A bird’s deployment of alulae is a remarkably transient event. If you blink, you will probably miss it, for each event lasts less than half a second. Yet, it is fun to spot these bits of aerodynamic wizardry which perform the same function for a bird as do the slats on an aircraft.
Half way along the leading edge of bird’s wing there is a bend that is analogous to a person’s wrist. An alula corresponds to a bird’s thumb. It is usually held flush to the wing to minimize drag during normal flight, and so passes unnoticed. Yet, birds have the same problem as do aircraft. To land, they must slow, but doing so decreases lift and control. To minimize this loss of lift, the bird tips back to increase its angle of attack. This can lead to a stall as the air ceases to flow smoothy over the upper surface of the wing. A bird’s deployment of alulae forces the airflow back over the wing’s upper surface, allowing the maintenance of adequate lift and control.
I was watching a heron hunting in a marsh. And although I felt that my twenty-five metre distance would not be threatening, the heron thought otherwise and flew farther away.
It flew low over the grass to a spot about double the distant. The Great Blue Heron cruises at about 40 km/hr and now must come to a graceful stop. If it were landing in a tree, it could approach from below and use gravity to slow. However, on a level surface, its wings need to do all the work.
It begins to tip back. The increased angle of attack increases drag slowing the bird and also partly compensates for the decreased lift at the lower velocity. Alas, it also increases the likelihood of a stall with its loss of control. At this time the alulae are deployed. They are the raised feathers halfway along the leading edge of the wing (at the wrists).
With its alulae still deployed, the heron tips back even further. The ruffled upper coverts (feathers that cover the wing) probably play the same role as vortex generators on aircraft wings.
Alulae are no longer needed as the heron swings its feet forward to alight.
And all of this was seen because the heron apparently wanted to double its distance from me. Successive landing pictures were separated by about a fifth of a second and the alulae were deployed in two pictures—about the time span of the blink of an eye.