CHAPTER
Birds
27-1
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27-2
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Diversity
Profile
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27-3
Only fishes have more species among
vertebrates
Birds live in all biomes, from mountains to
prairies, on all oceans, and from the North to
the South Pole
Some live in dark caves, and some dive to 45
meters depth
The “bee” hummingbird is one of the
smallest vertebrate endotherms
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Diversity
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The feather is the unique and essential
feature or hallmark of birds
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27-4
Some feathers were also present in some
theropod dinosaurs
These feathers were not capable of supporting
flight
Obviously served in other capacities such as
thermoregulation or mating behavior
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Diversity
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Uniformity in Structure
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27-5
Despite 150 million years of evolution, birds are
still readily recognized
Forelimbs are modified as wings, although not all
are capable of flight
Hindlimbs are adapted for walking, swimming or
perching
All birds have keratinized beaks
All birds lay eggs
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Diversity
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27-6
Driving force for this uniformity appears to be
adaptations necessary for flight
 Wings
 Present for support and propulsion
 Respiratory system
 Must meet high oxygen demands and cool
the body
 Bones
 Must provide a light but rigid airframe
 Digestion and circulation
 Must meet high-energy demands of flight
 Nervous system
 Must have superb sensory systems for highvelocity flight
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Origin and Relationships
History
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Discovery of the fossil of Archaeopteryx
lithographica in 1861 linked birds and
dinosaurs
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27-7
Skull resembled modern birds but had teeth rather
than a beak
Skeleton was reptilian with clawed fingers,
abdominal ribs, and a long bony tail
Feathers were unmistakably imprinted along wings
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27-8
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Origin and Relationships
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Zoologists had long recognized that birds
and reptiles shared many similarities
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Skulls that abut the first neck vertebra by a single
ball-and-socket joint
Single middle ear bone, the stapes
Lower jaw composed of five or six bones
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Excrete nitrogenous wastes as uric acid
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Mammals excrete urea
Similar yolked eggs
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27-9
Mammals have one mandibular bone
Embryo develops on surface by shallow cleavage
patterns
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Origin and Relationships
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Larger flightless birds such as the ostrich
and emu can outrun predators
Flightless birds are free from weight
restrictions of flight and some evolved to
very large sizes
27-10
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Structural and Functional Adaptations for Flight
Feathers
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Structure
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Feather is a special bird adaptation that
contributes to more power or less weight
Hollow quill emerges from skin follicle and
continues as a shaft or rachis
Rachis bears numerous barbs
Up to several hundred barbs are arranged to form
a flat, webbed surface, the vane
Each barb resembles a miniature feather
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27-11
Numerous parallel filaments or barbules spread laterally
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27-12
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Structural and Functional Adaptations for Flight
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Up to 600 barbules in each side of a barb
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Barbules from two neighboring barbs overlap
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27-13
May be over one million in the whole feather
“Zip” together with tiny hooks
When separated, they are “zipped” back together
by preening
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Structural and Functional Adaptations for Flight
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Types of Feathers
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Contour feathers
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Down feathers
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Hairlike, degenerate feathers with a weak shaft and tuft of
short barbs
Powder-down feathers
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27-14
Under contour feathers
Barbules lack hooks and function as insulation
Filoplume feathers
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Provide the form of the bird
Flight feathers are contour feathers that extend beyond
body
Herons and their relatives
Disintegrate and release a talc-like powder to waterproof
feathers
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Structural and Functional Adaptations for Flight
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Molting
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Fully-grown feather is a dead structure
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Except in penguins, molting is a gradual process
that avoids leaving bare spots
Flight and tail feathers are lost in pairs, one on
each side, to maintain balance
In some species, replacement is continuous
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27-15
Flight is unimpaired
In many water birds, primary feathers are molted
all at once
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Shedding or molting is an orderly process
Birds are temporarily grounded
Most birds molt once a year, usually in late
summer after the nesting season
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27-16
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Structural and Functional Adaptations for Flight
Skeleton
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Bone Weight
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Compared with the Archeopteryx
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27-17
Modern birds have light, delicate bones laced with air
cavities
Termed pneumatized bones
Very strong
Total weight of a bird’s feathers may outweigh
skeleton
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27-18
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Structural and Functional Adaptations for Flight
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Bird Skull
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27-19
As archosaurs, birds evolved from ancestors with
diapsid skulls
Skulls are so specialized
 Difficult to see the diapsid condition
Skull is fused into one piece
 Braincase and orbits are large to hold a larger
brain and eyes
While the skull is lighter
 Legs are heavier than in mammals
 Lowers the center of gravity
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Structural and Functional Adaptations for Flight
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Jaws
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27-20
In Archeopteryx
 Jaws contained teeth set in sockets
Modern birds have a keratinous beak molded
around bony jaws
Most birds have kinetic skulls
 In some, the upper jaw is hinged to the skull
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Structural and Functional Adaptations for Flight
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Vertebral Column and Appendages
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Vertebral column is very rigid
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Ribs are mostly fused with the vertebrae, pectoral
girdle, and sternum
Except in flightless birds
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Sternum bears a large keel to anchor flight muscles
Bones in the forelimbs
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27-21
Vertebrae fused except for cervical vertebrae
Highly modified for flight
Some bones reduced in number or fused
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Structural and Functional Adaptations for Flight
Muscular System
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Pectoralis muscles
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Supracoracoideus muscle
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Depress the wing in flight and are attached to the
keel
Raises the wing, is also attached to the keel
Lays under the pectoralis muscles
Pulls the wing up from below by way of a “ropeand-pulley” type of arrangement
Having both muscles low in the body
provides aerodynamic stability
27-22
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27-23
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Structural and Functional Adaptations for Flight
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Main leg muscle mass is in thigh with
connections by long tendons to feet and toes
Toe-locking mechanism prevents a perching
bird from falling off a branch while asleep
Lost the long reptilian tail
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Substituted a muscle mound where tail feathers
are rooted
As many as 1000 muscles may control the tail
feathers for steering in flight
Neck is thoroughly interwoven with stringy
muscles to provide great flexibility
27-24
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27-25
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Structural and Functional Adaptations for Flight
Food, Feeding and Digestion
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Insect Eaters
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27-26
Early in their evolution, birds were carnivorous
 Primarily feeding on the great variety of insects
Modern birds have specialized to hunt nearly all
types of insects in most habitats
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Structural and Functional Adaptations for Flight
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Other Diets
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27-27
Other animals joined the diet of birds, including
worms, molluscs, crustaceans, fish, frogs, etc
Nearly one-fifth of birds feed on nectar
Beaks of birds often reveal their food habits and
vary between seed-eaters, insect-eaters, etc.
 Woodpecker has a straight, hard, chisel-like
beak to expose insect burrows
 Long, flexible, barbed tongue seeks out insects
in wood galleries
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27-28
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Structural and Functional Adaptations for Flight
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Food Quantity
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Birds are voracious feeders
Have a high metabolic rate
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Hummingbirds use oxygen 12 times faster than a
pigeon and 25 times that of a chicken
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Eats 100% of body weight each day
Have rapid and efficient digestive systems
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27-29
Small birds need even more food per body mass
A shrike can digest a mouse in 3 hours
A thrush will pass berries through the tract in just 30
minutes
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Structural and Functional Adaptations for Flight
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Because birds lack teeth
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27-30
Foods that require grinding are cut apart in the gizzard
Many have a crop that serves to store food at
lower end of esophagus
Crop of pigeons, doves, and some parrots, also
produces a lipid- and protein-rich “milk”
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Structural and Functional Adaptations for Flight
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Stomach consists of
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Proventriculus
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Gizzard
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Serve as fermentation chambers
End of the digestive system is the cloaca
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Form a pellet of indigestible material in the
proventriculus and eject it
Paired ceca at the junction of the intestine and
rectum
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Grinds food
Birds may swallow pebbles or grit to assist
grinding in gizzard
Birds of prey such as owls
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Secretes gastric juice
Also receives products from genital ducts and ureters
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Structural and Functional Adaptations for Flight
Circulatory System
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4-chambered heart is large, with strong
ventricular walls
Share with mammals a complete separation
of respiratory and systemic circulations
Right aortic arch, instead of the left as in
mammals, leads to dorsal aorta
Brachial and pectoral arteries to wings and
breast are unusually large
27-32
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Structural and Functional Adaptations for Flight
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Heartbeat relatively fast compared to
mammals and inversely proportional to size
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Red blood cells (erythrocytes)
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Turkey heart beats 93 times per minute
Chicken heart beats 250 times per minute
A small black-capped chickadee heart beats 500
times per minute
Nucleated and biconvex
Mobile phagocytes are efficient in repairing
wounds and destroying microbes
27-33
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Structural and Functional Adaptations for Flight
Respiratory System
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Differs radically from lungs of reptiles and
mammals
Bird Lungs
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Finest branches of the bronchi do not terminate in
alveoli but are tube-like parabronchi
Air sacs
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27-34
Extend into thorax, abdomen, and long bones
Most efficient respiratory system of any vertebrate
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27-35
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Structural and Functional Adaptations for Flight
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An air sac system helps cool bird during
vigorous exercise when up to 27 times more
heat is produced
Air sacs extend into bones, legs and wings,
providing considerable buoyancy
27-36
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Structural and Functional Adaptations for Flight
Excretory System
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Pair of large metanephric kidneys is
composed of many thousands of nephrons
Each nephron has a renal corpuscle and a
nephric tubule
Birds use vertebrate pattern of glomerular
filtration and selective resorption
Urine flows through ureters to the cloaca
Uric Acid
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27-37
Birds also use the reptilian adaptation of
excreting nitrogenous wastes as uric acid
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Structural and Functional Adaptations for Flight
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In shelled eggs, all excretory products remain
within the eggshell
Uric acid is stored harmlessly
Uric acid has low solubility
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bird can use far less water to excrete wastes
Concentration of uric acid occurs almost entirely
in cloaca where water is absorbed
Bird kidney is less efficient than a mammal
kidney in removing ions of sodium, etc.
Mammal kidneys can concentrate solutes to
4–25 times that of the blood
27-38
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Structural and Functional Adaptations for Flight
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Avian kidneys concentrate solutes only a
little greater than the blood concentration
Marine birds excrete larger salt loads due
diet and seawater they drink
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27-39
Salt glands located above each eye excrete highly
concentrated solutions
Salt solution runs out the nostrils
Gulls and other sea birds have a perpetual “runny
nose”
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27-40
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Structural and Functional Adaptations for Flight
Nervous and Sensory Systems
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A bird’s nervous and sensory system must
accommodate the problems of flight and a
visual lifestyle
Bird’s brain has well-developed cerebral
hemispheres, cerebellum and midbrain
tectum
Cerebral cortex is thin, unfissured, and poorly
developed
Core of the cerebrum, the corpus striatum, is
enlarged into the principal integrating center
27-41
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27-42
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Structural and Functional Adaptations for Flight
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Size of the cerebral hemisphere directly
related to the intelligence of the bird
Cerebellum is where muscle-position sense
(proprioception), equilibrium sense and
visual cues are assembled
Optic lobes bulge to each side of midbrain
and form a visual association apparatus
Sense of smell is poorly developed except in
flightless birds, ducks, and vultures
Have good hearing and superb vision
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27-43
Best in the animal kingdom
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Structural and Functional Adaptations for Flight
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Ear is similar to that of mammals
External ear canal leads to an eardrum
 Inner ear has a short cochlea
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27-44
Allows birds to hear about the same range of
sound as humans
Bird ears do not hear as high a frequency
as do humans, but surpass us in ability to
distinguish differences in pitch and
intensities
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Structural and Functional Adaptations for Flight
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Eye is similar to mammal eye, but it is larger
for a relative to body size
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Less spherical and almost immobile
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Has both rods and cones
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Juts into the vitreous humor
May provide oxygen and nutrients to eye
Herbivores must avoid predators
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Diurnal birds have more cones
Nocturnal birds have more rods
A pecten is a highly vascularized organ attached
to the retina
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Bird turns its head rather than eyes
Eyes placed to each side to view all directions
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27-46
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Structural and Functional Adaptations for Flight
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Birds of prey have eyes directed forward
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Many birds have two foveae or regions of detailed
vision
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Provides better depth perception
Provides both sharp monocular and binocular vision
A hawk has eight times the visual acuity of a human
and can see a rabbit over a kilometer away
An owl’s ability to see in dim light is more than ten
times that of a human
Many birds can see partially into the ultraviolet
spectrum
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27-47
Can see flower nectar guides
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Structural and Functional Adaptations for Flight
Flight
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27-48
History
Early airspace was an unexploited habitat with
flying insects for food
Flight also provided rapid escape from predators
and ability to travel to better environments
2 hypotheses on the evolution of bird flight
 The “ground-up” (cursorial) hypothesis
 Based on running birds with primitive wings
to snare insects
 The “trees-down” (arboreal) hypothesis
 Has birds passing through tree-climbing,
leaping, parachuting, gliding, and finally
powered flight
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Structural and Functional Adaptations for Flight
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Basic Forms of Bird Wings
 Elliptical Wings
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High-Aspect Ratio
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27-49
Birds that must maneuver (prevent stalling at
low speeds with deep wing slots) in forested
habitats have elliptical wings
Birds that feed on the wing or make long
migrations have high-speed wings
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27-50
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Structural and Functional Adaptations for Flight
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Dynamic Soaring Wings
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High-Lift Wings
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27-51
Albatrosses and other oceanic soaring birds
have wings with long, narrow wings… allow
high speed, high lift and dynamic soaring
Vultures, hawks, eagles, owls and other birds
of prey that carry heavy loads have wings with
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Structural and Functional Adaptations for
Flight
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If flight is such an advantage, why
would wingless birds evolve?
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27-52
Flight is costly in terms of energy so birds
abandon flight if they live in areas with
food and without substantial predators,
such as islands.
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Migration and Navigation
Migration
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About half of all bird species migrate
southern wintering regions and northern
summer breeding regions
Can exploit seasonal changes in abundance
of insects and avoid bird predators
Appearing one time a year prevents buildup
of specialized predators and favors
homeostasis, allowing birds to avoid climatic
extremes and food shortages
27-53
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Migration and Navigation
Migration Routes
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Most migratory birds follow established
north-south routes
Some use different routes in the fall and
spring
Some aquatic species make rapid journeys
Smaller species migrate at night and feed by
day
Others are daytime migrants
Many birds follow landmarks
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27-54
Some fly over large bodies of water
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Migration and Navigation
Stimulus for Migration
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Long days of late winter and early spring
stimulate development of gonads and fat
Long day length stimulates the anterior lobe
of the pituitary
Release of pituitary gonadotropic hormone
sets in motion a complex series of
physiological and behavioral changes
resulting in
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27-55
Gonadal growth, fat deposition, migration,
courtship, mating behavior, and care of young
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Migration and Navigation
Direction Finding in Migration
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Experiments suggest birds navigate chiefly
by sight
Birds recognize topographical landmarks and
follow familiar migratory routes
This pools navigational resources and also
experience of older birds
Birds have a highly accurate sense of time
Research indicates they can navigate by the
earth’s magnetic field
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27-56
May be related to magnetite found in the neck
musculature of pigeons
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Migration and Navigation
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Sun-azimuth Orientation
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German ornithologists used special cages to show
birds navigate by sun at day and stars at night
Planetarium experiments revealed they use sun as a
compass
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27-57
An internal clock tracks position
These experiments suggest use of the North Star as
an axis at night
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Social Behavior and Reproduction
Cooperative Behavior
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Sea birds often gather in huge colonies to
nest and rear young
Land birds, except for birds such as starlings
and rooks, tend to seek isolation for rearing
their brood
Birds that isolate during breeding may
congregate for migration or feeding
27-58
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27-59
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Social Behavior and Reproduction
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Advantages for flocking together
 Mutual protection from enemies,
 Greater ease in finding mates,
 Less opportunity for an individual straying
during migration
 Mass huddling for protection against low
night temperatures during migration
27-60
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Social Behavior and Reproduction
Reproductive System
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Testes are very small until the approach of
the breeding season
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May then enlarge 300 times
Males of most species lack a copulatory
organ
 Mating involves bringing cloacal surfaces
in contact
In most birds, left ovary and oviduct develop
and right ovary and oviduct degenerate
27-61
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Social Behavior and Reproduction
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Expanded end of the oviduct, the
infundibulum, receives discharged eggs
Special glands add albumin or egg white to
the egg as it passes down the oviduct
Farther down oviduct, the shell membrane,
shell, and shell pigments are also secreted
Fertilization takes place in the upper oviduct
before albumin and shell are added
Sperm remain alive in the oviduct for many
days after a single mating
27-62
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27-63
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Social Behavior and Reproduction
Mating Systems
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Over 90% of bird species are monogamous
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Only mate with one partner each breeding season
In a few species, such as swans and geese,
partners are chosen for life
In monogamous birds, both male and
females are equally adept at most aspects of
parental care
27-64
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Social Behavior and Reproduction
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Bird Territories
 A male sings often to announce his
presence to females and drive away males
 Females wander about to select a male
that offers the best chance of reproductive
success
 Usually a male can defend an area that
provides just enough resources for one
nesting female
27-65
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Social Behavior and Reproduction
Nesting and Care of Young
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Nearly all birds lay eggs that must be
incubated by one or both parents
Eggs of most songbirds require 14 days for
hatching
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Those of ducks and geese may require a month
Often the female performs most of the duties
of incubation
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27-66
Rarely the male has equal or sole duties
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27-67
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Social Behavior and Reproduction
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Some birds merely lay eggs on bare ground
or rocks
Others build elaborate nests using mud,
lichens, brush, etc.
Nests are often carefully concealed from
enemies
Woodpeckers, chickadees, bluebirds and
others nest in tree hollows and other cavities
Cuckoos and cowbirds are nest parasite
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27-68
Lay eggs in other bird’s nests
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Social Behavior and Reproduction
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Precocial birds are able to feed and run or
swim as soon as they are hatched
Altricial birds are naked and helpless at birth
and must be fed in the nest for a week or
more
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Nesting success in altricial birds is very low
 Sometimes barely 20% of nests produce young
Causes of nesting failure include predators, nest
parasites and other factors
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
27-70
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bird Populations
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Recent Decline of Songbirds
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Some songbird species that were abundant 40
years ago are in decline
Agriculture has utilized once-fallow fields
Fragmentation of forests in the United States
exposes nests to nest predators
House cats are formidable predators that kill
many songbirds
Loss of tropical forests also deprives about 250
migratory songbirds of wintering homes. Birds
stressed in wintering grounds are in poor
condition to make northward migration