L14- Origin of Flight

• Flight has made birds successful

• Murmurations→ Starling birds move together in large groups, function is unknown, is not just individual flying that is important

Advantages of Flight:

1. Predator escape

2. Foraging

3. Cost-efficient locomotion→ gliding, moving long distances with low energy

4. Colonisation→ of islands

5. Migration→ to escape the winter

Flight also imposes constraints:

• birds are diverse but uniform in morphology

• need major physiological and anatomical adaptations for light

• birds have a limited size and weight range (~12kg)

• the muscle power needed to take off increases by x2.25 as the biomass double

  → physically cannot get a bigger bird than this as it would not be strong enough able to fly

The evolution of feathers:

• Some theropods had feathers

• Dromaeosaurs had feathers

• Feathers are homologous to reptilian scales

• Feathers did not evolve for flight→ evolved for insulation or display

• Now feathers have multiple functions e.g. insulation, display, thermoregulation, flight…

Hypotheses for the origin of flight:

1. Cursorial Theory-

   • wings helped with stability when running or catching flying prey (insects)

   • ground-based theory

2. Arboreal Theory-

   • wings helped with stability for locomotion in trees

   • tree-based theory

3. Wing-Assisted Incline Running (WAIR)-

   • wings evolved to assist climbing trees

   • is seen in extant juvenile birds, pigeons using wings for extra thrust

4. Pouncing Pro-avis Hypothesis-

   • wings evolved to jump and bounce on prey→ facilitate hunting

   • is the alternative to cursorial theory

5. Raptor Prey Resistant Model (Ripper Behavioural Model)-

   • wings were used to pin down prey on the floor→ facilitates hunting/foraging

Which theory of the origin of flight is right?

• Cursorial theory was initially favoured, evidence:

  • Archaeopteryx morphological evidence shows it had gliding abilities but not flying abilities

  • Archaeopteryx had no trees where they were found

  • The spreading of feathered forelimbs (which turned into wings) was probably originally used for stabilisation whilst running

• But there are also problems with the cursorial theory:

  • Velocity gap→ Archaeopteryx ran up to 2m/s but it would have needed to run at least 6m/s to get off the ground

  • There are high energy demands needed for flight that Archaeopteryx may not have had

  • Cannot explain the origin of lifting off the ground in an earthbound organism

• There is also evidence for the arboreal theory:

  • The Hoatzin bird has retained its claw at the end of its wings

    • in the juveniles, it is used to help move in trees

    • it is similar to the claw in Archaeopteryx:→ but there were no trees where Archaeopteryx are found

  • Pigeons have an alula

    • may have been the remains of the claw that evolved into a feather

    • is used to reduce turbulence when flying

  • Feduccia’s Claw Arc Analysis

    • analysed different traits of the claw and compared to many species of different habitats

    • found that ground-dwellers had flatter claws than perchers

    • Archaeopteryx front claw is like a climber but the back claw is like a percher

      → Archaeopteryx evolved in the trees, perched and used wings to climb

• Main issue with the arboreal theory is that there were no trees where Archaeopteryx fossils are found

Structure of feathers:

• Have a shaft down the middle

• On each side of the shaft are barbs

• From the bards are barbules

• Barbules are tiny hooks that stick to each other to maintain waterproof properties

• Birds preen→ put oil onto feathers to maintain structure and keep waterproof:

• Components of feathers:

  • keratin

  • lipids

  • water

  • pigments

There are many types of feathers in extant birds e.g.:

• Down feathers→ thermoregulation

• Semiplume feathers→ insulation

• Tail feathers→ stability

• Wing feathers→ flight

• Filoplume and bristle feathers, e.g.:

  • Common Poorwill bird use bristles around their beak to detect prey movements

How does flight work?

• the wings act as an aerofoil and a propeller→ lift and move forward

• the shape, area and angle of the wings change whilst flying:

  • the top of the wing has a larger surface than the underwing

  • air moves slower at the top of the wings

  • this creates a pressure gradient of low pressure above the wing and high pressure below the wing

  • the pressure gradient creates lift

  • can also use temperature gradients in the air to create lift

• the wing tips (the primary feathers) are responsible for thrust and moving forward

• there are different forces at play during flight:

• different parts of the wings have different functions:

  • the part of the wing closest to the body remains flat during flying

  • at downstroke, the end of the wing has a steep angle which allows converting uplift force into downward and sideways movement too

• different wing shapes are associated with different types of flight

  • aspect ratio (length-width ratio) is a strong indicator of the type of flight used:

• Birds have a unidirectional air flow in their lungs and a higher oxygen concentration than mammals, needed for flight

• Colours and patterns in feathers are used for:

  • sexual selection

  • crypsis→ predator avoidance mechanism

  • pigments→ display and efficiency of flying

  • iridescence→ colour changes depending on the angle

    → colours are made of pigments and structural properties

• Feathers can be used for insulation:

  • different colours have different absorption properties

  • darker colours keep more heat in than lighter ones

  • white feathers with black tips have a difference in temperature that creates a conductive current towards the tip to allow lift and help with flight