- This article is about the Alula feather. For the Finnish ornithological journal of the same name, see Alula (journal); for the Hawaiian plant, see Brighamia insignis.
The alula, or bastard wing, is a small projection on the anterior edge of the wing of modern (and some ancient) birds. The alula is the freely moving first digit, a bird's "thumb," and is typically covered with three to five small feathers, with the exact number depending on the species. 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.
In most situations, the alula is held flush against the wing; however, it can be manipulated. When flying at a 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
. During stretching of the wing down toward the ground, the alula is abducted from the wing and can be clearly viewed.
Alula in ancient birds
The presence of an alula has been confirmed in several now-extinct
ancient relatives of modern birds
, including Eoalulavis hoyasi
from the mid-Cretaceous
, 115 mya
) and the earlier Protopteryx fengningensis
. Since these species
are not closely related to modern birds, either the alula evolved
twice, or it did so more than 130 mya.
Alula in flies
In flies (Diptera
), the alula is a lobe of the posterior margin of the wing bounded proximally
by the upper calypter
by the axillary incision, and anteriorly by the base of the anal vein. The term has also been applied to the upper and lower calypters collectively.