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:
- Birds and mammals both have enlarged cerebral and cerebellar hemispheres; the brain in both Classes makes up 2-9% of total body weight.
- 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
- Some birds
have larger olfactory bulbs than typical for birds and are capable of smelling very effectively (e.g., Turkey Vultures, kiwis).
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.
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.