Matt Johnson
Lecture Notes
August 25, 1999
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 8 - BRAIN AND SENSORY ADAPTATIONS
I.
II.
III.
Introduction.
 Bird brain.
 Just ain't right. Bird brains are 2-9% of body mass, about the same as mammals;
much higher than reptiles. What's more, they're smart:
 Learning by imitation is a relatively advanced form of intelligence, and birds excel at
it.
 Many birds also have good counting abilities.
 However, there is wide variation among bird taxa in intelligence, with the overall
trend being that long-lived species and/or species with highly variable food resources
or environments are, by necessity, the most intelligent birds. The corvids and parrots
are among the bird brain champions, the African gray parrot may be the world's
smartest bird.
 Indeed, some birds are pathetically stereotyped in their behaviors……so neophobic
they can’t solve even the simplest of “problems”
Avian Brain. Birds share with the other vertebrates the following sections of the brain
and their associated functions: OVERHEAD
A. Forebrain - controls complex behaviors, instincts, sensory integration and learning
(includes olfactory bulbs, cerebral hemispheres)
B. Midbrain - integrates muscular coordination with vision (flight), controls
physiological and neuro-endocrine function (includes optic lobes, chiasma,
cerebellum).
C. Hindbrain - links spinal cord and peripheral nervous system to brain (medulla
oblongata and base of spinal cord).
D. Intelligence developed in different places in birds vs. mammals:
1. Cerebral cortex in mammals (outside layer o’ brain)
2. hyperstriatum in birds…and inner layer poorly developed in other vertebrates
Vision
A. Very well developed. Most birds are diurnal; they rely on vision as their primary
sense.
B. Specifications.
1. Birds have huge eyes, accounting for up to 50% of cranial volume. If human's
had eyes that big they would be the size of tennis balls.
2. Most birds have more or less monocular vision; their eyes are on either side of
their head. Gives them panoramic views, but poor depth perception. Close one
eye demonstration and bring pinky and dot (or pencil - partners?) on paper. Now
IV.
both eyes. Exceptions are birds that need to judge distance accurately, such as
raptors and fast-flying swallows. Woodcock actually has binocular vision to the
rear.
3. To help judge distance, many birds bob their heads. Now try to bob head in
demonstration - better yet, try to do it while :)
4. Visual acuity:
a. visual acuity is sharpest in the fovea centralis, a region of the retina with
especially densely packed rods and cones. Raptors often have 6-8x as many
rods and cones as humans. Most birds, like humans, have one fovea, but some
have more than one, namely raptors (central and to the side so peripheral
vision is acute -- good for judging distance) and shorebirds (ribbon-like strip
fovea to perceive horizon clearly even on cloudy or misty days).
b. Cones >> rods. Most birds are active in the day and have advanced color
perception. Owls are an exception (more rods).
c. Great ability to resolve movement - movement exceeding ~60 Hz (cycles per
second) are a blur to humans; most birds can resolve up to 100 Hz. Makes
sense given their ultra fast-paced lifestyle.
5. Pecten - a highly vascularized structure in the eye is attached to and protrudes
from the retina. Its purpose is poorly understood and lots of creative ideas have
been proposed, but most scientists now think that it has the boring function of
simply supplying O2 to the retina.
C. Eye shapes. There are three general types of eye shape: OVERHEAD
1. Flattened. Adapted for quick scanning of large expanses. Short optical axis so
image projected on retina is not enlarged (small). Present in small passerines.
Like a wide-angle lens.
2. Globose. Adapted for a magnified image. Length and width of eye are about
equal, which enlarges retinal image. High cone density, strongly curved cornea
and large diameter pupil gather lots of light. Present in focused, keen sighted
birds (raptors, swallows). Like a zoom lens.
3. Tubular. Adapted for low light levels. Long optical axis and huge cornea and
pupil serve to increase light gathering ability. High rod density, but retinal image
somewhat blurred by large cornea. Low depth of field. Present in owls.
Hearing
A. Specifications
1. Based on their well-developed vocal communication, we might guess that bird
hearing is more advanced than in other vertebrates…but we would guess wrong.
2. Most birds have good, but not outstanding, hearing. They probably can detect
difference in timing better that we can, but their range (1-5kHz) is not wider than
other vertebrates, overall.
3. Like humans, birds have binaural hearing, that is, they locate the positions of
sounds by integrating differences in timing and intensity of sounds in the left vs.
right ears.
4. Unlike in humans though, many birds left and right ears are not at exactly the
same vertical position (or orientation)…so the can detect vertical position of
sounds much better than we can (we use timing to detect horz. Position of
sounds…but we suck at telling if a sound comes from above or below us).
V.
VI.
B. Barn Owl Hearing
1. Best studied in Barn Owls. Barn Owls can pinpoint the location of a mouse in
complete darkness – by hearing alone.
2. The Barn Owl has an asymmetrical skull; the left ear is higher than the right and
oriented downward, whereas the right ear is lower and is oriented up.
OVERHEAD
3. Using earphones, researchers have:
a. held volume constant and varied timing – head moves horizontally
b. held timing constant and varied volume – head moves vertically
c. varied both – head moves diagonally
d. how distance is estimated, and the integration of this information is
unclear
Mechanoreception
A. Barometric pressure – very poorly studied, but most birds are probably sensitive to
changes in barometric pressure
1. they feed fast when storms are approaching
2. fly at consistent altitudes
3. only studied to any degree in Homing Pigeon, which can, in fact, detect
differences in air pressure corresponding to only 5 m
B. Pressure sensitive corpuscles (Herbst) in bill and tongue while feeding
1. These monitor changes in muscle tension – they sense where muscles, feathers,
and limbs are in space: proprioception.
2. Many Herbst corpuscles located at feather follicles, especially at base of
filoplumes, which you may recall monitor the positions of flight feathers.
3. Also found in high numbers in bills of probe-feeding sandpipers and tongues of
woodpeckers.
Taste and smell - chemoreception
A. In general, birds have poorly developed senses of smell.
B. Lots of variation among taxa though, with predictable patterns: those taxa whose
primary foods are aromatic have good senses of smell….
1. Cathartidae – they can smell putrid meat; engineers sometimes find leaks in
pipelines by pumping ethyl mercaptan (which simulates smell of rotting meat)
through pipes and then following the vultures.
2. Procellariiformes – tubed-nosed seabirds can small carboxylic acids released by
krill, an important food source. Especially in Hydrobatidae…which are thought
to re-located their nesting burrows (which they return to at night) by smell.
3. Honeyguides can smell, you guessed it, honey.