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Alula
The alula, or bastard wing, (plural ''alulae'') is a small projection on the anterior edge of the wing of modern birds and a few non avian dinosaurs. The word is Latin and means "winglet"; it is the diminutive of ''ala'', meaning "wing". The alula is the freely moving first digit, a bird's "thumb," and typically bears three to five small flight feathers, with the exact number depending on the species. There also are minor covert feathers overlying the flight feathers. Like the larger flight feathers found on the wing's trailing edge, these alula feathers are asymmetrical, with the shaft running closer to anterior edge.
==Function==
In most situations, the alula is held flush against the wing; however, it can be manipulated. When flying at slow speeds or landing, the bird moves its alula slightly upwards and forward, which creates a small slot on the wing's leading edge. This functions in the same way as the slats on the wing of an aircraft, allowing the wing to achieve a higher than normal angle of attack – and thus lift – without resulting in a stall. The tip of the alula forms a tiny vortex that forces the airflow over the wing to better bind to it. 〔(【引用サイトリンク】title=Small vortex on wing makes the elegance of birds' flight )〕 During stretching of the wing down toward the ground, the alula is abducted from the wing and can be clearly viewed.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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