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Digit of a bird wing homologous to the thumb.



(also known as bastard wing or spurious wing), a group of feathers attached to the first digit of bird wings. The alula usually consists of four feathers, but sometimes there are three (certain passerines and Picidae) or six (the touraco). It can move forward and upward, preventing disruption of the airstream from the wing at large angles of incidence. The alula is most developed in forest birds that have rounded wings and are capable of maneuverable flight.

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