Birds

• Over 10,500 bird species have been described worldwide and they outnumber most vertebrates except the fishes
• Habitats where birds are found

  • Mountains
  • Prairies
  • Deserts
  • Oceans
  • North Pole
  • South Pole
• Some birds live in dark caves, and some dive down to 45 m in search of food
• The bee hummingbird is one of the smallest endothermic vertebrates, weighing only 1.8 g
• Feather


  • Unique feature that only birds possess
  • Some extinct theropod dinosaurs also possessed feathers

  
• Despite 150 million years of evolution, birds are still immediately recognisable due to uniformity in structure

  
• Bird structure


  • Forelimbs are modified as wings, although not all are capable of flight
  • Hindlimbs are adapted for walking, swimming or perching
  • All birds have horny, keratinised beaks without teeth
  • All birds lay eggs (oviparous)

  
• Wings


  • Used for lift and propulsion
• Respiratory system


  • Meets the high oxygen demands of flight and cools the body
• Bones


  • Light but provide a rigid frame
• Digestion and circulation


  • Fast and efficient to process energy-rich foods and meet the high energy needs of flight
• Senses


  • Acute sense of vision
• The discovery of the Archaeopteryx lithographica fossil in 1861 dramatically demonstrated the phylogenetic relationship between birds and theropod dinosaurs

  
• Archaeopteryx


  • Skull resembled modern birds but had a beak-like jaw with teeth in sockets
  • Skeleton was reptilian, with clawed digits, abdominal ribs and a long bony tail
  • Feathers were unmistakably imprinted along the wings

  
• Shared features between birds and non-avian reptiles

  • Single occipital condyle
  • Single middle-ear bone, the stapes
  • Lower jaw made from multiple bones
  • Nitrogenous waste excreted as uric acid
  • Produce large, yolky eggs; embryo develops on the surface by shallow cleavage patterns

  
• Thomas Henry Huxley classified birds with theropod dinosaurs due to numerous similarities

  
• Similarities between birds and theropod dinosaurs

  • Long, mobile, S-shaped neck
  • Dromeosaurs (including Velociraptor) share derived characters like furcula and lunate wrist bones

  
• Fossils in China from the late Jurassic and early Cretaceous period had hollow filaments and symmetrically vaned feathers on short forelimbs

  
• Early dromeosaurs


  • Could not fly but probably used the early feathers and filaments for thermoregulation, camouflage, and courtship displays

  
• Later dinosaurs like Microraptor sp.


  • Probably took short controlled jumps and glided from trees with their developing wings and feathers
• Evolution of modern birds from ancient dinosaurs

  1. Feathers were exapted for powered flight

  2. With a keeled sternum, loss of teeth, alula, and the fusion of bones, the ancient dinosaurs evolved into the modern birds

  
• Modern birds (Neornithes)


  • Paleognathae (flightless birds with flat sternum and poorly developed pectoral muscles)
  • Neognathae (flying birds with keeled sternum and strong flight muscles)

  
• Flightlessness has appeared independently among many groups of birds, from ancient to modern, even in the Neognathae
• Penguins


  • Flightless but use wings to "fly" in the water
• Birds that evolved on islands lacking terrestrial predators

  • Became flightless, some able to run at great speeds like the ostrich
• Contour feathers


  The most typical type of bird feather, vaned and streamlines the bird's body

  
• Quill


  Hollow structure that emerges from a skin follicle and continues as a shaft (rachis)
• Barbs


  Branches from the rachis
• Barbules


  Branches from the barbs that overlap and are locked together by tiny hooks
• Vane


  Flat surface formed by the interlocking barbs and barbules
• Types of bird feathers

  • Contour feathers
  • Down feathers (soft tufts without prominent rachis, act as heat insulators)
  • Filoplume feathers (hairlike with weak shaft and tuft of short barbs)

  
• Molting


  • Gradual process that prevents leaving bare patches of skin
  • Flight and tail feathers lost in pairs to maintain balance
  • Replacement is continuous in some species so flight is unimpaired
  • Most birds molt once a year, usually in late summer after nesting season
• Bird skeleton


  • Light, hollow bones supported by internal struts (pneumatised)
  • Total weight of feathers may outweigh skeleton
  • Skull is light and fused into one piece with large braincase and orbits

  
• Legs


  • Relatively heavy, lowering centre of gravity for improved aerodynamic stability
• Jaws


  • Toothless, with horny keratinous beak molded around bony jaws
  • Mandible made of hinged bones for double-jointed action and wide mouth opening
  • Most birds have kinetic skulls with flexible attachments between upper jaw and skull

  
• Vertebral column


  • Very rigid, with most vertebrae fused together
  • Caudal vertebrae fused into pygostyle, remaining vertebrae fused into synsacrum
• Ribs


  • Mostly fused with vertebrae, pectoral girdle and sternum to increase stiffness
• Furcula


  • Fused clavicles that store energy as it flexes during wing beats
• Sternum


  • Bears a large keel to anchor flight muscles, except in flightless birds
• Forelimbs


  • Highly modified for flight, with some bones reduced in number or fused together, but all major tetrapod limb components still visible