I tend to throw some speculative fiction into my personal fantasy story ideas from time to time, and I've made myself very curious as to how and if certain features could develop. For example, how feasible is a creature that is covered in feathers as a juvenile, but has only scales as an adult? In what environment would that trait evolve? Is it possible to support a stable body temperature caused by warm-blooded traits in a creature with no fur or feathers? What environment would that kind of trait develop from? And what about intelligence and social behavior? What circumstances cause those kinds of traits to develop? What about the hollow bones of birds? How and why did they develop? How did the lungs of birds start to develop? How did the varied teeth of mammals?

I get the general idea of "to gain an advantage in their environment and live to produce offspring" that you see fairly often in response to this type of stuff, but I want to see some specifics as to why each trait developed in real animals, as well as an explanation of how and if they could exist in the same creature all at once. I understand that this is a lot to ask all at once, but I want to see if I can make at least a tiny part of my idea scientifically viable and learn some neat facts in the process.

sorry no-one seems to know the answer to your question.