Big Picture

Birds are feathered, bipedal endothermic vertebrates. Birds are one of the most diverse classes of vertebrates. Bird species have an incredible array of diets and live in many different terrestrial habitats, including the Arctic and the tropics. Birds are believed to have descended from dinosaurs, one of their recent ancestors being the Deinonychus.

Key Terms

Bird: Bipedal, endothermic, tetrapod vertebrate that lays amniotic eggs and has wings and feathers.

Bipedal: Organism that walks or moves on two legs.

Crop: Pocket-like structure that stores food for digestion later.

Gizzard: Food-grinding organ in the digestive systems of birds and some other animals.

Courtship: Behavior that precedes mating and are performed with the intent of attracting a mate.

Cloaca: Body cavity that stores waste/gametes and has an opening that allows them to leave the body.

Incubation: Period of bird reproduction when one or both parents sit on, or brood, the eggs in order to keep them warm until they hatch.

Generalist: An organism that has a varied diet.

Structure & Function

Birds are tetrapods (four limbs), but they are bipedal. All have:

  • Front legs are modified and become wings
  • Have two basic types of feathers:
  • Flight feather: Stiff and waterproof feathers that aid the bird in flight
  • Down feather: Short and fine feathers that help keep the bird warm

A lightweight body is needed for the bird to fly.

  • Bones are light and are filled with air
  • Have beak instead of teeth
  • Digestive system includes crop and gizzard
  • Gizzard may contain swallowed stones that help digest food more quickly
  • Have air sacs in the lungs that store air and help meet the bird’s high oxygen demands during flight

Birds have well developed brains, especially in the region that control flight. Birds are indeed very intelligent(not bird-brained at all!).

Image credit: CK-12 Foundation, CC-BY-NC-SA 3.0

Reproduction & Development

  • Male and female birds have distinct sexual organs and undergo sexual reproduction and internal fertilization.
  • Courtship usually occurs before birds mate and reproduce
  • Sperm passes from the male bird’s cloaca into the female bird’s cloaca
  • Birds often build nests to incubate their eggs in
  • Nests can be made up of twigs, leaves, and organic matter, or they may simply be a depression in the ground
  • During the incubation period, birds often sit on the eggs, an act known as brooding
  • Hatchlings (newborn) that nest on the ground are often more mature than hatchlings of birds that nest off the ground.


Birds cont.

Classification, Evolution, Diet

  • Flightless birds: These birds are adapted for other modes of movement other than flight. For example, the ostrich is adapted for running and the penguin is adapted for swimming.
  • Flying birds: There are 29 orders of flying birds. Seven common orders are below.
  • Landfowl: Spend most of their time on the ground and tend to be relatively big in size compared to other birds. Landfowl include turkeys and chickens.
  • Waterfowl: Spend most of their time on the water and tend to be relatively big in size compared to other birds. Waterfowl include ducks and geese.
  • Shorebirds: Live near the water and tend to be strong swimmers. Shorebirds include gulls.
  • Diurnal raptors: Birds that hunt mostly hunt other vertebrates and are active during the day. Diurnal raptors include hawks and eagles.
  • Nocturnal raptors: Birds that are active and hunt during the night. Nocturnal raptors include horned owls and barn owls.
  • Parrots: Tropical birds that tend to be brightly colored. Parrots include cockatoos and parakeets.
  • Perching birds: Small birds that, as their name suggests, tend to perch on various surfaces. Perching birds include sparrows and crows.

Diurnal means active during the day, nocturnal means active during the night.

Evolution of Birds

  • The ancestors of modern birds were thought to be theropods, a group of bipedal dinosaurs
  • Deinonychus was an extinct bird relative that provided convincing evidence that birds evolved from dinosaurs. Bird-like features of Deinonychus included feathers and wings.
Image credit: Nobu Tamura and Dinoguy2,
CC-BY 2.5


  • Some birds are generalists. The picture below shows the beak of a generalist bird and those of birds with specialized feeding patterns.