Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180



** 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).

I. CENTRAL NERVOUS SYSTEM

1) Brain = same basic plan as in reptiles and mammals, 3 Divisions:

  • a) Forebrain (Cerebrum) = integration, instinctive behavior, intelligence
  • b) Midbrain = vision, muscular coordination, physiological control
  • c) 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.

A) Forebrain

- Pallial domains are responsible for learning and intelligence in vertebrate brains. In mammals, the outer layer (cerebral cortex) greatly enlarges to become dominant and serves as the seat of higher intelligence. Provides a great capacity for learning.
- In birds, the cortex is thin and relatively undeveloped (thought to be the seat of conditioned behavior),
•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 (= mainly 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
•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

B) Midbrain

- 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.

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


II. PERIPHERAL NERVOUS SYSTEM - similar to the situation in other vertebrates.


III. SENSES

1) Smell - olfactory lobes are generally small and, as a result, birds have historically been regarding as having a generally poor sense of smell. More recent research, however, have shown that most birds are capable of detecting at least certain odors with similar abilities to mammals and birds appear to use odors in a number of daily activities (e.g., feeding, orientation). Also, number of functional olfactory receptor genes is roughly similar to that in humans, suggesting that sense of smell is fairly well developed in birds.

- Some birds have larger olfactory bulbs than typical for birds and are capable of smelling very effectively (e.g., Turkey Vultures, kiwis).

2) Taste - all birds can taste; birds are equally or less sensitive to certain ingredients than mammals.Birds have fewer taste buds than do mammals.

3) Mechanoreception

    a) Touch - possess typical touch, pressure, temperature, and pain receptors.
  • b) Birds are also sensitive to barometric pressure.
    • (i) Many birds can sense oncoming storms and modify foraging behavior accordingly.
    • (ii) Pigeons and thrushes can select proper altitude for migratory flights, presumably this is true for other birds as well.
  • c) Magnetism - 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.


IV. HEARING

  • a) 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.
  • b) 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. (SEE PG. 193, GILL).

  • c) Owls with specializations allowing them to detect and capture prey by hearing alone.
    • (i) Detect low frequency sounds effectively
    • (ii) Have high numbers of auditory neurons
    • (iii) Facial discs act as sound collectors and aid in focusing sound to ear
    • (iv) 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.

  • d) 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.


V. VISION

1) Vision is the most important sensory input for birds, as they are visual animals.

2) Birds have large eyes relative to other vertebrates (e.g., starlings have an eye that makes up 15% of the head mass, humans = 1%)

3) Shapes of avian eyes vary.

  • a) Globular = diurnal birds with high resolution over great distances (hawks, etc.)
  • b) Flattened = most birds
  • c) Tubular = nocturnal birds, allows increased accommodation (focusing) and light-gathering

4) 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
 
5) 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.

6) 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 (e.g., 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 species, but not in others. UV reflectance of plumage can also influence reproductive output. For example, in Blue Tits, females increase the number of males in their clutch when mated to males with high UV reflectance in their crest.

- Diurnal birds have colored oil droplets in the eyes, probably functions to enhance contrast by filtering out background "noise"

7) 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.

8) Pecten = structure composed of blood vessels and supporting stromal cells, present at exit of optic nerve from eye and 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: (1) reduce glare, (2) regulate pressure or temperature within the eye, (3) perception of movement, (4) light absorption.