31-2 Birds
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What Is a Bird?
What Is a Bird?
Class Aves
Nearly 10,000 modern species.
Birds are reptile-like animals that maintain a constant
internal body temperature.
Birds have:
•outer covering of feathers
•2 legs covered with scales, used for walking/perching
•front limbs modified into wings
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What Is a Bird?
Feathers are the single most important characteristic
that separates birds from all other living animals.
Feathers are made mostly of protein and develop
from pits in the birds' skin.
Feathers help birds fly and also keep them warm.
The two main types of feathers are contour and
down.
Some birds, like herons, that live on or in water also
have powder down, which releases a fine powder to
repel water.
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What Is a Bird?
Feathers
Barb
Contour feather: Contour
feathers provide the lifting force
and balance needed for flight.
Down feather: Down
feathers trap air close to the
body and keep the bird warm.
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Barbule: The hooks on
each barbule fit together,
holding them flat.
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Evolution of Birds
Evolution of Birds
Paleontologists agree that birds evolved from extinct
reptiles.
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Embryos of both develop within amniotic eggs.
Both excrete nitrogenous wastes as uric acid.
Bones that support the limbs, and other skeleton
parts, are similar in both groups.
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Evolution of Birds
Archaeopteryx was the first birdlike fossil discovered.
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Dates from the late Jurassic Period, ~150 mya
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It had teeth in its beak, a bony tail, and toes and
claws on its wings.
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It may be a transitional species between dinosaurs
and birds with characteristics of both groups.
Looked like a small, running dinosaur with welldeveloped feathers covering most of its body.
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Evolution of Birds
Other fossil evidence leads some to hypothesize that
birds and dinosaurs both evolved from an earlier
common ancestor.
The origin of birds is still not completely resolved.
New fossils of ancient birds are being found all the
time.
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31-2 Birds
Form, Function, and Flight
Form, Function, and Flight
Birds have a number of adaptations that enable them
to fly, including:
• highly efficient digestive, respiratory, and
circulatory systems
• aerodynamic feathers and wings
• strong, lightweight bones
• strong chest muscles
Most birds have these characteristics, even though
some cannot fly.
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Form, Function, and Flight
Body Temperature Control
Birds generate their own body heat and are called
endotherms.
Endotherms have a high rate of metabolism
compared to ectotherms (reptiles).
Metabolism is the sum of chemical and physical
processes that go on inside the body. Metabolism
also produces heat.
Feathers insulate a bird enough to conserve most of
its metabolic energy, allowing it to keep warm more
efficiently.
A bird’s body temperature is ~41oC (or 105.8oF).
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31-2 Birds
Form, Function, and Flight
Feeding
Any body heat that a bird loses must be regained
by eating food.
Birds need to eat a lot of food to produce the heat
energy they need to maintain metabolism.
Small birds lose heat relatively faster than large
ones, so they must eat more relative to their body
size.
The phrase “eats like a bird” is misleading,
because most birds are voracious eaters.
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Form, Function, and Flight
Birds’ beaks, or bills, are adapted to the food they eat.
• short, fine bills that pick insects
off leaves and branches or catch
flying insects
• short, thick bills crack seeds
• carnivorous birds shred prey
with hooked bills
• long, thin bills to reach into
flowers or probe mud for worms
and shellfish.
• Large, long bills used to pick
fruits or long, flat bills for
grasping fish.
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Form, Function, and Flight
Birds do not have teeth and cannot chew.
Many birds have specialized structures to help digest
food.
The crop is a structure at the lower end of the
esophagus in which food is stored and moistened.
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Form, Function, and Flight
In some birds, like pigeons, the crop has a 2nd function
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During nesting season, the crop produces a
substance rich in protein and fat.
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Parents regurgitate this to feed their newly
hatched young.
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This provides young birds with materials they
need to grow.
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Form, Function, and Flight
The form that the stomach takes depends on the
feeding habits of the bird.
Birds that eat meat or fish have an expandable area
in which large amounts of soft food can be stored.
Birds that eat insects or seeds have a muscular
organ called the gizzard that helps in the mechanical
breakdown of food by grinding it.
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Form, Function, and Flight
First chamber of stomach
Gizzard
Moistened food passes
to the stomach, a twopart chamber.
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31-2 Birds
Form, Function, and Flight
First chamber of stomach
Gizzard
The first chamber
secretes acid and
enzymes.
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Form, Function, and Flight
First chamber of stomach
Gizzard
The partially digested
food moves to the 2nd
chamber, the gizzard.
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31-2 Birds
Form, Function, and Flight
The muscular walls of the gizzard
squeeze the contents, while small
stones grind the food.
Gizzard
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Form, Function, and Flight
As digestion continues, the food moves
through the small intestine, where the
breakdown of food is completed and food is
absorbed.
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Form, Function, and Flight
The large intestine then stores
undigested waste until it is eliminated.
Undigested food is expelled through the
cloaca
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Form, Function, and Flight
Respiration
Birds have a highly-efficient way of taking in oxygen
and eliminating carbon dioxide.
Air enters air sacs.
It flows through the lungs in a series of small tubes
lined with specialized tissue, where gas exchange
takes place.
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Form, Function, and Flight
Air flows in a single direction.
The one-way flow of air:
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constantly exposes the lungs to oxygen-rich air.
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maintains a high metabolic rate.
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provides efficient extraction of oxygen, which
enables birds to fly at high altitudes where the
air is thin.
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Form, Function, and Flight
Circulation
Birds have four-chambered hearts and two
circulatory loops.
There is complete separation of oxygen-rich and
oxygen-poor blood.
Oxygen-poor blood from the body is pumped to the
lungs.
Oxygen-rich blood returns from the lungs and is
pumped to the rest of the body.
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31-2 Birds
Form, Function, and Flight
Bird Heart
Pulmonary – pumps from right ventricle to lungs
Systemic – pumps from left ventricle to body
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Form, Function, and Flight
Excretion
Excretion in birds is similar to that of most living
reptiles.
Nitrogenous wastes are removed from the blood by
the kidneys, converted to uric acid, and deposited in
the cloaca.
Most of the water is reabsorbed, leaving uric acid
crystals in a white, pasty form.
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Form, Function, and Flight
Response
Birds have well-developed sense organs, which
are adaptations that enable them to coordinate the
movements required for flight.
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31-2 Birds
Form, Function, and Flight
Birds’ brains can
quickly interpret and
respond to signals. It
is relatively large for
their body size.
The cerebrum controls
behavior such as
flying, nest building,
care of young,
courtship, and mating
and is quite large.
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Form, Function, and Flight
• The cerebellum coordinates the movement of the
wings and legs; it is larger in birds than in reptiles.
• The medulla oblongata coordinates basic body
processes such as the heartbeat.
• Birds have well-developed eyes which allow them
to see color very well and therefore have sizable
optic lobes.
• Most bird species can hear quite well.
• Taste and smell are not well developed in most
birds, therefore the olfactory bulbs are reduced.
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Form, Function, and Flight
Movement
Some birds, like ostriches and penguins, cannot fly.
They get around mainly by walking, running, or
swimming.
Most birds, however, can fly.
The skeletal and muscular systems of flying birds
exhibit adaptations that enable flight.
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31-2 Birds
Form, Function, and Flight
Skeletal System
of a Bird
Vertebra
Skull
Collarbone
(wishbone)
Pelvic girdle
Strut
Tailbone
Pectoral griddle
Air
space
Sternum
Rib cage
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Form, Function, and Flight
In flying birds, many large bones are fused together,
making the skeleton rigid.
These form a frame that anchors the muscles used for
flight.
Bones are strengthened by struts.
Air spaces between the struts make
bones lightweight.
Birds have large chest muscles that power the upward
and downward wing strokes necessary for flight.
Muscles attach to a keel that runs down the front of
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an enlarged breastbone, or sternum.
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Form, Function, and Flight
Birds move their wings in a circular pattern similar to
rowing. On the downstroke, feathers are held together,
and the wings move down and rotate forward.
This pushes air down and back, providing lift and
propelling the bird through the air.
During the upstroke, feathers are opened to allow air
to move through them , making it easier to move the
wings upward. The wings bend up, moving closer to
the body, and rotate backward.
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Form, Function, and Flight
Reproduction
Both male and female reproductive tracts open into
the cloaca.
Sex organs often shrink in size when not breeding.
Mating birds press their cloacas together to transfer
sperm from male to female.
Some male birds have a penis that transfers sperm to
the female.
In females, a single ovary produces an amniotic egg
with a hard outer surface made of CaCO3.
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Form, Function, and Flight
Bird lay amniotic eggs that have hard outer shells.
Most birds incubate their eggs until the eggs hatch.
When a chick is ready to hatch, it makes a hole in the
shell with a small tooth-like structure on its bill called
the egg tooth.
Once the bird has hatched, it rests for a while and
lets its feathers dry.
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Groups of Birds
Groups of Birds
There are nearly 30 different orders of birds.
The largest order of birds is the passerines, or
perching birds, including over 5000 species.
Other groups of birds include: pelicans, parrots, birds
of prey, cavity-nesting birds, herons, and ostriches.
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Ecology of Birds
Ecology of Birds
Birds interact with ecosystems and humans in many
ways.
• Hummingbirds pollinate flowers.
• Fruit-eating birds disperse seeds in their droppings.
• Insect-eating birds catch insects, controlling
populations.
• Food!
• Sport – hunting
• Pillows/coats – down feathers
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Ecology of Birds
Many birds migrate long distances, usually
seasonally.
Some species use stars and other celestial bodies as
guides.
Others use a combination of landmarks and cues
from Earth’s magnetic field.
Magnetic sense: allows them to sense rotation of
Earth; used for migration
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Diagram
Labels:
A = esophagus
B = crop
C = liver
D = gizzard
E = stomach
F = intestine
G = cloaca
H = colon
I = pancreas
J = air sac
K = kidney
L = heart
M = lung
N = brain
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31-2
A bird with a short, thick bill probably eats
a. fish.
b. seeds.
c. insects.
d. fleshy fruit.
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Archaeopteryx has characteristics of both
a. modern birds and ancient birds.
b. amphibians and reptiles.
c. reptiles and modern birds.
d. amphibians and modern birds.
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31-2
Which of the following bird adaptations is NOT
associated with flight?
a. bones with many hollow air spaces
b. air sacs in addition to lungs
c. gizzard
d. contour feathers
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The largest order of birds is the
a. perching birds.
b. birds of prey.
c. pelicans and relatives.
d. penguins.
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31-2
Which of the following birds assists in pollinating
flowering plants?
a. pelican
b. hummingbird
c. raptor
d. heron
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