How do the bird kids in maximum ride hides their wings? I have come to a conclusion. Skin pockets. They are made of highly elastic yet thick skin, which is very rubbery and is able to compress the wings (which can be uncomfy). In order to have the wings fit to the sides of the body, the ribcage is much thinner. To make up for reduced area for them to exist, the lungs have a much much much higher surface area than that of a typical human. Additionally, instead of a keel, their bones are artificially enhanced with carbon fiber, and they are much stronger than human bone, as well as light, making them able to support the powerful flight muscles while staying fairly light. The muscles are artificially enhanced as well, being much much stronger than what can occur naturally, so the flight muscles can be way smaller than that of naturally evolved flying vertebrates. The flight muscles are more batlike versus birdlike due to no place really to arrange the muscles for a pulley-like system like birds have, due to how low the wings are on the back. This also contributes to the chest being slightly flatter, as some of the flight muscle is on the back.