No one's pushing my swings! I've already accounted for the input, it's the great mass of the swaying tree (and in turn the wind etc.), and like the tides' moon it's the same input to both swings, only one's them is swinging like the clappers whilst the other barely moves. Even though the former clearly has more energy at a given point in time most of that is a one off build up, and there can't be any more repeatedly extractable energy in the big swinger than the small.