks, etc.)– Flattened = most birds– Tubular = nocturnal birds, allows increased

accommodation (focusing) and light-gathering

The three general categories of avian eyes

Flattened Globular Tubular

Avian Vision• Birds with higher visual acuity (resolving

power) than mammals because of higher numbers of photoreceptors and a slight magnifying effect of the fovea. – Raptors and passerines = 2 - 3 times human

abilities• Generally, birds have higher powers of

accommodation as both the cornea and the lens change curvature while focusing. Only the lens changes curvature in mammals.

The avian retina is relatively thick compared with thatin other vertebrates. The increased thickness results in

increased refraction and increased magnification.(P. 187, Gill)

Avian Vision• Color Vision - birds have very high numbers of

cones (diurnal birds), which suggests well-developed color vision.

• Birds are sensitive to UV light.– UV light probably more important to short-range visual

communication (such as mate choice) than long-range communication, because UV wavelengths are more highly scattered than longer wavelengths in air.

– UV reflectance of plumage increases female preference for males in some spp., but not in others.

– In Blue Tits, females increase the number of male offspring in clutch when mated to males with high UV reflectance in their crest.

Avian Vision• Diurnal birds have colored oil droplets in the eyes,

probably functions to enhance contrast by filtering out background "noise."

• Birds do not have stereoscopic vision as do mammals.

• Optic nerve tracts project only to the opposite brain hemisphere.

• Most birds see laterally better than forward due to the lateral position of eyes on the head and little overlap in fields of view.

Avian Vision• Pecten = structure composed of blood vessels and

supporting stromal cells; present at exit of optic nerve, projects into vitreous chamber of eye.

• May serve a nutritive role for the avascular retina since it is highly vascularized, but the exact function is unknown.

• Other proposed functions include: – reduce glare– regulate pressure or temperature within the eye– perception of movement– light absorption

Pecten