Alula

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.

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.

In falcons, the alula is more prominent, and provides a degree of control when the wing is cupped or tucked in a dive/stoop.

[citation needed] The presence of an alula has been confirmed in several now-extinct ancient relatives of modern birds, including Eoalulavis hoyasi (an enantiornithine from the mid-Cretaceous, 115[3] mya) and the earlier Protopteryx fengningensis.

[4] Since these species are not closely related to modern birds, either the alula evolved twice, or it did so more than 130 million years ago.