A bird not being able to fly because of underdeveloped chest muscles is a myth. All that means is that he wont be able to fly miles on end for hours and hours. However if a good gust of wind comes along or if he hits good thermal drafts he could still end up miles away. Strong chest muscles are needed for strong take offs from the ground. If he is elevated from the ground (your arm, stand, tree branch etc.) a fully flighted bird thats never flown can still go pretty far. Everything about a bird is designed to keep them in the air. Next time your outside look at the vultures, they fly for miles and seldom flap their wings. Thats because they ride on thermal drafts that are invisible to us, it gives them the lift they need without working hard to fly. A bird that has not been taught to fly, or that has not been taught recall, is at a much higher risk of flying unexpectedly and not being found. This is because it is so unfamiliar to the bird that its sesory overload and their gone. Its in your birds genetic makeup to fly, underdeveloped muscles or not. Every bird that doesn't fly on a regular basis is going to have underdeveloped muscles but still has the full capability of flight, its instinctual. Its like saying because a person has never been in water before that they cant swim, everyone is born with the ability to do it (it may not be well, or beautiful).