From John Henson
This little American Kestrel came flying straight at me, and for a split second I wasn't sure if I was about to get the shot or get buzzed!
They're tiny, unbelievably fast, and almost impossible to predict. Capturing one head-on like this takes a little luck, a lot of patience, and complete trust in your autofocus.
Those little nubs sticking out of our bird friend's wings here are something very special. Those are the alulae, also called "bastard wings", and they are the birds equivalent of thumbs, with 3-5 flight feathers on them.
This may seem somewhat insignificant, but they serve a very important purpose. They are so important that we humans stole the idea.
I'm no aviation expert, so perhaps someone may be able to provide more details, but the alulae are used during takeoff, landing, and certain aerial maneuvers where the main wings no longer produce enough lift, what would be called a "stall" in an aircraft. It basically keeps them from just dropping out of the air when they're moving too slowly to maintain adequate air passing over the wings.
Thinking about it, all feathers are there to save energi in one way or another.
And they evidently developed well before flight, for completely different reasons.