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Well adapted to marine, freshwater, & terrestrial habitats
Bodies adapted for flight
Endothermic – body temperature controlled by metabolism


  • Evolved from reptiles
  • Few fossils due to lack of preservation of feathers or thin, hollow bones
  • Archaeopteryx:
    1. Possible link between birds & reptiles
    2. Lived during Jurassic period
    3. Large skull with reptile like teeth
    4. Bones not hollow
    5. Claws on forelimbs
    6. Long tail
    7. Strong legs & rounded wings for gliding
    8. Feathers
    9. Furculum – fused collarbone or wishbone

Archaeopteryx Fossil

  • Hesperonis:
    1. Bird fossils from Cretaceous period
    2. Large, flightless bird
    3. Had teeth like reptiles

kish-02.jpg (71663 bytes)

  • Ichthyornis:
    1. Smaller, tern like bird
    2. Lived during Cretaceous period
    3. Had large flight wings

Characteristics of Birds:

  • Body covered with feathers made of protein called keratin
  • Thin, hollow bones
  • Some bones fused for extra strength
  • Forelimbs modified into wings for  flight
  • Two hind limbs with claws to support upright body
  • Scales on legs
  • Toothless, horny beak
  • Additional air sacs  with lungs for more oxygen
  • Endotherms (40 to 41 degrees Celsius body temperature)
  • Four chambered heart with single, right aortic arch
  • Amniote egg with calcium carbonate shell
  • Oviparity with both parents often caring for eggs
  • Eggs usually incubated within a nest


  • Modified scales
  • Function to provide lift for flight & help conserve body heat
  • Five kinds of feathers —– down, contour, flight, filoplume, & bristles

Types of Feathers

  • Down feathers:
    1. Soft & fluffy
    2. Cover the body of nestlings
    3. Provide an undercoat insulating adult birds
  • Contour Feathers:
    1. Give streamline shape to body
    2. Provide coloration to adult birds
    3. Give additional insulation to body
  • Flight Feathers:
    1. Specialized contour feathers
    2. Found on wings & tail

  • Filoplumes:
    1. Called pin feathers
    2. Hairlike feathers under contour feathers on body

Parts of a feather:

  • Develop from tiny pits in the skin called follicles
  • Shaft emerges from the follicle
  • Two vanes develop on either side of shaft
  • Barbs branch off of each vane & have projections called barbules
  • Barbules have microscopic hooks to hold barbules together

Parts of a Flight Feather

Microscopic Hooks on Barbules

  • Birds preen their feathers to clean them & coat them with oil
  • Preen glands – oil glands located at the base of the tail
  • Birds shed or molt feathers periodically:
    1. Molting usually in late summer between breeding & migration
    2. Flight feathers replaced
    3. Some birds molt before courtship

Beaks and Feet:

  • Adapted to habitat & feeding
  • Hawks & eagles have hooked beaks & talons for tearing meat


Gentoo Penguin
TalonsHooked BeakPenguin Flippers


  • Swifts have tiny beaks that open wide to catch insects in midair
  • Flightless birds like ostriches have legs & feet modified for running & walking
  • Penguins have wings modified into flippers for swimming
  • Ducks & geese with webbed feet


Running Legs of OstrichWebbed Feet on Duck


  • Legs of some birds such as herons &  egrets turn vivid colors to attract mates; caused by hormones

Skeleton and Muscles:

  • Pelvic & pectoral girdles fused for strength
  • Bones thin & hollow so bird lighter

A birdbone(notice the honey combed shape)
Hollow Bones

  • Furculum or wishbone is a fused collarbone that stabilizes bird in flight
  • Lighter beak replaces heavy teeth & jaws
  • Lower vertebrae fused so no heavy ligaments needed
  • Enlarged eye sockets reduce skull weight
  • Keeled sternum for attachment of large flight muscles
  • Pygostyle – terminal vertebrae support tail & aids in flight (lift, steering, & braking)
  • Two digits in forelimbs lost & other three digits fused to form wings
  • Wings shaped like air foils (thicker in front & tapering to back) so air moves faster on top causing lift

  • Powerful muscles make up 50% of body weight
  • each wing movement uses different set of muscles
  • Flight muscles called pectorals & are attached to wing & keeled sternum
  • When large pectorals contract, wings move down
  • When large pectorals relax & small pectorals contract, wings move upward

Body Temperature:

  • Metabolism generates body heat (endothermic)
  • Enables birds to survive in warm & cold environments
  • Rapid breathing & increased air sacs in lungs bring in more oxygen

Diagram of a bird's lung and air sac system, and countercurrent exchange
Air Sacs in Bird Lungs

  • Ingest large amounts of food for energy
  • Fluff out feathers to trap air for insulation
  • Aquatic birds have thin layer of fat for insulation

Digestive System:

  • Fast & efficient digestion (mouse digested in 3 hours)
  • No chewing
  • Crop for temporary food storage
  • Two part stomach — proventriculus & gizzard
  • Proventriculus is 1st chamber where digestive juices added
  • Gizzard is 2nd part for crushing food
  • Small stones & gravel eaten by birds aids grinding in gizzard
  • Pyloric sphincter valve at lower end of gizzard controls food movement into intestines
  • Duodenum – beginning of small intestine where bile (digests fats) & pancreatic juice are added & digested food is absorbed

birdanat.gif (87464 bytes)

Excretory System:

  • Paired kidneys filter nitrogen wastes (uric acid) from blood
  • No urinary bladder to store liquid wastes
  • Uric acid travels down ureters to cloaca where intestinal wastes & reproductive products added
  • Uric acid secreted in white, semi solid mass
  • Shorebirds have salt secreting glands above the eyes & secrete excess salt through their nostrils

Respiratory System:

  • Fly at high altitudes where there is less oxygen so need efficient respiratory system
  • High metabolic rate requires large amount of oxygen
  • Nine air sacs associated with lungs increase oxygen level & decrease density
  • Air sacs connected to air spaces in hollow bones
  • One way flow of air in lungs & air sacs so more oxygen is removed
  • Air pathway:
    air enters body through nostrils on beak  trachea (windpipe) syrinx (voice box) 2 primary bronchi 75% of air into two posterior air sacs and 25% of air into lungs air from lungs into other seven air sacs
  • When carbon dioxide exhaled, oxygen from posterior air sacs moves into lungs to always keep fresh oxygen supply

Circulatory System:

  • Four chambered heart
  • Right side of heart pumps deoxygenated blood from body cells to lungs
  • Left side of heart receives oxygenated blood from lungs & pumps it to the body cells
  • Single aortic arch
  • Rapid heartbeat (hummingbird 600X/minute & chickadee 1000X/minute)
  • Less active birds such as ostrich have slower heart rates (70X/minute)

Nervous System:

  • Large brains relative to size of bird
  • Most highly developed brain areas control flight
  • Cerebellum coordinates movement
  • Cerebrum controls navigation, mating, nest building, & care of young
  • Optic lobes receive & interpret visual stimuli
  • Keen vision
  • Have color vision for locating food
  • Large eyes located on side of head for wide field of vision in most birds
  • Some birds such as owls with eyes on front of head for binocular vision (depth perception)
  • No external ears, but have feathers around ear openings to direct sounds into ear canals
  • Tympanic membrane or eardrum for picking up sound vibrations
  • Semicircular canals in inner ear regulate balance
  • Poorly developed sense of smell except in ducks & flightless birds
  • Sense of taste helps avoid bitter tasting or toxic foods

Reproductive System:

  • Testes in males produces sperm that travels by the vas deferens to cloaca
  • Females have single ovary that makes eggs
  • Eggs are fertilized in the oviducts
  • Shell added by shell gland & then egg moves into
  • In mating, male presses cloaca to female to transfer sperm (internal fertilization)
  • Lay an amniote egg:
    1. Embryo suspended in fluid called albumen (white of egg)
    2. Chalaza – rope like strands suspending embryo in albumen
    3. Chorion is membrane inside of shell
    4. Yolk is stored food surrounded by yolk sac

Bird Egg

Incubation & development of Egg:

  • Eggs incubated by one or both parents
  • Brood patch – thickened, featherless patch of skin on abdomen of bird used to warm eggs
  • Membranes grow out of embryo’s digestive tract & surround yolk
  • Membranes make digestive enzymes to dissolve proteins & lipids in yolk
  • Yolk sac has blood vessels to carry food to embryo
  •  Wastes from embryo collect in membrane called allantois
  • Chorion membrane lines the shell & allows gas exchange
  • Young birds may be precocial or altricial
  • Precocial young:
    1. Have longer incubations
    2. More eggs laid
    3. Active as soon as hatch
    4. Nestlings can swim, walk, & feed themselves
    5. Need some parental care
    6. Includes ducks, geese, & swans
  • Altricial young:
    1.Lay fewer eggs
    2. Hatch quickly
    3. Hatchlings are blind, naked, & helpless
    4. Depend on parents for warmth & food for several weeks
    5. Includes songbirds, woodpeckers, hawks, pigeons, doves, raptors


Dunnock & Cuckoo
Altricial YoungPrecocial Young



  • Longer parental care allows more complex learning (courtship, nesting, migration, etc.)
  • Territoriality allows males to establish & defend breeding areas
  • Courtship behaviors are used by males to attract mates:
    1. Brightly colored feathers
    2. Flight displays
    3. Songs

Male Scarlet Tanager Breeding Plumage

  • Nest building holds eggs, conceals & shelters young birds, may help attract mates
  • Nests are built in sheltered, well-hidden spots in trees, on the ground, etc. & are made of twigs, mud, grass, feathers…

  • Migration to new areas is triggered by dropping temperatures & dwindling food supplies
  • Birds use migration clues including:
    1. Position of sun & stars
    2. Topographical landmarks
    3. Magnetic clues
    4.Air pressure changes
    5. Low frequency sounds


Section 2 Review


  • Class Aves
  • 27 orders
  • Gaviiformes – loons
  • Pelecaniformes – pelicans & cormorants
  • Ciconiiformes – wading birds like ibises & herons
  • Anseriformes – ducks, geese, & swans
  • Falconiformes – falcons, eagles, hawks, vultures
  • Galliformes – turkey, quail, pheasants
  • Gruiformes – cranes, coots, & rails
  • Charadriiformes – snipes, sandpipers, gulls, terns
  • Columbiformes – pigeons & doves
  • Psittaciformes – parrots, parakeets, & macaws
  • Cucluiformes – cuckoos & roadrunners
  • Strigiformes – owls
  • Caprimulgiformes – whippoorwill & nighthawk
  • Apodiformes – hummingbird & swifts
  • Coraciiformes – kingfishers
  • Piciformes – woodpeckers, sapsuckers, & flickers
  • Passeriformes – perching birds like robins, cardinals, blue jays


Pelican at Oranjestad waterfront
Pygmy OwlBrown Pelican
photograph of macawFemale Northern Cardinal Photograph
MacawFemale Cardinal


Food & Habitat Adaptations:

  • Anseriformes (ducks, geese, & swans) have webbed feet for swimming & flattened bills; young are precocial but need some parental care
  • Strigiformes (owls) have sharp, hooked beaks & talons (claws) for meat eating, keen hearing & eyesight, & forward facing eyes
  • Apodiformes (hummingbirds) are small, fast-flying birds with tiny feet & long tongues for drinking nectar; found only in western hemisphere
  • Psittaciformes (parrots, cockatoos, parakeets…) have a strong, hooked beak for seed opening & two forward & two rear facing toes for perching & climbing
  • Piciformes (woodpeckers, toucans, & flickers) have two rear facing toes for dwelling in tree cavities & sharp, chisel like bills for drilling into trees
  • Falconiformes or raptors ( hawks, eagles, vultures) have hooked beaks & talons & keen vision for seeing prey
  • Passeriformes or songbirds (blue jays, cardinals, sparrows, robins …) have enlarged rear facing toe to grip branches, a syrinx or voice box in males to produce songs, & a variety of beak shapes to feed on seeds, nectar, fruits, & insects; known as passerines or perching birds
  • Columbiformes (pigeons & doves) have small heads & bills, a crop that makes “pigeon’s milk” for feeding young, short incubation period (2 weeks)
  •  Ciconiiformes (herons, ibises, & egrets) have long legs for wading & sharp pointed bills for piercing frogs & fish
  • Galliformes (turkeys, quail, pheasants, & chickens)  have plump bodies with limited flying &a large gizzard for grinding grains
  • Sphenisciformes (penguins) have wings modified into flippers, an extra layer of body fat for insulation, & webbed feet for swimming
  • Struthioniformes (ostrich) are the largest birds that can’t fly but have long legs with only two toes adapted for fast running
Section 3 Review