rationalskepticism.org

Sorry to be such a "damp duvet", but baby dinosaurs were very small, and benefited from downy insulation. Thus birds inherited the much of the genetic structure for flight feathers from their dinosaur ancestors, and, because they remained very small as they became adults, there was the possibility of flight when new mutations changed down into flight-feathers (via their use for gliding down onto prey, presumably) in the adult.