Page 229 - The Origin of Birds and Flight

The wing structure in Eoalulavis hoyasi is also present in certain present-day
flying birds. The feathers on this bird’s wing contain a small bunch of feathers
attached to the “finger”. When the bird wishes to slow down or descend to earth, it
decreases the angle of the wing to the horizon. This allows air to flow over the
wing’s top surface and to stop without falling.