The underside of a bird’s wing is called the underwing. It consists of the flight feathers that provide lift and thrust to enable flight. The underwing displays a variety of patterns and colors that serve different functions for birds.
Anatomy of the Underwing
The underwing consists of the flight feathers that are attached to the ulna and manus bones of a bird’s wing. The ulna and manus form the main framework of the wing. The flight feathers emerge from follicles in the skin and fan out to form the shape of the wing.
The flight feathers can be divided into three main groups:
- Primary flight feathers – Attached to the “hand” and digits and stretch along the rear edge of the wing. They provide thrust and lift.
- Secondary flight feathers – Attached to the forearm (ulna) and stretch along the front edge of the wing. They provide lift.
- Tertiary flight feathers – Additional smaller feathers towards the base of the wing. They help smooth airflow over the wing.
The rachis is the central shaft of the feather. The vane branches out from either side of the rachis and consists of smaller barbs and barbules that interlock to form a flat surface.
The shape, size and pattern of the flight feathers are tailored to a bird’s size, weight and flight style. For example, large soaring birds like eagles have long, broad wings suited to gliding. Small songbirds have shorter, more rounded wings for flapping flight. The differences in underwing pattern and design reflect adaptations for specialized modes of flight.
Colors and Markings
Many birds display vivid colors and striking patterns on their underwings. These serve a variety of purposes:
- Camouflage – Underwing patterns like bars, stripes and splotches can help break up a bird’s outline and allow it blend into the environment when wings are folded.
- Signaling – Bright patches are used in courtship displays and aggression. Flashing the underwing signals identity or serves as a threat display.
- Aerodynamics – Dark wing tips are thought improve airflow over the wing in some species. They may also help reduce wear and tear.
- Thermoregulation – Darker feathers may help absorb heat, while light colors reflect solar radiation away from the body.
Some examples of birds with patterned underwings include:
- Barn owl – Golden buff underparts with dark bars and speckles.
- Northern goshawk – White underwings with dark grey barring.
- Eurasian magpie – Vivid blue and white pattern.
- Indian peafowl – Peacock’s train feathers are underwing coverts.
Functions of the Underwing
The underwing serves several crucial functions in bird flight:
Lift Generation
The flight feathers generate lift as the bird flaps its wings. As air flows over the wing, the shape causes air pressure above the wing to drop while air below the wing is pushed up. This creates lift.
The wing’s angle of attack – or angle relative to oncoming air – can be adjusted to maximize lift. The underside of the wing plays a key role in engineering the optimal airflow for generating lift.
Propulsion
The flight feathers provide thrust as the bird flaps it wings. Thrust is generated by accelerating air backwards as the wings push downward and backward in the power stroke phase of flapping.
The underside of the wing plays an integral role in flap propulsion. The complex underwing patterns help smooth and guide airflow to boost propulsive efficiency.
Maneuvering
By adjusting the orientation and angle of attack of the wings, birds can maneuver and steer in flight. The wings act similar to an aircraft’s control surfaces.
The expansive surface area and articulations of the underwing give birds exceptional maneuverability and agility when flying through complex environments.
Braking and Landing
Extending the wings helps slow a bird’s speed prior to landing. The broad surface area creates substantial air resistance to slow descent. Angling the wings forward acts as an aerodynamic brake.
The underside of the wing essentially acts as a landing surface as birds alight on branches or the ground. Flexible flight feathers cushion the landing impact.
Evolution of Underwing Patterns and Functions
The evolution of ornate underwing patterns and diverse feather adaptations provided major advantages to early birds.
Feather Asymmetry
Primitive bird feathers were symmetrical with equal-length vanes projecting from the central rachis. This began to change around 150 million years ago in the late Jurassic.
Asymmetrical feathers evolved where the leading vane was shorter and more rigid than the trailing vane. This helped streamline feathers against oncoming airflow and improve aerodynamic functions. The specialized shape enhanced lift generation and propulsion.
Camouflage and Signaling
Intricate barring and striping patterns on the underwing likely evolved as a form of camouflage in early raptors and other predatory birds.
Display features like vivid patches subsequently developed as sexual selection shaped courtship displays. Bright underwing patches helped attract mates and signal dominance.
Specialization for Flight Styles
Underwing designs adapted over time to suit different flight styles. Large soaring birds developed slotted, elongated feathers to enhance glide performance. In songbirds, shorter rounded feathers enhanced maneuverability.
Underwing features continue to specialize and diversify along with birds’ lifestyles and ecological niches. Form and function are closely intertwined.
Importance for Bird Identification
The underwing pattern and coloration provides important clues for bird watching and identification. Subtle differences in underwing plumage can distinguish similar species or males from females.
By noting underwing features like feather shape, wing bars, patches and colors, birdwatchers can identify species in the field more reliably.
Some examples of birds where underwing patterns help identification:
- Red-tailed hawk vs red-shouldered hawk
- Peregrine falcon vs prairie falcon
- Cooper’s hawk vs sharp-shinned hawk
- Merlin vs American kestrel
- Marsh wren sexes – male has white underwing stripes, female is plain.
Careful observation and comparison of underwing plumage provides valuable clues for identification. High-quality photographs or views through binoculars may be needed.
Mimicry Using Underwing Patterns
Some species have evolved to mimic the underwing patterns of other dangerous birds. By copying conspicuous underwing markings, harmless mimics can avoid predation.
Some examples of underwing mimicry include:
- The hornet moth mimics yellowjacket patterning.
- Viceroy butterflies mimic the underwing of monarchs.
- The hooded pitohui mimics the vivid black and orange underwing of orioles.
- Daggerwings copy the barred underwings of forest raptors.
Mimicking the threatening flash patterns of predatory birds gives mimics a survival advantage. Predators avoid targeting them by mistake.
Conclusion
The underwing is a multifunctional structure critical to avian flight. Its anatomy provides the airfoil surface needed to generate lift and thrust. Underwing colors and patterns serve functions ranging from camouflage to courtship displays.
The underwing’s features continue to adapt and diversify alongside birds’ ecological niches. Careful examination of underwing plumage provides valuable identification clues for birdwatchers.
From aerodynamics to avian evolution, the intricate design of the underwing reveals birds’ extraordinary adaptation for flight.
