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Chapter 19
BIRDS
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Chapter opener 19
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Diversity
• Over 9,700 species have been described worldwide
• Only fishes have more species among vertebrates
• Birds inhabit 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 feather is the unique and essential feature or
hallmark of birds
– Some feathers were also present in some theropod
dinosaurs
– These feathers were not capable of supporting flight
19-3
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Diversity
– 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 horny, keratinized beaks
– All birds lay eggs
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Diversity
– A birds entire anatomy is designed around flight
• Wings
– Present for lift and propulsion
• Respiratory system
– Must meet intense metabolic demands of flight
• 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 headfirst, highvelocity flight
19-5
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Origin and Relationships
• Discovery of the fossil of Archaeopteryx
lithographica in 1861
– 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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19-7
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Origin and Relationships
• Zoologists had long recognized that birds and nonavian reptiles shared many similarities
• Thomas Henry Huxley classified birds with theropod
dinosaurs
– Group of dinosaurs with a long, mobile, S-shaped neck
– Dromeosaurs, a group of theropods that includes
Velociraptor, share many additional derived characters
with birds
• Including a furcula (fused clavicles) and lunate wrist
bones that permit swiveling motions used in flight
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19-9
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19-10
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Origin and Relationships
– Additional evidence linking birds to dromeosaurs comes
from recently described fossils from late Jurassic and early
Cretaceous deposits in China
– Additional dromeosaurs like fossils were recently
unearthed in China, such as Sinosauropteryx covered with
filaments
– These fossils, including Proachaepteryx and Caudipteryx,
are dromeosaurs-like theropods, but with feathers
– Feathers of dromeosaurs could not have been used for
powered flight
• May have been used in social displays
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Origin and Relationships
• Modern birds include Paleognathae with a flat
sternum and Neognathae with a keeled sternum
• Paleognathae, or ratite lineage, are large, flightless,
orstrichlike birds and kiwis
• Smaller birds can revert to flightlessness on islands
that lack terrestrial predators
• Larger flightless birds such as the ostrich and emu
can outrun predators
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Structural and Functional Adaptations for Flight
Feathers
• Structure
– Feather is a special bird adaptation that contributes to
more power or less weight
– Most feathers are contour feathers
– Hollow quill or calamus 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
• Numerous parallel filaments or barbules spread laterally
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Structural and Functional Adaptations for Flight
– Up to 600 barbules in each side of a barb
• May be over one million in the whole feather
– Barbules from two neighboring barbs overlap
• “Zip” together with tiny hooks
– When separated, they are “zipped” back together by
preening
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Structural and Functional Adaptations for Flight
• Origin and Development
– Bird feather is homologous to reptile scale
– Feather develops from the epidermis overlying a
nourishing dermal core
– Rather than flattening, feather bud rolls into a hollow
cylinder
– Near the end of its growth, soft shaft and barbs transform
into hard structures of keratin
– When the protective sheath splits apart, the feather
protrudes and barbs unfold
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Structural and Functional Adaptations for Flight
– Fully-grown feather is a dead structure
• Shedding or molting is an orderly process
– Flight and tail feathers are lost in pairs, one on each side,
to maintain balance
– In some species, replacement is continuous
• Flight is unimpaired
– In many water birds, primary feathers are molted all at
once
• Birds are temporarily grounded
– Most birds molt once a year, usually in late summer after
the nesting season
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Figure 19_04
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Structural and Functional Adaptations for Flight
Skeleton
Compared with the Archeopteryx
• 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
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
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Structural and Functional Adaptations for Flight
– 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
• Vertebral Column and Appendages
– Vertebral column is very rigid
• Vertebrae fused except for cervical vertebrae
• Sternum bears a large keel to anchor flight muscles
– Ribs are braced against each other with uncinate processes
• Supports legs and provides rigidity for flight
• Fused clavicles form an elastic furcula that apparently stores energy
as it flexes during wing beats
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Structural and Functional Adaptations for Flight
– Bones in the forelimbs
• Highly modified for flight
• Some bones reduced in number or fused
– All elements of basic vertebrate limb are
represented in modified form
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Figure 19_05
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19-23
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Structural and Functional Adaptations for Flight
Muscular System
• Pectoralis muscles
– Depress the wing in flight and are attached to the keel
• Supracoracoideus muscle
– Raises the wing, is also attached to the keel
– Lays under the pectoralis muscles
– Pulls the wing up from below by way of a “rope-andpulley” type of arrangement
19-24
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19-25
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Structural and Functional Adaptations for Flight
• 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
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19-27
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Structural and Functional Adaptations for Flight
Food, Feeding and Digestion
• Insect Eaters
– The first 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
19-28
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Structural and Functional Adaptations for Flight
• Other Diets
– 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
19-29
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Figure 19_09
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Structural and Functional Adaptations for Flight
Have rapid and efficient digestive systems
• A shrike can digest a mouse in 3 hours
– A thrush will pass berries through the tract in just 30 minutes
– Because birds lack teeth
• Foods that require grinding are cut apart in the gizzard
– Has a slender horn-covered tongue
– Few taste buds
– A long, muscular esophagus extends from pharynx to
stomach
– 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 fluid
19-31
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Structural and Functional Adaptations for Flight
– Stomach consists of
• Proventriculus
– Secretes gastric juice
• Gizzard
– Grinds food
– Birds may swallow pebbles or grit to assist grinding in
gizzard
– Birds of prey such as owls
• Form a pellet of indigestible material in the proventriculus and
eject it through the mouth
– Paired ceca at the junction of the intestine and rectum
• Serve as fermentation chambers
– End of the digestive system is the cloaca
• Also receives products from genital ducts and ureters
19-32
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Structural and Functional Adaptations for Flight
Circulatory System
• 4-chambered heart is large, with strong ventricular
walls
• Share with mammals a complete separation of
respiratory and systemic circulations
• Heartbeat relatively fast compared to mammals and
inversely proportional to size
– 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
19-33
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Structural and Functional Adaptations for Flight
Respiratory System
• Differs radically from lungs of nonavian reptiles and
mammals
• Bird Lungs
– Finest branches of the bronchi do not terminate in alveoli
but are tube-like parabronchi
– Air sacs
• Extend into thorax, abdomen, and long bones
– Large portion of air bypasses lungs and flows directly into
posterior air sacs
– On expiration, oxygenated air flows through lungs and
collected in the anterior air sacs
• Continuous air flow
– Takes 2 respiratory cycles for a single breath of air to pass
through system
19-34
– Most efficient respiratory system of any vertebrate
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19-35
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Structural and Functional Adaptations for Flight
Excretory System
• Pair of large metanephric kidneys is composed of
many thousands of nephrons
- In shelled eggs, all excretory products remain within the
eggshell
– Uric acid is stored harmlessly
– Uric acid has low solubility
• 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.
• Avian kidneys concentrate solutes only a little greater
than the blood concentration
19-36
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Structural and Functional Adaptations for Flight
• Marine birds excrete larger salt loads due diet and
seawater they drink
– Salt glands located above each eye excrete highly
concentrated solutions of sodium chloride
– Salt solution runs out the nostrils
– Gulls and other sea birds have a perpetual “runny nose”
19-37
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Figure 19_11
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Structural and Functional Adaptations for Flight
Nervous and Sensory Systems
• 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 optic lobe
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Structural and Functional Adaptations for Flight
• Cerebellum is where muscle-position sense,
equilibrium sense and visual cues are assembled
• Optic lobes bulge to each side of midbrain and form a
visual apparatus comparable to the visual cortex
• Sense of smell is poorly developed except in flightless
birds, oceanic birds, and waterfowl
• Have good hearing and superb vision
– Keenest in the animal kingdom
19-40
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Structural and Functional Adaptations for Flight
– Cochlea-organ of hearing
• 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
• Eye resembles that of other vertebrates in gross
structure, but it is larger for a relative to body size
– Less spherical and almost immobile
• Bird turns its head rather than eyes
– Light sensitive retina has both rods and cones
• Diurnal birds have more cones
• Nocturnal birds have more rods
– A pecten is a highly vascularized organ attached to the
retina
• Juts into the vitreous humor
• May provide oxygen and nutrients to eye
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19-43
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Structural and Functional Adaptations for Flight
• Many birds have two foveae or regions of detailed vision
– 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
– Can see flower nectar guides
19-44
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Structural and Functional Adaptations for Flight
Flight
– 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
19-45
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Structural and Functional Adaptations for Flight
• Bird Wing as a Lift Device
– Wing is streamlined with a concave lower surface
(cambered)
– Leading edge of the wing has small tight-fitting feathers
– Over two-thirds of the total lift comes from negative
pressure from the airstream flowing a longer distance over
the top of the wing, the convex surface
19-46
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Structural and Functional Adaptations for Flight
– Lift-to-drag ratio
• Determined by angle of attack and airspeed
– At high speeds, sufficient lift is generated so that wing is
held at a low angle of attack, creating less drag
– When the angle of attack becomes too steep and stalling
occurs
– Stalling is delayed or prevented by a wing slot along the
leading edge to direct rapidly moving air across the leading
surface
– In some birds the alula, or group of small feathers on the
“thumb,” provides a midwing slot
19-47
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Figure 19_13
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Structural and Functional Adaptations for Flight
• Flapping Flight
–
–
–
–
–
Requires a vertical lifting force and a horizontal thrusting force
Primary feathers at the wing tips provide most of the thrust
Lift is provided by the secondaries
Greatest power is provided by downstroke
Primary feathers are bent upward and twist to a steep angle of
attack
– On the upstroke, the primary feathers bend in the opposite
direction so that upper surfaces twist to produce thrust
– Powered upstroke is essential for hovering and fast, steep
takeoffs
19-49
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19-50
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19-51
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Structural and Functional Adaptations for Flight
• Basic Forms of Bird Wings
– Elliptical Wings
• Birds that must maneuver in forested habitats have
elliptical wings
• Has a low-aspect ratio
• Elliptical wings have both an alula and slotting between
primary feathers to prevent stalling at low speeds, etc.
• The small chickadee can change its course within 0.03
seconds
19-52
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Structural and Functional Adaptations for Flight
– High-Aspect Ratio
• Birds that feed on the wing or make long migrations
have high-speed wings
• These wings sweep back and taper to a slender tip
• Reduces “tip vortex” turbulence
19-53
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Structural and Functional Adaptations for Flight
– Dynamic Soaring Wings
• Albatrosses, gannets and other oceanic soaring birds
have wings with long, narrow wings
• The high-aspect ratio of long, narrow wings lack wing
slots and allow high speed, high lift and dynamic
soaring
• These birds exploit the highly reliable sea winds and air
currents of different velocities
19-54
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Structural and Functional Adaptations for Flight
– High-Lift Wings
• Vultures, hawks, eagles, owls and other birds of prey
that carry heavy loads have wings with slotting, alulas
and pronounced camber
• Produces high lift at slow speed
• Many are land soarers
– broad, slotted wings allow sensitive response for static soaring
19-55
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19-56
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Migration and Navigation
Migration Routes
• Most migratory birds follow established north-south
routes
• Some use different routes in the fall and spring
• Some aquatic species make rapid journeys
• Others such as warblers take 50–60 days to migrate
• The Arctic tern circles from North America to
coastlines of Europe and Africa to winter quarters, a
total of 18,000 kilometers (11,200 miles)
19-57
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19-58
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Migration and Navigation
Stimulus for Migration
• 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
– Gonadal growth, fat deposition, migration, courtship,
mating behavior, and care of young
19-59
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Migration and Navigation
Direction Finding in Migration
• 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
19-60
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19-61
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Migration and Navigation
• Sun-compass navigation
– 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
– These experiments suggest use of the North Star as an axis at
night
• Migration involves a combination of environmental and
innate cues
• Natural selection culls individuals that make errors
– Only the best navigators leave offspring
19-62
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Social Behavior and Reproduction
• 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
19-63
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Social Behavior and Reproduction
• 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
19-64
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Social Behavior and Reproduction
• Pelicans use organized cooperative behavior to feed
• Organized social interactions of birds are most
noticeable during breeding season
– They stake out territory, select mates, build nests, incubate
and hatch eggs, and rear young
19-65
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Figure 19_19
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Social Behavior and Reproduction
Reproductive System
• Testes are very small until the approach of the
breeding season
– May then enlarge 300 times
• Males of most species lack a penis
– Mating involves bringing cloacal surfaces in
contact
• In most birds, left ovary and oviduct develop and
right ovary and oviduct degenerate
19-67
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19-68
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Social Behavior and Reproduction
• Fertilization takes place in the upper oviduct
• 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
• Sperm remain alive in the oviduct for many days
after a single mating
19-69
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19-70
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Social Behavior and Reproduction
Mating Systems
• Over 90% of bird species are monogamous
– Only mate with one partner each breeding season
– In a few species, such as swans and geese, partners are chosen for life
• Recent DNA analyses have shown many species of songbirds
frequently are “unfaithful,” engaging in extra-pair copulations
– Nests of many of these species may contain 30% of young with fathers
other than attendant male
• In monogamous birds, both male and females are equally
adept at most aspects of parental care
19-71
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Social Behavior and Reproduction
• Some birds are polygamous
– Individuals mate with two or more partners each breeding
season
– Polygyny
• Most common form of polygamy
• One male mates with many females
– Male grouse collect at a lek (collective display ground) where
each has a small territory
• Vigorously defended
– The male grouse does not care for young
– Competition for females is intense and females appear to
choose the dominant male for mating
19-72
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Figure 19_22
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Social Behavior and Reproduction
• 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
19-74
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Social Behavior and Reproduction
Nesting and Care of Young
• Nearly all birds lay eggs that must be incubated by
one or both parents
• Often the female performs most of the duties of
incubation
19-75
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19-76
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Social Behavior and Reproduction
• 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
• Cuckoos and cowbirds are nest parasite
– Lay eggs in other bird’s nests
• Altricial birds are naked and helpless at birth and
must be fed in the nest for a week or more
• Precocial birds are covered with down and run or
swim as soon as they are hatched
19-77
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19-78
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Humans and Bird Populations
• Since the dodo went extinct in 1695
– More than 140 bird species since 1681 have also become
extinct due to human influence
• Causes of bird extinction include habitat destruction
and hunting
• Modern hunting interests have helped recover
wetlands
– No legally hunted birds are endangered
19-79
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19-80
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Figure 19_26
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Humans and Bird Populations
• Recent Decline of Songbirds
– 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
– Some birds such as robins, sparrows and starlings can
accommodate these changes
– Some species are adversely affected by deforestation
19-82
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19-83