Fighting/Flying
I actually think this is an awesome idea as far as flavour go. Going from primarily fighting to primarily flying could really symbolise...something.
Fighting/Flying
Well 3 options to me really.
Flying/Fighting
Normal/Fighting
Flying
Mono Fighting just don't make alot of flavor sense as it would mean the primary turns to secondary typing upon evolving.
On the contrary, it makes total sense when you look at the whole flavor design rather than just precedence in typing. Like a couple people mentioned, "the pokemon's quills harden with age". Also, what birds come straight out of a nest knowing how to fly? They need to be taught how to fly.
PS: If you want an example SOMEWHAT along the same lines, onyx is rock/ground -> steel/ground and swablu is normal/fly -> dragon/fly, so it's not unprecedented for the primary type to change.
To all the people who want pure Flying, while I apprecaite the sentiment to be sure what with the lack of Pokemon of the type, I just can't really agree with that. Looking at the Pokemon that CAN be pure Flying in addition to Tornadus, you have all the type-changers, none of which really have wings (you can hardly count Kecleon's vestigial winglike things). This kind of leads me to believe that pure Flying is more indicative of a wind element-based creature than an actual bird, all of have historically been Flying-type hybrids with Normal or some other type.