Birds
Chapter 27
I. Diversity
 9000 species worldwide; second to fishes in
number of species of the vertebrates
 All land biomes; all oceans; both poles
II. Origin
 Thomas Huxley recognized that birds share
many characteristics with theropod dinosaurs
 Theropods belong to archosaurian diapsid
reptiles (crocodiles are living member)
 Similarities with reptiles: 1st neck vertebra
single ball-socket joint, single middle ear
bone (stapes), lower jaw composed of 5-6
bones, excrete waste as uric acid, similar
eggs and development
 Fossil Archaeopteryx lithographica has bird
skull with teeth in beak, wings with feathers
but end in reptilian clawed fingers, abdominal
ribs, long bony tail
Archaeopteryx
III. General Characteristics
 Feathers unique to birds
 Forelimbs wings, but not all capable of flight
 Hindlimbs adapted for walking, swimming,
perching
 Horny beaks
 Lay eggs
 Adaptations for flight have created a
uniformity in structure
IV. Feathers
A. Origin
 Homologous to reptile scale
 Epidermis rolls into cylinder as develops,
forming rachis and barbs
 Pigments develop
 Rachis and barbs formed from hardened
keratin
 Splits out of sheath, feathers protrude
B. Structure
 Feather developed to add
more power with less weight
 Quill emerges from skin
follicle, becoming rachis as
it extends up feather
 Barbs extend from rachis,
each with numerous barbules
spreading laterally
 Several hundred barbs form
flat, webbed surface, the
vane of the feather
 Neighboring barbs “zip”
together with tiny hooks;
preening keeps barbs
“zipped” and vane surface
intact
C. Types
 Contour—outermost feathers forming shape of
bird
 Flight—extend off wing in flight
 Down—under contour, lack hooks, provide
insulation
 Filoplume—hairlike with weak shaft and tufty
barbs
 Powder-down—disintegrate and release a
powder that waterproof feathers
Feather Types
Contour
Filoplume
Down
D. Molting
 Once a year, usually in
late summer after nesting
season
 Gradual process that
avoids leaving bare spots
 Flight and tail feathers
are lost in pairs to
maintain balance for
flight
 In water birds, takes
place all at once leaving
them grounded for a
period of time
E. Coloration
 Lipochromes in feathers give
them red, orange, or yellow
coloration
 Melanin colors them black,
brown, red-brown, or grey
 Blue color is caused by the
structure of the feather
scattering light
V. Skeleton
 Modern birds have
light, delicate bones
with air cavities
(pneumatized bones)
 Arching shape of
cavities create
strong bones and
rigid structure
 Weight of feathers
may outweigh
skeleton
A. Weight
B. Skull
 Evolved from
ancestors with diapsid
skulls
 Fused into one piece
 Braincase and orbits
are large to hold
larger brain and eyes
 Horny beak molded
around bony jaws
 Kinetic skull; some
have upper jaw hinged
to skull
C. Vertebral Column and Appendages
 Column is rigid with all vertebrae fused except cervical
 Additional bony structures called unicate process are
fused to pelvic girdle to provide more rigidity for flight
and support legs (legs are proportionately heavier in
birds than mammals)
 Large keel on sternum provides attachment site for
flight muscles
 Forelimb bones fused or absent for flight
 Muscles in neck provide flexibility
 Pectoralis muscles on keel lower
wings
 Beneath these,
supracacoracoideus muscles pull
wing up by a “rope and pulley”
system
 Having both sets of muscles below
wings provides more flight
stability
 Thigh has most muscle mass in leg
 Toe-locking mechanism prevents
birds from falling off perch when
asleep
 Almost 1000 muscles control tail
feathers for steering
VI. Muscles
VII.Food & Digestion A.Food Sources
 Early birds primarily carnivorous, feeding on
insects
 Now other animals eaten like worms, molluscs,
crustaceans, fish, frogs, birds, carrion, etc.
 1/5 of all birds eat nectar
 Other birds eat seeds and fruit
 Euryphagous—eats a variety of items and
switches to whatever is in season
 Stenophagous—specialize in particular food
source; most vulnerable to extinction
 Beaks are adapted to food source
Bird Feet
Feeding
B. Feeding
 Fast efficient digestive system provides quick energy
for flight
 Birds have high metabolic rate and must eat a
quantity of food; generally the smaller the bird, the
more food it must eat
 A hummingbird must eat 100% its own body weight
but a chicken only 3.4%
 Taste buds on tongue, salivary glands lubricate food
and tongue
 Pharynx, esophagus, crop (some birds produce a
lipid-rich “milk” here), stomach ( 2 parts:
proventriculus secretes gastric juice and gizzard
grinds food), intestines, rectum, cloaca
 Some birds swallow grit or pebbles to aid grinding in
gizzard
Bird Anatomy
Bird Digestive System
Bird Beaks
VIII. Circulatory System
 Adapted to provide
high oxygen demands
of flight
 4-chambered heart
with strong ventricular
walls
 Separate pulmonary
and systemic circuits
 Heart beat faster as
bird gets smaller:
turkey 93 bts/min;
chickadee 500 bts/min
 Active immune system
IX. Respiratory System
 Unlike mammal lungs, bronchii do not end in
alveoli but in tube-like parabronchii so that
they can squeeze into available small spaces
in thorax, abdomen, even wing and leg bones!
 This system of air sacs provide more
buoyancy and also enables the bird to stay
cool even during flight when 27x more heat is
produced
 On inspiration, air flows directly to air sacs,
bypassing lungs; on expiration air flows out
through lungs giving continuous air flow
 Most efficient respiratory system of all
vertebrates
Respiratory System
X. Excretory System
 Pair of kidneys which filter
blood and resorb some
substances
 Urine flows through ureters
to cloaca
 Like reptiles, excrete uric
acid to cloaca where water
is resorbed
 Marine birds have salt
glands located above each
eye that secrete excess
salts ingested from food
and saltwater; the salt
solution runs out of the
nostrils
XI. Sensory System
A. Brain
 Brain has well developed cerebrum, cerebellum, and
hind brain to accommodate requirements for flight
and visual lifestyle
 The size of the cerebral hemisphere is directly
related to intelligence of bird; corpus striatum serves
as integrating center rather than the cortex in
mammals
 Cerebellum provides sensing for muscle-position,
equilibrium, and vision cues
 Optic lobes bulge on either side of midbrain,
providing birds with best vision of all animals
 Poor sense of smell except in vultures, flightless
birds, and ducks
Bird Brain
B. Hearing
 Excellent hearing
 Ear similar to mammals: external ear canal,
eardrum, middle ear, inner ear, cochlea
 Hear same range of sounds as humans but not
as high of frequencies; however better able
to distinguish differences in pitch and
intensities
C. Vision
 Eye similar to mammals except larger in proportion
to body size
 Eye almost immobile; bird turns head instead
 Retina with rods and cones; diurnal birds have more
cones; nocturnal have more rods; some birds can see
UV—aids in seeing honey guides on flowers
 2 foveae regions of vision give birds excellent
monocular and binocular vision
 Vegetarian birds have eyes on sides of head; birds
of prey have them placed in front for better depth
perception
 Hawk has 8x better visual acuity than humans and
can see a rabbit 1 km away; an owl can see 10x
better in dim light than human
XII. Flight
A. Evolution
 Feathers preceded flight and arose for
thermoregulatory purposes
 Flight would provide birds with the ability to
travel to better environments, exploit flying
insects, and provide a means to escape form
predators
 Ground-up hypotheses—running birds with
primitive wings to snare insects
 Trees-down hypotheses—birds passed from
tree-climbing, leaping, parachuting, gliding
and finally flight
B. Lift
 Lift is provided by secondary feathers midarm
 Wing is streamlined with concave lower
surface; as air takes longer to flow over the
convex top surface, it creates a negative
pressure generating 2/3 of the lift
 Lift/drag ratio determined by tilt and
airspeed: high speeds/small angles or low
speeds/high angles but at 15o stalling will
occur
 Slotting in primary feathers can slow down
stalling
Wing Movements
C. Thrust
 Besides vertical lift, flight requires forward
thrust
 Provided by hand bones with attached primary
feathers; these bend and twist with upstroke
to provide thrust
 Upstroke needed for fast, steep takeoffs and
hovering
 Greatest power on downstroke
D. Wing Shape
1. Elliptical
 Used for flying in
forested habitats
 Slotting in primary
feathers prevents
stalling at slow
speeds
 High degree of
maneuverability
2. High Speed
 Wings sweep back
and taper to slender
tip which reduces
vortex turbulence
 Relatively flat and
lack slotting
 Seen on birds that
feed while on wing or
make long migrations
3. Soaring Wings
 Long narrow shape
without wing slots
 Allows high speed,
high lift, and dynamic
soaring
 Most efficient but not
very maneuverable
 Seen on ocean birds
(albatrosses, gannets,
etc) who can exploit
the sea winds and
wide open spaces
4. High Lift
 Broad, slotted wings
allow birds to carry
heavier body mass
and provide sensitive
response to static
soaring
 Vultures, hawks,
eagles, owls all have
this wing type
XIII. Migration
A. Reasons
 Avoid climatic extremes
 Exploit seasonal changes in abundance of food
sources reducing food shortages
 Can avoid build-up of specialized predators
 Expands living space reducing aggressive
territorial behavior
B. Routes
 Lengthening days stimulates pituitary gland to
promote gonad growth, fat deposits, migration, and
even controls courtship, mating behavior, and care of
young
 Most follow north-south routes usually in fall and
spring; lanes may be narrow or very broad
 Many birds follow landmarks, but some birds fly over
large water bodies
 Aquatic species usually make rapid migrations; others
may take months
 The artic tern travels from North America to
coastlines of Europe and Africa for the winter,
covering 11,200 miles!
C. Navigation
 Chiefly by sight using topographical landmarks
 Flocks rely on older experienced birds for
route finding
 Earth’s magnetic field
 Also use the sun’s position to navigate and tell
time
 Use position of stars at night
 By natural selection, only best navigators
leave offspring
XIV. Reproduction
 Testes are small until breeding season when they may
enlarge 300x; no penis
 Fertilization occurs by bringing cloacal surfaces in
contact; must occur in upper oviduct before albumin
and shell added
 Left ovary and oviduct develop; right side
degenerates
 Glands add albumin (egg white) to eggs as passes
down oviduct; farther down shell membrane, shell,
and shell pigments are also secreted
 Sperm may remain alive in oviduct many days after
single mating
Reproductive Organs
Egg Structures
XV. Social Behavior
A. Cooperative
 Flocking provides mutual protection from enemies,
greater ease in finding mates, less chance of getting
lost during migration, protection against low
temperatures
 Flocking occurs typically in sea birds during nesting
and rearing young; in other land birds it may during
feeding and roosting
 Most land birds seek isolation for rearing young
B. Mating
 90% of species are monogamous during breeding season
some such as swans and geese, a partner is chosen for
life
 Male sings to announce his availability and drive away
other males from territory; female make selection based
on faithfulness and best chance for reproductive success
 Both birds care for young
 Polygyny most common form of polygamy (1 male mates
with many females); male does not care for young
C. Nesting & Care
 Nests may be laid on bare dirt or rocks or may be
made of mud, lichen, brush, grasses; some are made
in hollows and cavities of trees; most are carefully
concealed
 Nearly all eggs must be incubated by one or both
parents; usually female does most or all
 Most songbird eggs require 14 days to hatch; ducks
may take 1 month
 Precocial birds—able to feed, run, swim at birth
 Altricial birds—naked and helpless at birth; must be
fed in nest week or more; success of these birds low
(about 20%)
 Nesting failure caused by predators, nest parasites
and other factors
Young
Altricial Young
Precocial
Young
Nests
XVI. Populations
A. Fluctuations
 Population size varies year to year depending
on abundance of food resources
 Bird of prey populations cycle with food
supply
 Other birds may relocate to another area to
locate alternative food supplies
B. Declines
 Since Dodo went extinct in 1695, more than 80
identified species have gone extinct due to human
influence
 Common cause habitat destruction and hunting
 Modern hunting techniques have restored wetlands;
no legally hunted birds are endangered
 Recent decline of songbirds over last 40 years due
to no fallow agricultural land, fragmentation of
forests, housecats, loss of tropical forests in
wintering grounds, use of pesticides and hunting in
wintering grounds
 Introduced species such as sparrows and starlings
can accommodate and are widespread now
XVII. Classification A. Aves
 1. Superorder Paleognathe—
modern birds with primitive
archosaurian palate
 Ratites—all flightless
Ostriches—Africa
Rheas—South America
Cassowaries, emus—
Australia, New Guinea
Kiwis—New Zealand
 Tinamous
Flightless
Central and South
America
2. Superorder Neognathe
 Modern birds with
flexible palate
a. Order
Sphenisciformes
Penguins
Wings used for
paddles
Southern oceans
b. Order Pelecaniformes
 Fish-eaters with
throat pouch
 55 species
worldwide, especially
in tropics
 Pelicans, cormorants,
gannets, boobies
c. Ciconiiformes
 Long-legged, longnecked waders and
vultures
 90 species worldwide
 Herons, bitterns,
storks, ibises,
spoonbills,
flamingoes, and
vultures
d. Anseriformes
 Broad bills with
filtering ridges
 Webbing only on
front toes
 Long breast bone
with low keel
 150 species
worldwide
 Swans, geese, ducks
e. Order Falconiformes
 Diurnal birds of prey
 Strong fliers with
keen vision
 270 species
worldwide
 Eagles, hawks,
falcons, condors,
buzzards
Coopers
Red
tailed
Sharp
shinned
f. Order Columbiformes
 Short necks, short legs, short slender bill
 290 species worldwide
 Pigeons and doves
g. Order Psittaciformes
 Hinged moveable upper beak with fleshy
tongue
 320 species in tropics
 Parrots and parakeets
h. Order Strigiformes
 Nocturnal predators
with large eyes
 Powerful beaks and
feet
 Silent flight
 135 species worldwide
 Owls
i. Order Apodiformes
 Small with short legs
and rapid wingbeat
 Hummingbirds and
swifts
 40 species worldwide
but hummingbirds
mainly found in the
tropics
j. Order Piciformes
 Highly specialized bills
 Two toes extend forward and two backward
 Nest in cavities
 380 species worldwide
 Woodpeckers, toucans, puffbirds,
honeyguides
K. Order Passeriformes








Perching songbirds
Largest order containing 60% of all bird species
Well developed syrinx (for singing)
Feet adapted for perching on small twigs and thin
stems
Altricial
Singers such as thrushes, warblers, mockingbirds
Nonsingers such as swallows, magpies, crows, jays
5000 species worldwide
l. Order Galliformes
 Chicken-like ground-nesting herbivores
 Strong beaks and heavy feet
 250 species worldwide
 Quail, grouse, pheasant, turkey, domestic
fowl