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Class Aves: Birds
Characteristics of Class Aves
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Adaptations for flight (feathers/wings)
Endothermic/High metabolic rate
Vertebral column modified for flight
Bones lightened by numerous air spaces
Horny bill/beak
Lack teeth
4 chambered heart
Super efficient respiratory system
Lay amniotic eggs
Miscellaneous
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Largest class of terrestrial vertebrates
Evolved from small carnivorous dinosaurs
about 150 mya.
Oldest known bird is archaeopteryx, which
had many similarities with dinosaurs
(teeth, claws on wings, long tail)
External Structures: Integumentary
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A bird’s feathers are called its plumage.
Feathers have 3 main purposes:
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Provide lift and aid steering during flight
Waterproofing and preventing excessive heat loss
Courtship
Feathers are modified scales made of keratin which are molted
annually
Feathers consist of a central shaft (rachis) and the vane (the rest of
the feather)
The vane is made of barbs and barbules that interlock to provide
strength and catch air
Thin skin and scales on legs and feet
Cere – skin around the beak
Types of Feathers
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Contour feathers – cover the body wings and tail
Down feathers – insulating feathers
Filoplume feathers – sensory functions
Birds preen in order to stay clean, remove dead
skin cells, and remove parasites
An oil gland at the base of the tail provides
natural lubrication and waterproofing for skin
and feathers
The Skeleton & Muscles
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Lightweight, yet strong due to air sacs
Flight birds have MANY mitochondria and
produce LARGE amounts of ATP in their
muscles. Why might this be? What about
chickens and turkeys?
Flight: The Airfoil
Nutrition & Digestion
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Most birds are ALWAYS hungry! They have
to eat a lot to keep up with the energy
necessary for flight. They have ridiculously
high metabolisms.
A bird’s beak and tongue are indicative of
its food source. For example, the beak of
a bald eagle if specialized for tearing meat
off live prey. (Remember the activity.)
Nutrition & Digestion Continued
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Crop – stores & moistens
food, does not digest it; it
allows birds to eat even
when their stomach is full
so that food is constantly
moving through their
body and providing
energy.
Proventriculus – stomach
Ventriculus – gizzard
kneads and crushes food;
birds swallow rocks/sand
in order to enhance the
function of the gizzard
Circulatory System
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4 chambered heart that is large and beats very rapidly
Veins return blood TO the heart, arteries take blood
away FROM the heart to the rest of the body; Pulmonary
vessels carry blood to and from the lungs.
Respiratory System
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Birds have a greater rate of oxygen
consumption than any other vertebrate,
because they must maintain a high
metabolic rate.
Birds have air sacs that extend from lungs
that constantly cycle air with every breath
in and out.
Respiration
Thermoregulation
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Birds are endothermic. What does this
mean?
When birds get hot, they don’t sweat, but
rather pant like a dog.
In order to stay warm, they will tuck their
bill under their feathers and possibly pull
one leg, then the other, close to their
body.
Nervous System
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Birds have big brains relative to their size. The
bigger brains function in visual learning, feeding,
courtship, nesting, and migration.
Vision – birds see in color, have eyelids AND a
nictitating membrane. Birds that hunt prey
usually have eyes that face forward. Birds that
eat insects, fruit, or seeds usually have eyes on
the sides of their heads. WHY?
Smell – birds do not have a highly developed
sense of smell
Hearing – Birds can hear EXTREMELY well
Excretion
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Birds have a cloaca and NO bladder.
They excrete uric acid.
Reproduction & Development
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All birds are oviparous – they lay eggs that
develop outside of the body
Most birds compete for mates through
courtship behavior.
Over 90% of birds are monogamous for a
season and some are monogamous for
life.
Chicks
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Altricial birds – helpless at
hatching; must be fed,
kept warm, and grow
rapidly.
Precocial birds – almost
independent at hatching;
usually covered with
down and can walk, run,
swim, and feed
themselves almost
immediately.
Birds that don’t fly are
usually precocial. Why
might this be?
Migration & Navigation
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The instinct to migrate is
usually initiated by the
changes in the length of
daylight that accompanies
the change in seasons.
Birds use two forms of
navigation:
 Route-based navigation
– keeping track of
landmarks
 Location-based
navigation – using the
angle of the sun,
changes in air pressure,
and the Earth’s
magnetic field to guide
them