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Birds
Chapter 27
Class Aves – Birds
 Birds (class Aves) are archosaurs but almost every
feature of their reptilian anatomy has undergone
modification in their adaptation to flight.
Class Aves – Birds
 Birds are found in most every habitat from forests to
deserts, even in caves.
 Some birds dive in the ocean to 45 m to catch prey.
 Birds have visited both the North & South poles.
 The bee hummingbird of Cuba weighs 1.8 g and is one of
the smallest vertebrate endotherms.
Derived Characters of Birds
 A bird’s most obvious adaptations for flight are
its wings and feathers.
 Feathers are the feature that set birds apart
from other vertebrates.
Derived Characters of Birds
 Some other theropod dinosaurs had feathers, but they
were not capable of supporting flight.
 Insulation – provides support for the idea that some
dinosaurs were endotherms.
 Bright colors may have been used to attract mates.
Characteristics of Birds
 All birds also have hindlimbs
adapted for walking,
swimming, or perching.
 Foot structure in bird feet
shows considerable variation.
 All have keratinized beaks.
 All lay shelled amniotic eggs.
The Origin of Birds
 Birds descended from theropods – a group of small,
carnivorous dinosaurs.
 By 147 million years ago, feathered theropods had
evolved into birds.
The Origin of Birds
 Archaeopteryx
 The oldest bird known.
 Skull similar to modern birds but with thecodont
teeth.
 Wings with feathers were present.
The Origin of Birds
 Much of the skeleton was
that of a theropod
dinosaur.
 Long bony tail
 Clawed fingers
 Abdominal ribs
 S-shaped, mobile neck
 This fossil demonstrated
the connection between
theropods & birds.
The Origin
of Birds
 Archaeopteryx
arose from the
theropod lineage.
 Closely related to
Dromaeosaurs.
 More shared
derived
characters.
 Many had
feathers used for
insulation and/or
social display.
Living Birds
 The ratites, superorder
Paleognathae, are all
flightless.
 Primitive archosaur
palate.
 Ostriches, emus, rheas,
kiwis, tinamous.
 Flat sternum, poorly
developed pectoral
muscles.
Living Birds
 All other birds are superorder
Neognathae, having a flexible
palate.
 The demands of flight have
rendered the general body form
of many flying birds similar to
one another.
 Flying birds have a keeled
sternum with well developed
pectoral muscles.
Living Birds
 Flightlessness has
evolved in many
groups of birds.
 Penguins (use wings
to swim through
water).
 Many fossil forms
including flightless
owls, pigeons,
parrots, cranes,
ducks, & auks.
 Usually occurs on
islands with few
predators.
Form & Function – Feathers
 Feathers are lightweight,
yet tough, consisting of:
 A hollow quill emerges
from the skin.
 This becomes the shaft
which bears numerous
barbs that form a flat,
webbed surface, the
vane.
 Each barb contains many
barbules.
Form & Function – Feathers
 Contour feathers are vaned
feathers that cover and
streamline a bird’s body.
 Called flight feathers if they
extend beyond the body.
 Down feathers are soft and
have no hooks on barbules.
 Filoplume feathers are hairlike – function unknown.
 Powder-down feathers
disintegrate as they grow,
releasing powder that aids in
waterproofing.
Form & Function – Feathers
 Feathers are homologous to reptiles’ scales.
 It develops from an epidermal elevation overlying a
nourishing dermal core.
 In reptiles, this elevation flattens into a scale.
 In birds, it rolls into a cylinder and sinks into the follicle
from which it will grow.
Form & Function – Feathers
 As a feather nears the
end of its growth,
keratin is deposited to
make some of the
structures hard.
 The protective sheath
surrounding the new
feather splits open,
and the feather
unfurls.
Form & Function – Feathers
 When fully grown, feathers are dead – like mammalian
hair.
 Birds molt to replace worn out feathers.
 Usually feathers are discarded gradually to avoid bare
spots.
 Flight feathers & tail feathers are lost in pairs to maintain
balance.
 Many water birds lose all their primary feathers at once
and are grounded during the molt.
Form & Function – Feathers
 Colors in birds may be pigmentary or structural.
 Red, orange, & yellow are colored by pigments called
lipochromes.
 Black, brown, & gray are produced by the pigment
melanin.
 Blue is created structurally by the scattering of shorter
wavelengths of light by particles within the feather.
Form & Function – Skeleton
 A light, yet still strong skeleton is a requirement for
flight.
 Bird bones are laced with air cavities.
Form & Function – Skeleton
 Birds are archosaurs, and had ancestors with diapsid
skulls.
 Bird skulls are highly specialized – mostly fused into one
piece.
 Leg bones in birds are heavier – this helps lower the
center of gravity giving aerodynamic stability.
Form & Function – Skeleton
 Modern birds are toothless.
 Instead they have a keratinized beak.
 Most birds have kinetic skulls.
 They have a wide gape.
 Upper jaw is attached loosely increasing the gape.
Form & Function – Skeleton
 All birds that can fly
have a large, thin
keel on their
sternum that
provides area for the
large flight muscles
to attach.
Food & Feeding
 Early birds were carnivorous, feeding mostly on
insects.
 Many birds are still insectivores.
 Other foods include nectar, seeds, berries, worms,
crustaceans, molluscs, fish, frogs, small birds &
mammals.
Food & Feeding
 Some birds are generalists, feeding on a wide range
of food items.
 Perhaps more competition for food, but less danger of
something happening to the food source.
 Others are specialists, only feeding on one type of
food.
 Less competition, more danger of losing the food source.
Food & Feeding
 The beaks of birds are strongly adapted to
specialized food habits.
Digestion
 At the end of the esophagus of many birds is the crop.
 Used for storage.
Digestion
 The stomach has two compartments:
 The first secretes gastric juices.
 The second, the gizzard, is lined with keratinized plates
that serve as millstones for grinding food.
 Birds swallow small stones to help this process.
Digestion
 Owls can’t digest the bones & fur or feathers of their
prey.
 These materials are bundled together and ejected
through the mouth.
 Owl pellets can be used to determine what the owls in a
particular area have been eating.
Circulatory System
 Birds have a four-chambered heart.
 Separate systemic and respiratory circulations.
 Fast heartbeat – faster in smaller birds.
 Red blood cells are nucleated and biconvex.
 Mammals are enucleated and biconcave.
Respiratory System
 The highly adapted respiratory system of birds is
adapted for the high metabolic demands of flight.
 The finest branches of the bronchi are developed as
tubelike parabronchi through which air can flow
continuously – instead of ending in saclike alveoli as in
mammals.
Respiratory System
 There is an
extensive system
of nine
interconnecting air
sacs that connect
to the lungs.
 Air flows to the
posterior air sacs,
to the lung, then to
the anterior air
sacs and out.
http://youtu.be/kWMmyVu1ueY
Respiratory System
 The result is that there is an almost continuous stream
of oxygenated air passing through the highly
vascularized parabronchi.
Excretory System
 Urine is formed in large, paired metanephric kidneys.
 There is no urinary bladder.
 Nitrogenous wastes are secreted as uric acid rather than
urea.
 Bird kidneys can only concentrate solutes to 4-8 times
that of blood concentration.
Excretory System
 Some birds,
including marine
birds, have a salt
gland to help rid the
body of excess salts.
 Salt solution is
excreted from the
nostrils.
Nervous System
 Birds have well
developed cerebral
hemispheres,
cerebellum
(important for
coordinating
movement &
balance), and optic
lobes.
Senses
 Birds usually have poor sense of smell & taste.
 Some, carnivores, waterfowl, flightless birds have well
developed sense of smell & taste.
 Birds have the keenest eyesight in the animal kingdom
and also very good hearing.
 A hawk can clearly see a crouching rabbit a mile away!
Flight
 To fly, birds must generate lift forces greater than their
own mass and they must provide propulsion to move
forward.
 Bird wings are designed to provide lift.
Flight - Wings are Specialized for
Particular Kinds of Flight
 Elliptical wings are good for maneuvering in forests.
 High speed wings are used by birds that feed during flight
or that make long migrations.
 Dynamic soaring wings are used by oceanic birds that
exploit the reliable sea winds.
 High lift wings are found in predators that carry heavy
loads. Soaring over land with variable air currents.
Migration
 Many species of birds
undergo long
migrations using well
established routes.
 Some species make
the trip quickly, others
stop along the way to
feed.
 Often, they follow
landmarks such as
rivers and coastlines.
Migration
 The stimulus for
migration has to do
with changing
hormone levels
brought about by a
change in day length.
Migration
 Birds navigate using a number of cues:




Visual cues – landmarks.
Accurate sense of time.
Some may use the Earth’s magnetic field.
Celestial cues – sun by day, stars at night.
Social Behavior – Mating Systems
 Two types of mating
systems found in
birds:
 Monogamy where
an individual has
one mate.
 Rare in animals,
common in birds.
 Seasonal or lifelong
Social Behavior – Mating Systems
 Birds have a high incidence of monogamy because
both parents are equally able to perform most aspects
of parental care.
 Often success of the hatchlings requires care from two
parents.
Social Behavior – Mating Systems
 Polygamy where an individual has more than one
mate during a breeding season.
 Polygyny – one male, many females
 Polyandry – one female, many males
Social Behavior – Mating Systems
 The most common
form of polygamy in
birds is polygyny.
 In some species,
such as grouse,
males gather in a
display area or lek.
Each male defends
part of the lek and
displays for the
females.
 Only females care for
young.
http://www.youtube.com/watch?v=s2_wdMmEupQ
Social Behavior – Mating Systems
 An example of polyandry occurs in spotted
sandpipers.
 Females defend territories and mate with several males.
 Each male incubates a nest of eggs in the female’s
territory and does most of the parental care.
 This system may have evolved in response to high
predation rates.
Nesting
 Most birds build nests
in which to lay eggs.
 Often great care is
taken to hide the nest,
or make it inaccessible
to predators.
 When the young hatch,
they usually must be
fed by one or both
parents.
Nesting
 Precocial young, such as
ducks, water birds, fowl
and quail are covered with
down when they hatch
and can run or swim as
soon as their down dries.
 Most precocial young
must still be cared for by
the parents for a time.
Nesting
 Altricial young are naked and
unable to see or walk at
hatching.
 They must remain in the nest
for a week or more.
 Parents must spend lots of
time & energy bringing food to
hatchlings.
 There is a continuum with the
young of many species falling
in between the two extremes.