What is a bird ?

  • Birds are endothermic reptile-like animals with an outer covering of feathers, two legs used for walking or perching, and front limbs modified into wings that usually do not have useful claws
  • Most important characteristic that separates birds from reptiles is feathers
  • Many feathers are hollow, light and strong
  • Contour feathers are large feathers that cover a bird’s body and wings
  • Certain contour feathers, known as flight feathers, are long and stiff
  • Flight feathers on the wing and tail provide the lifting force and balance needed for flight
  • Both sides of the flight feather quill, grows branches called barbs
  • From the barbs, smaller structures grow called barbules
  • The hooks on a barbule catch on the hooks of nearby barbules, holding the barbs together in flat veins
  • Grooming of the feathers is called preening
  • One of the reasons why birds preen themselves after each flight is to realign any vanes whose barbules may have been split apart during use
  • General body feathers have fluffy barbules at their base and are not as long and stiff as flight feathers
  • General body feathers are usually brightly colored and help determine the shape of a bird’s body
  • Down feathers - grow underneath and between the contour feathers
  • Short, soft, and fluffy
  • These feathers trap warm air close to a bird;s body, insulating the bird
  • Baby birds are covered with down feathers for a period do time after they hatch
  • Powder feathers - important to birds that live on or in water
  • These feathers release a fine white powder that repels water and keeps it from penetrating the layer the layer of down feathers
  • Birds also produce a waterproof oil in special glands near their tail
  • When ducks, geese, or other water birds preen themselves, they rub this oil over their feathers
  • This oil makes water “roll off a duck’s back”

Evolution of Birds

  • First fossil ever found of an early bird-like animal is called Archaeopteryx and dates back from late in the Jurassic Period
  • The Archaeopteryx had teeth in its beak unlike modern birds
  • It also had toes and claws on its wings
  • Would be classified as a dinosaur except one important feature: it had well-developed feathers covering its entire body
  • A lot of controversy over which fossils are those of birds and when birds first appeared on earth because the fossils of early birds are very rare, often poorly preserved, and are very similar to many small dinosaurs
  • Birds evolved from ancient reptiles

Form and Function in Birds

  • Have feathers, wings, bones, beaks, and legs--differ dramatically among species adapted to different ways of life
  • Feeding - high metabolic rate and burn many calories just to keep warm
  • Birds need to eat large amounts of food
  • Have evolved many specialized organs and behaviors that help them feed in a variety of ways
  • Woodpecker - hears insects chewing beneath the bark of a tree, their beak is strong enough to pierce the bark which then reveals the bug beneath it
  • Vulture - featherless head is easy to be kept clean because they often put their head into the carcass of dead animals, cleanliness is important
  • Huge talons of this soaring eagle help the bird catch fish
  • Carnivorous birds catch prey in razor-sharp talons and slice them to pieces with pointed beaks
  • Insect-eating birds do everything from picking insects off leaves and branches to catching them on the fly
  • Pollen and nectar feeders, such as hummingbirds, have long, probing beaks with which they reach deep into flowers
  • Their tongues are often equipped with a brushlike structure at the tip for lapping up nectar and fruit juices
  • Fruit and seed-eating birds may have short, stout beaks or long, sharp ones depending on the fruits or seeds they commonly eat
  • Filter feeders such as ducks and flamingoes have broad beaks with strainers built into the upper and side parts of the bill
  • These birds sift through murky water to filter out plankton a mouthful at a time
  • Crop - an enlarged area of the esophagus, where food can be stored and moistened before it enters the stomach
  • In some species, the food in the crop is later regurgitated for feeding to a bird’s young
  • Gizzard - specialized muscular part of the stomach that often contains small bits of gravel swallowed by a bird
  • The muscular walls grind the gravel and food together, crushing food particles and making them easier to digest


  • Very efficient at taking in oxygen and eliminating carbon dioxide
  • High metabolic rate of birds demands an efficient gas exchange system
  • The reason for this efficiency is that bird lungs are connected at both the anterior and posterior to large air sacs in the body cavity and bones
  • The air sacs make the bird’s body more buoyant, allowing the bird to fly more easily

Internal Transport

  • Four chambered heart, and two circulatory loops
  • One half of the heart receives oxygen-poor blood from the body and pumps this blood to the lungs
  • Oxygen-rich blood returns to the other side of the heart to be pumped throughout the rest of the body
  • This dual-loop system ensures that oxygen collected by the lungs is distributed with maximum efficiency to the tissues that need it
  • Bird’s heart beats from 150 to more than 1000 beats per minute


  • Birds eliminate nitrogenous wastes by filtering them from the blood in the kidneys
  • Most water is reabsorbed in the cloaca, leaving uric acid crystals in a white paste-like form
  • Bird species that have very little source of fresh water, their diets contain larger amounts of salt than needed
  • These species have evolved special salt glands near their eyes
  • These salt glands specialize in excreting salt


  • Intelligent animals
  • The birds cerebrum, which controls such behaviors as flying, nest building, care of young, courtship, and mating, is quite large
  • The medulla and spinal cord are much like those of reptiles
  • Excellent eyesight, reflected in a pair of sizable optic lobes in the brain
  • Can see colors better than humans
  • Predatory birds fly high in the air to spot mice on the ground
  • Senses of taste and smell are not well-developed
  • Many bird species can hear quite well
  • Owls can hear mice crawling through dead leaves from a distance
  • Migratory birds can hear the pounding of waves on the shore many kilometers away
  • Some migratory birds use a magnetic sense to navigate
  • This magnetic sense acts like a built-in compass located inside the head, which responds to the Earth’s magnetic field
  • Many other migratory birds use a combination of keen eyesight, instinct, and a built-in clock to navigate by the sun and starts


  • Many and diverse species of birds travel through different environments with wings, bodies, legs, and feet adapted for various types of locomotion
  • Different shapes of wings to serve in flight
  • The air sacs used in respiration extend inside certain bones, making the bones lighter
  • These air sacs help cool off a bird’s body by getting rid of excess heat generated by the flight muscles
  • Flying birds have an adaptation that decreases the weight they carry
  • They shrink the size of their sex organs during the time the birds are not breeding
  • In strong flying birds, the chest muscles may account for as much as 30% of the animal’s mass
  • Many birds use their flying ability to migrate, or travel long distances, between summer breeding grounds in the North and winter resting grounds in the South or the tropics
  • Some birds have lost their ability to fly, such as ostriches. They spend their time walking or running on a powerful pair of hind legs
  • These birds have small wings, and have no need to minimize their mass
  • Penguins - have given up flying in favor of swimming, their wings, legs, and feet are reduced in size
  • Their feet and wings are powerful flippers which enable them to “fly” in water


  • Both male and female reproductive tracts open into the cloaca
  • In many female birds, only one side of the reproductive tract develops to minimize body weight
  • Male birds have no external reproductive organs
  • Instead, male birds press the lips of their cloacas close together to transfer sperm from male to female
  • Most birds incubate their eggs until the eggs hatch
  • Depending on the species, it could take 13 to more than 50 days til the egg will hatch
  • When the chick is ready to hatch, it uses a small egg tooth on its bill to make a hole in the shell. After it hatches, the exhausted bird collapses for a while and allows its feathers to dry
  • Within hours or a day, they walk on their own and feed themselves
  • The baby birds stay close to their mother for protection, but she rarely feeds them
  • Other newly hatched birds, such as robins and sparrows, are blind and totally helpless when they hatch
  • For days or even weeks, they sit in the nest with their mouth wide open, screaming for food
  • Some species, such as swans, mate for life
  • Others pair up briefly to mate, the female goes off on her own to find a nest, while the male continues to mate with many other females
  • Peacocks - use their brightly colored feathers to attract females and warn off other males during the breeding season
  • Male canaries and similar species sing to attract females and ward off other males

How Birds Fit into the World

  • Humans admire birds for their beauty and their ways of flight
  • Hummingbirds are pollinators for a number of temperate and tropical plants
  • Fruit eating birds disperse plant seeds when the seeds pass through their digestive tract unharmed
  • Keeps the insect population from over populating
  • People use down feathers as insulators in cold climates
  • Down feathers are used in making comforters and jackets
  • Many birds are favorite foods all over the world
  • Bird meat is a healthful source of protein in a balanced human diet
  • Birds such as chickens and turkeys have been specially bred for their meat