Class Aves

Class Aves

Birds belong to class AvesAbout 9,700 speciesLargest class of terrestrial vertebratesAlso most recently evolved group of

vertebratesAppeared only about 150 mya

Oldest known bird fossils are species Archaeopteryx lithographica

Date from late Jurassic period, about 150 mya

Feathers clearly visible

Characteristics

7 important characteristics1. Feathers2. Wings3. Lightweight, rigid skeleton4. Endothermic metabolism5. Unique respiratory system6. Beak7. Oviparity

Feathers

Most characteristic adaptation of bird

Protect the birds body against weather and form a smooth, low friction flying surface

Made mainly of protein keratinEssential for flightInsulate against heat lossFeathers must be shed or

molted once or twice every year

Feathers

Modified scales that have 2 functions1. Provide lift for flight2. Conserve body heat Soft, fluffy down feathers cover body of

nestling birds and provide insulating undercoat of adults

Contour feathers give adult birds streamlined shape and provide coloration and additional insulation

Flight feathers are specialized contour feathers on wings and tail

Feathers

Structure combines maximum strength and minimum weight

Develop from tiny pits in skin called folliclesShaft comes out of follicleTwo vanes develop on opposite sidesAt maturity, each vane has many branches, called

barbsBarbs have many projections called barbules

with microscopic hooksHooks interlock and give feather sturdy but

flexible shape

Feathers need careIn preening birds use beaks to rub feathers

with oil secreted by preen gland located at base of tail

Birds periodically molt – shed – their feathers

In temperate climates, flight feathers replaced during late summer

Major molt

Wings

Forelimbs modified into wingsFeathers cover most of surface area

Flight involves series of complex wing movements

Each one uses different set of musclesOn downstroke – wings cut forward and

downward through air

During upstroke, they move up and backFlight muscles can make up 50% of birds

weight

Lightweight, rigid skeleton

Reflects requirements of flight

Many bones are thin-walled and hollow

Air sacs from respiratory system penetrate some bones

Many bones are fused – resist forces made by flight muscles

Skeleton and Muscles

Avian skeleton combines lightness with strength

Bones are thin and hollowMany are fused so skeleton is more rigid than

reptile or mammalProvides stability during flight

Metabolism

Rapid metabolism supplies energy for flightMaintain high body temps – 40-41°CBirds are endothermic – generate heat to

warm body internallyRapid breathing and digestion of large

quantities of food support high metabolic rateBirds can’t go long time without eatingConserve body heat by fluffing out feathersAquatic birds have thin layer of fat to give

additional insulation

Beak

No modern bird has teethJaws covered by tough horny sheath called a

beak

Unique Respiratory System

Have most efficient respiratory system of all terrestrial vertebrates

Lungs connected to several sets of air sacs

Ensures oxygen-rich air always in lungs

Respiratory System

High metabolism needs a lot of oxygenSome birds migrate thousands of miles at

high altitudes where oxygen is lowAir enters through paired nostrils at base of

beak

Air passes down trachea and enters 2 bronchi

From bronchi, air moves to lungs

75% bypasses lungs and goes directly to posterior air sacs

9 sacs extend from lungs

Gas exchange doesn’t happen in air sac

Function – to store airWhen bird exhales, CO2-

rich air from lungs forced into anterior sacs

O2-rich air forced into posterior sacs

This way, oxygenated air is in lungs during inhalation and exhalation

Digestive and Excretory Systems

High energy required gotten by quick and efficient digestive system

Birds don’t have teeth

Food passes from mouth cavity straight to esophagus

Enlargement of esophagus called crop stores and moistens food

Then passes into 2 part stomach

First chamber – proventriculus – acid and enzymes begin breaking down food

Then goes to gizzard – muscular portion of stomach

Gizzard squeezes and crushes food

Often has small stones that bird has swallowed

Helps in grindingAct like teeth

From stomach to small intestine

Bile from liver and enzymes from pancreas break down food

Nutrients absorbed into blood

Passage usually very quick

Can be in less than 1 hour (humans about 6 hours)

Excretory system is efficient and lightweight

Most do not store liquid waste in urinary bladder

Two kidneys filter uric acid from blood

Concentrated uric acid travels through ducts called ureters to cloaca

Mixed with undigested matter from intestines

Mixture of feces and uric acid

Circulatory System

Like crocodiles and mammals, have heart with 2 ventricles

Deoxygenated blood always separate from oxygenated blood

Most birds have faster heartbeat

Hummingbird – 600 times a minute

Chickadee – 1,000 times a minute

Human – about 70 (like ostrich)

Nervous System and Sense Organs

Relative to body size, birds have large brains

Most highly developed areas control flight-related functions

Ex. Cerebellum – coordinates movement

Cerebrum also large – controls complex behavior patterns like navigation, mating, nest building and caring for young

Most birds have good color vision that helps them find food

Most species have large eyes near side of head – wider field of vision

Birds that have eyes closer to front of head have better binocular vision

Higher depth perceptionHearing is important in songbirds and nocturnal

species that rely on sound to hunt preyBirds don’t have external earsOwls have feathers shaped to create channel

around ear openingsHelps direct sound into earSmell is also developed in many birds

Oviparity

All birds lay amniotic eggsEncased in hard, calcium-containing shellMost species incubate eggs in nest

Reproduction

In male bird, sperm made in 2 testes that lie anterior to kidneys

Sperm passes through tubes called vasa deferentia into cloaca

During mating, male presses cloaca to female’s cloaca and releases sperm

Most females have single ovary on left side of bodyOvary releases eggs into long funnel-shaped oviduct

where they are fertilizedFertilized eggs move down oviduct where they receive

protecting covering and shell

Egg passes out of oviduct and into cloaca, then out of bird

Nest Building and Parental Care

Nests hold eggs, conceal young birds from predators, provide shelter and sometimes used to attract mate

Most build nests in sheltered, well-hidden spots

Ex. Woodpeckers – build in hole they have drilled in a tree

One or both parents incubate eggs by sitting on them and covering them with thickened, featherless patch of skin on abdomen

Brood patch

Classification

Most taxonomists divide 10,000 species of living birds into 29 orders

Traditionally used morphological evidence from beaks, feet, plumage, bone structure, etc. to classify

Technological advances (protein, DNA, etc) also used

Order Anseriformes

Swans, geese, ducks (waterfowl)160 speciesUsually aquaticWebbed feet for paddling and swimmingFeed on variety of aquatic and terrestrial

foodsBill (beak) usually flattenedParental care usually by female

Order Strigiformes

Contains owlsHave sharp, curved beakSharp talonsLarge, forward-facing eyes that provide

improved night visionRely on keen sense of hearing to locate prey

in darkAbout 180 species

Order Apodiformes

Hummingbirds and swifts420 small, fast-flying birds

with tiny feetSwifts capture insects in

flightHummingbirds feed on

nectarSwifts worldwideHummingbirds in western

hemisphere

Order Psittaciformes

Parrots and their relatives – parakeets, budgerigars, cockatoos, cockatiels

Most of 360 species live in tropics

Parrot characterized by strong hooked beak for opening seeds and slicing fruit

Upper mandible of parrots is hinged on skull and movable

Have 2 toes that point forward and 2 that point backward – adaptation for perching and climbing

Most birds – 3 front and 1 back

They are vocal birdsMany gather in large, noisy

flockLiked as pets because of

colorful plumage and intelligence

Some species can be taught to mimic human speech

Excessive collecting for pet trade and habitat destruction now threaten many parrot species with extinction

Order Piciformes

Tree-dwelling birds including woodpeckers, honeyguides and toucans

Like parrots, have 2 forward toes and 2 backward toes

About 350 species throughout world except Australia

Diversity of food reflected in diversity of bills

Ex. Woodpecker – strong, sharp, chisel-like bill – drill holes in trees to capture insects

Ex. Toucan – long bill – pluck fruit out of trees

Order Falconiformes

Known as raptorsHave sharp, curved beak and sharp talonsIncludes ospreys, hawks, falcons, vultures,

and eagles

About 310 species distributed throughout the worldMost are diurnal (daytime) hunters with good visionVultures feed on dead animals and use sense of

smell to detect odor of decaying flesh

Order Passeriformes

About 5,700 species (about ½ the species of birds)

Common North American birdsRobins, warblers, blue jays, wrens, etc.

3 toes forward, 1 toe backwardBackward toe is enlarged and flexible to

give better grip on branches in this groupSometimes called perching birdsFeed on variety of foods

Many called songbirds b/c males make songsSing to warn away other males and attract

femalesSong produced in structure called syrinx

located at base of bird’s tracheaRegulating flow of air through syrinx can

generate range and complexity of songs

Order Columbiformes

About 310 species of pigeons and dovesUsually plump-breasted and have relatively

small headsShort necks, legs, and beaksShort, slender billsMost feed on fruit or grain

Crop secretes nutritious milk like fluid called crop milkBoth sexes produce to feed youngUsually lay clutch of 2 eggsHatch after 2 week incubation periodYoung usually leave nest 2 weeks after hatching

Order Ciconiiformes

Long-necked, long-legged birdsAbout 120 species of herons, storks, ibises,

egrets

Have long, flexible neck and long bill

Many are wading birds

Feed on fish, frogs, others in shallow water

Some members grow large

Marabou stork of Australia can be more than 1.5 m tall

Order Galliformes

Includes turkeys, pheasants, chicken, grouse, quails

Commonly called fowl

Terrestrial birdsPlump bodiedLimited flying abilityGrains are large part of dietAll species have large, strong

gizzardAbout 260 species worldwide

Order Sphenisciformes

Flightless marine birdsPenguins17 species in Southern

HemisphereWedge-shaped wings

modified into flippersFeet are webbedMost have thick coat

of insulating feathers and layer of fat beneath skin

Maintain fat layer by eating large amounts of fish and krill

Order Struthioniformes

Only the ostrichCannot flySpecialized as high-speed runnersCan reach 55 kmphEach foot has 2 toesNative to Africa