The bird

I learned where that term came from recently. The fletching on an arrow was called "the bird." During one of those English/French wars, the French expected to win, by larger numbers, but they were being hurt by the English long bowmen. the French said that when they won, they were going to cut off the bowmen's "bird" finger or middle finger. When, by the archers' help, the English won, the archers saluted the defeated French by showing them they still had that finger... So they were showing them their shooting finger or their bird finger.