Gerardo asked about the Mechanics behind the flapping bird While I don't have anything to add to what has been said (very enlighting by the way), I'd like to share something regarding mechanics and the flapping bird.
A few years ago I thought that since the flapping action includes friction, it would somehow produce heat at some point, so I flapped a bird for about 5 minutes and then checked with an infrared camera expecting to spot some areas hotter than others (the hinges I thought). To my surprise no area of the bird showed a different temperature than the rest except for the two places where I had my fingers on. This camera I used has a 0.1 C (0.18 F) accuracy so this result (or lack of it thereof) wasn't due to the use of the proper tool. I always thought it was odd and was never able to find an explanation for this. -- Jorge Jaramillo