Chapter 8

• Birds


  • Vertebrates with feathers
  • Modified for flight
  • Active metabolism
• Birds are a monophyletic lineage, evolved once from a common ancestor, and all birds are related through that common origin
• Some birds don't fly, but their ancestors did, and these birds have secondarily lost the ability to fly
• Modern bird traits related to hot metabolism and flight

  • Horny beak, no teeth
  • Large muscular stomach
  • Feathers
  • Large yolked, hard-shelled eggs
  • Parent bird provides extensive care of the young
  • Strong skeleton

  
• Birds


  • Amniote homeothermic vertebrates
  • Skin with feathers
  • Forelimbs wings, with three fused fingers in hand
  • Hindlimbs legs, each with four or fewer toes
  • Horny beak, teeth absent
  • Heart four-chambered, single aortic arch on right
  • Lungs with extended air pouches
  • Syrinx at base of trachea
  • 12 pairs of cranial nerves
  • Pelvis fused to sacrum, pubic bones typically not fused ventrally
  • Without external ear lobes
  • Without external genitalia or urinary bladder
  • Urine semisolid
  • Endothermal
  • Fertilization internal
  • Eggs amniote, oviparous
• Pigeon (Columba livia)


  • Skin very thin, glandless except for uropygial gland
  • Eyes surrounded by three eyelids
  • Eyes rimmed with bony sclerotic plates
  • External auditory aperture as circular depression
  • Trunk divided into large thorax and small abdomen
  • Sternum with keel or carina
• Classification of birds


  • Superorder Palaeognathae: Walking birds, or "ratites"; generally flightless; sternum typically without keel
  • Superorder Neognathae: Flying birds, generally capable of flight, sternum typically with keel

  
• Bird beak


  • Unique and multi-functional tool
  • Can help gather or capture food, communicate, groom feathers, defend territories, and attack rivals
  • Shape designed for eating particular types of food

  
• Beak shapes for different food types

  • Hawks, owls, and other birds of prey: sharp, "hooked" beaks to bite skull/neck and tear body
  • Hummingbirds: long, thin tubular bills to sip nectar
  • Woodpeckers: long, strong beaks that taper to a chisel for pecking holes in trees
  • Seed eaters like sparrows and pigeons: short, thick conical bills for cracking seed
  • Some ducks: long, flat bills that strain small plants and animals from water
  • Herons: beak with serrated edges and hooked tip to catch fish

  
• Feathers


  • Prominent feature of bird anatomy, unique to birds
  • Arise from defined tracts on skin
  • Aid in flight, thermal insulation, and waterproofing
  • Coloration helps in communication and protection
  • Evolved from reptilian scales
  • Replaced regularly through moulting

  
• Feather structure


  • Central hollow supporting shaft called rachis
  • Fine side branches called barbs
  • Barbs linked by barbules with hooklets called barbicels
  • Rachis and vane (all barbs and barbules) are visible parts
  • Base called calamus with hollow entrance for blood vessels
• Types of feathers


  • Contour feathers: largest, give shape and color, include flight and tail feathers
  • Down feathers: smaller, lack barbules, soft and fluffy, provide insulation

  
• Bird legs


  • Digitigrade: walk on toes, not entire foot
  • Feet covered in horny scales, digits end in horny claws
  • Rudimentary wing digits may have nails or claws
  • Spur: horny sharp claw on tarsometatarse, characteristic of males
  • Webbed feet in ducks for swimming, flat beak to filter food
  • Hooked beak and powerful claws in raptors to seize prey
  • Long legs in wading birds, small thin legs in perching birds
  • Most have 4 toes, 3 forward and 1 backward
• Bird flight adaptations


  • Not overly large or small
  • Fusiform body shape
  • Cannot store much body fat, must eat continuously
  • High metabolic level for efficient flight
  • Feathers for heat regulation and flight
  • Wing muscles maintain overlap between adjacent feathers on downstroke, feathers canted on upstroke
  • Uropygial gland maintains flying capacity
  • Light, hollow, delicate bones
  • Fused skull bones to minimize wind pressure
  • Eyes rimmed with bony sclerotic plates to minimize wind pressure
  • No external ear lobes for smooth flight
  • Neck vertebrae lock together firmly during flight
  • Large keel on sternum for flight muscle attachment
  • Arms transformed into wings
  • Oviparous to avoid overweight from pregnancy
  • Reduced tail vertebrae, tail feathers function as rudder
  • Air sacs inside body to aid breathing and reduce weight
  • No external genitalia to reduce wind resistance