With the concept phase completed, I set out gathering my usual reference material for all parts of the illustration. I use indirect reference for nature-based content, like wings. Keep in mind that wings are just big flappy hands, which is a helpful basis for anatomy and disturbing thoughts of chicken people.
Above is my page of reference just for wings. I look up the birds that the figures are based on and use a flying fox for the dragon wings. Pro tip is to use big bats instead of small bats for dragon wing reference as the proportions and physics will be closer. Big bat wings are really bendy under all that weight and the anatomy is easier to see. Also they're cute, so treat yourself.
You might ask why I have so many bird wing references? This is because there is no perfect reference wrapped into one image (unless you pose a model or maquette), so I have one photo that gives insight into back-lighting, another for the general pose, and then one photo might be perfect for just one of the wings.