These bird-like creatures look exactly like two hands with the thumbs hooked together.
They sometimes use one 'hand' to hold prey such as mice, and flap the other to fly. Naturally this is clumsier and slower than their normal, two-handed flying.
Every so often two of these creatures will find a private place, upon which the hands will unlock, and swap, forming two new creatures. Thus we might sometimes see a creature formed from one old and one young hand, or one male and one female. They are, however, almost always careful to swap hands for one of like size, lest they be rendered unable to fly straight.
It is not known how the creatures breed, or if the above practice is related.
A Bird-In-Hand, with one human and one goblin hand, is an emblem of the Conspiracy of Equals.