Diversity of Birds

Characteristics of Birds

  • bipedal (two-legged)
  • vertebrates (have backbones)
  • have feathers (evolved from specially modified scales)
    • dead structures, must be replaced constantly
    • necessary for temp regulation and flight
  • high body temperatures (insulated by feathers)
    • 40o - 44oC (humans are at 37oC)
  • have bills
    • toothless, covered in a horny sheath
    • teeth would require a heavy jawbone - modified for flight
    • variety of form and function
  •   - modified digestive system
    • gizzard - large, strong, muscular structure for grinding food
      • replaces molars!
      • in granivores, has folded surfaces - even some tooth-like projections
      • may contain grit to help with grinding
  • modified skeleton
    •      - bones hollow, spongy, with support struts
    • some bones are fused to reinforce the skeleton
    • ribs have overlapping projections to support the body
    • wing is a modified arm/hand
  • center of balance between feet by equal length of two main leg bones
    • loons and other diving birds modify this - exchange balance on land for powerful swimming
  • perching birds have feet that grip branches tightly
    • foot tendons attach to back of ankle joint
      • when bird squats, toes grip and lock around branch
      • when bird rises, toes open
      • allows birds to sleep while perching
    • opposable rear toe - hallux
  • four-chambered heart (like mammals)
  • efficient lungs
  • lay eggs - no bird species bears live young
    • eggs and young are cared for
    • most birds form monogamous bonds (some for life), although adultery is common - clutches of eggs may have mixed parentage.
  • large, well-developed brains with left and right hemispheres having different specializations
    • left side: learning & developing songs/calls
    • demonstrate complex motor skills, social behavior, communication
  • syrinx - sound-producing structure - much more complex than larynx
  • can naviagate by magnetic fields, celestial cues, polarized light
  • can see into the ultraviolet spectrum
  • can hear sounds below the range of human hearing


Diversity of Birds

There are about 300 billion birds currently on the planet.  They are classified into 30 orders, 193 families, 2099 genera, and around 9700 species.

Adaptive radiation is the process by which new species arise in response to changes in the environment or changes in the behavior of a group of individuals.

  • bill size/shape adapts to type of food eaten
  • leg length adapts to habits of perching or walking
  • wing shapes change in relation to flight patterns

Bird bills are the primary adaptation for feeding.  Diet determines and is determined by the size, shape, and strength of the bill.

  • raptor bills tear flesh
  • duck bills strain food from mud
  • woodpecker bills dig into trees to find insects

Why are there few bird browsers/grazers?  In most places in the world, these niches have been filled by mammals (bison, antelope, deer, moose, etc.).  Also, grazing requires a long, complex digestive tract the would weigh down the bird and inhibit flight.  An example of a bird grazer would be the moa.

Cranial kinesis is the term used to describe a bird's ability to bend/flex the upper half of the bill (maxilla).  The maxilla is hollow, but reinforced by bony struts, called trabeculae.  This allows the bill to be strong but lightweight.  Both halves of the bill are covered in a horny sheath (rhamphotheca).

Locomotion is determined by wing, leg, and toe structures.

  • long, narrow wings for soaring
  • short, round wings for maneuvering
  • flipper-like wings for swimming
  • long legs for wading/running
  • short legs for perching
  • long toes for walking on soft surfaces
  • lobed/webbed toes for swimming
  • claws for climbing

Natural Selection is the process by which individuals with advantageous traits have a higher rate of survival and reproducing than individuals with disadvantageous traits.  (Camoflaged chicks are more likely to grow up and have kids of their own than boldly colored chicks.)

Convergence is the process by which unrelated species evolve similar appearances and behaviors.  (Penguins in the southern hemisphere, auks in the northern.)

Biogeography is the study of geographical distributions of plants and animals.  This divides the Earth into six major regions:

  • Nearctic - North America
  • Neotropical - Central & South America
  • Palearctic - Europe & Asia
  • Ethiopian - Africa (sub-Sahara)
  • Australasian - Australia & New Guinea
  • Oriental - Southeast Asia

Each region has its own set of characteristic bird species (avifauna).