Birds have feathers that allow them to fly. These specialized feathers are called flight feathers. Flight feathers have a unique structure that makes them light, flexible and aerodynamic, enabling birds to take to the skies. Understanding the structure of flight feathers provides insights into the evolution of bird flight and avian biology.
What are flight feathers?
Flight feathers are the large, stiff asymmetrical feathers on a bird’s wings and tail that provide the lift, thrust and control necessary for flight. They include the wing feathers (remiges) that attach to the hand and arm bones and the tail feathers (rectrices) that grow from the tailbone.
While body contour feathers help insulate birds, flight feathers allow birds to fly. Their unique structure makes them resistant to airflow on the downstroke and helps create lift on the upstroke. This gives birds the aerodynamic properties necessary to fly. Flight feathers are only present in birds. They differentiate birds from other feathered dinosaurs.
Types of flight feathers
Flight feathers on the wing are divided into three major groups:
Primary feathers
The primary wing feathers attach to the “hand” bones of a bird’s wing. They are numbered from the wrist (the closest primary) towards the tip of the wing. There are typically 10-11 primaries in most birds. Primary feathers provide thrust on the downstroke and allow the wings to form a smooth airfoil for generating lift. The outermost primaries (the “pinions”) are the longest and give the wing its pointed shape.
Secondary feathers
Secondary feathers attach to the forearm bones and help provide lift and control. Most birds have 10-20 secondaries on each wing. Secondary feathers tend to be shorter than primaries. They provide lift by curving over the primaries when spread.
Tertial feathers
Tertials are located at the base of the wing, attached to the humerus bone. They help smooth airflow over the wing. Most birds have 3-6 tertials per wing. Tertials may overlap and cover secondary feathers when the wing is folded at rest.
Structure and composition of flight feathers
Flight feathers have a central shaft called a rachis. Thin lateral branches called barbs extend outward from the rachis at an angle. The barbs have tiny hooklets called barbules that zip them together, forming a continuous vane that allows air to flow over the feather.
Rachis
The rachis is the central shaft of the feather. It is made of a stiff yet flexible material called β-keratin. The rachis is thickest at its base near the calamus where it inserts into the skin. It tapers towards the tip of the feather. The tapered shape reduces weight, allowing flight feathers to be lightweight despite their large size.
Barbs and barbules
Barbs branch off the rachis at steep angles, giving flight feathers an asymmetrical look. The leading vane (towards the front edge) has shorter barbs, while the trailing vane has longer barbs. This helps reduce airflow disruption and turbulence.
Barbules are tiny hooklets that extend from each barb. They zip adjacent barbs together, forming a unified vane surface. The tight zipping allows air to flow smoothly over the feather. If barbules become damaged or unzipped, airflow will become turbulent, reducing flight efficiency.
Specializations for flight
Several adaptations give flight feathers the needed aerodynamic properties:
Asymmetrical shape
The rachis is off-center, making flight feathers asymmetrical. The broader leading vane helps cut through the air on each downstroke. The narrower trailing vane decreases drag and turbulence on the upstroke.
Stiffness and flexibility
The rachis made of β-keratin provides necessary stiffness to prevent feathers from bending under air pressure, while still allowing flexibility. The base is stiffest, while the tip can flex and spread.
Twist and curvature
When viewed from above, flight feathers have a slight twist. This maintains optimal airflow to generate lift throughout the entire stroke cycle. Many flight feathers also curve backwards at the tip, further improving aerodynamics.
Slotting
Slots form where the trailing edge of one feather overlaps the leading edge of the next feather. This slot separates airflow over the top and bottom of the wing, delaying stall at high angles of attack.
Coloration
While color is determined by pigments like melanins and carotenoids, most of the visible color in flight feathers is due to microscopic structural properties.
The barbules within flight feathers contain nanoscale arrays that reflect certain wavelengths. The specific array geometry determines the color produced through iridescent structural coloration.
Slight variations in barbule nanostructures between feathers creates subtle color differences. This gives each feather a distinctive look depending on the viewing angle.
Some feathers may have specialized color patterns. For example, a white speculum patch on ducks improves flight signaling. Other feather colors aid in camouflage or signaling.
Molt
Because feathers are subject to wear-and-tear, they are replaced periodically through molting. Adult birds will replace all their flight feathers gradually over several months as part of their annual molt cycle.
Shedding only a few flight feathers at a time allows birds to maintain flight capacity. New feathers grow in while old ones are molted. However, until molting is complete, flight ability is somewhat reduced.
Young birds may replace flight feathers multiple times as juveniles. Their first true flight feathers are called juvenal plumage. These are typically duller than adult plumage.
Evolution
Flight feathers likely evolved from more primitive feathered body coverings. Selective pressures for flight led to aerodynamic specializations.
Flight feathers are unique to birds and are not seen in non-avian feathered dinosaurs. Their presence in Archaeopteryx and other early birds shows that flight feathers were an early evolutionary innovation tied to avian flight.
Subtle differences in flight feather shape, structure and arrangement provide clues to evolutionary relationships between modern bird groups. The anatomy of fossilized flight feathers allows scientists to trace the evolution of birds back over 100 million years.
Roles in flight
Different types of flight feathers serve distinct roles in powering and controlling flight:
Generating thrust and lift
Thrust on the downstroke and lift on the upstroke are created primarily by the large primaries. The wings form an airfoil that produces unequal pressure above and below.
Reducing drag
The narrower, stiffer primaries cut through the air with minimal drag. Slots between the primaries allow smooth airflow. The trailing vane of each feather reduces turbulence from the leading vane ahead of it.
Providing stability and control
The outer primaries spread to steer turns and pitch the wing. Secondary feathers and tertials allow twisting and flexing to control glides and landing. The fanned tail rectrices aid in braking, steering and stabilization.
Specialized alula feathers on the wrist act as aerodynamic slots and air brakes to prevent stalls.
Conclusion
Flight feathers allow birds to fly through their specialized asymmetrical shape, aerodynamic properties and ability to generate thrust, lift and control. The structure of the rachis, barbs and barbules give flight feathers the needed combination of stiffness, flexibility and reduced weight. Adaptations like twist, curvature and slotting smooth airflow over the wing surface. Flight feathers evolved early in the avian lineage and were key innovations that allowed primitive birds to unlock the skies. Their unique structure provides insights into the biology, ecology and evolution of birds.
