NewScientist is carrying this summary of a new theory on bird evolution. A group of scientists is now putting forward the hypothesis that birds actually started out with four wings for gliding, which later developed into the two-wing flapping model we're all familiar with today. The description made my eyes cross trying to imagine a bi-planed bird, but in actuality they're thinking the "extra" wings covered the rear legs. They've got some fossil evidence, but not a lot, as bird fossils are quite rare. More will need to be discovered for confirmation, but it is definitely an interesting idea.