The upturned wing tip seen on many soaring birds like eagles, hawks, and vultures is called the alula. It is a small feathered projection on the leading edge of the wing near the wrist joint. The alula plays an important role in helping soaring birds utilize columns of rising air called thermals to gain altitude and remain aloft while expending minimal energy.
What is the alula?
The alula is a thumb-like projection found on the front margin of the wings of most birds. It consists of 3-5 feathers that can be moved independently of the rest of the wing. In soaring birds like eagles, hawks, and vultures, the alula feathers are especially elongated and prominent.
The alula is sometimes referred to as the “bastard wing” because it resembles a smaller secondary wing growing off the front of the main wing. It may also be called the alular digit, pollex, or preaxial digit.
Alula feather anatomy
The feathers that make up the alula are asymmetrical with curving shafts and stiffened rachises. They have short soft barbs on the inner vane but long hooked barbules on the outer vane. This creates a rigid trailing edge ideal for increasing the lift of the leading wing edge.
The alula feathers attach to well-developed alular quills which articulate with the alular digit – a small bony prominence on digit I of the manus or hand-wing of birds. Small muscles allow the alula feathers to be controlled independently of the wrist and spread or folded at will.
Alula size in soaring birds
In soaring birds that utilize thermals, the alula is especially long and prominent, consisting of 5-6 elongated feathers forming a larger surface area. For example:
- Bald eagles have 5-6 alula feathers up to 8 inches long
- Turkey vultures have 5-6 alula feathers around 5 inches long
- Red-tailed hawks have 5-6 alula feathers up to 6 inches long
This well-developed alula is thought to help these soaring birds maneuver and stabilize at slow speeds and steep angles of attack.
How do birds use the alula in flight?
Birds use the alula in multiple phases of flight to fine-tune their aerodynamics:
1. Taking off
As the wings are spread and flapped for take-off, the alula is extended to increase the wing area for extra lift and thrust. Once the bird is airborne, the alula is folded back against the wing to reduce drag.
2. Landing
Prior to landing, the alula is extended again to effectively enlarge the wing area and create greater lift and drag, allowing the bird to slow down and control its descent to the landing perch or ground.
3. Soaring and gliding
In soaring birds like eagles, hawks, and vultures, the alula plays a key role once the bird is aloft. When catching rising thermals and making tight turns, the alula is spread to maintain airflow over the wing at steep angles of attack. This prevents stalling and loss of lift.
When gliding between thermals, the alula stays tucked in to reduce drag like an airplane’s flaps. The alula essentially acts as adjustable “leading edge slats” to modulate stall conditions.
4. Maneuvering
By independently controlling each alula feather, a bird can subtly adjust the lift and drag forces along different parts of its wing. This permits precision maneuvering in complex wind conditions and through cluttered environments.
Being able to tweak the alula may help birds make quick turns, hold steady courses, land on branches, and expertly maneuver down to snatch prey.
Why do birds like eagles have an upturned alula?
In soaring birds, the feathers of the alula are especially elongated and have an upward curved, upturned shape. This morphlogy is thought to have several aerodynamic advantages:
- The upturned shape increases the effective camber or curvature of the wing’s leading edge. This enhances lift at the slow speeds and high angles of attack used in thermalling and soaring flight.
- The alula’s upward slant splits and redirects airflow over the wing to stay attached even when stalled. This maintains lift through a greater range of attack angles useful in thermalling.
- The alula’s curvature mimics traditional upturned wing tips. These wing tip modifications can reduce induced drag and wingtip vortices which is useful when soaring.
Additionally, the alula’s upturned form may help protect the fine feather tips from surface abrasion when birds are perched or landing.
Unique benefits of the alula for soaring flight
The alula provides unique benefits tailored to soaring flight techniques used by birds like eagles, hawks, vultures, and ravens:
Tight thermalling turns
When circling tightly within rising thermals, the alula provides key lift stabilization at the wing leading edge to prevent slip-stall at slow speeds and steep bank angles. This allows maximizing altitude gain within the narrow updraft.
Slow speed gliding
Between thermals, the alula enables smooth, controlled gliding at minmum sink speeds despite the wings functioning in an stalled condition. This extends the time and distance birds can glide between patches of lift.
Stall resistance
The alula generates a helpful vortex and redirects airflow to postpone stall far beyond wing angles that would cause an aircraft wing to lose lift. This gives soaring birds a wider operating range at high angles of attack.
Gust response
The ability to iteratively adjust each alula feather allows rapid modification of wing shape to counteract unpredictable gusts and turbulence. This helps eagles maintain steadier, less disrupted flight within thermals and winds.
Low speed precision
By fine-tuning the alula, birds can execute tight, controlled turns and precisely alter course and speed as needed for navigating cluttered terrain and positioning during hunting.
Braking and acceleration
Extending the alula on demand provides greater air drag to slow down for landing or drop altitude, while retracting it reduces drag to accelerate and build speed in a stoop or dive.
Similarities and differences to aircraft aerodynamic devices
The alula functions analogously in some ways to advanced high-lift devices used on aircraft wings and wingtips:
Similar to leading edge slats
Slats are extension surfaces on the leading edge of a wing that provide a high-lift benefit. Like slats, an extended alula increases camber and helps airflow adhere to the top of the wing at higher angles of attack before stalling.
Comparable to split-flaps
Aerodynamic “split-flaps” placed along parts of a wing’s trailing edge can enhance maneuverability. The alula feathers essentially act as small independent split-flaps to provide localized control of lift.
Analogous to wingtip devices
Upturned wingtips like winglets reduce induced drag from wingtip vortices. The alula’s upturned form helps minimize drag in a similar manner.
Differences from aircraft wings
Unlike rigid aircraft wings, an alula can be controlled dynamically during flight to provide adaptive high-lift and drag-tuning ability surpassing aircraft wing devices:
- Each alula feather acts independently like multiple movable control surfaces along the wing edge
- The alula can reverse positions from retracted to extended instantly on demand
- The alula is movable in both horizontal and vertical planes for precise control
Additionally, the alula produces a tip vortex that actually helps keep airflow attached, unlike aircraft wingtip vortices which increase drag. This demonstrates the complex and subtle aerodynamics made possible by the articulated alula.
Conclusion
The alula � or bastard wing � is an articulated, feathered projection on the leading edge of bird wings. In soaring birds, the alula feathers are elongated and upturned.
This distinctive alula morphology generates high lift, prevents stalling, comtrols drag, and permits precision maneuvering at the slow speeds, steep angles, and high turbulence associated with exploiting thermals and wind currents for sustained soaring flight.
The alula acts as a dynamic, multi-functional high-lift device – continuously adjustable to provide birds unique flight performance and efficiency advantages for their ecology and evolution.