Not sure about which feats would best help out, but I do seem to recall that most creatures with flying, can't actually take off if they are carrying anything more than a light load. So that might have more of an impact on what you can get away with.
An average Strix (STR 10) wouldn't be able to take off with more than about 33lbs, and even a pretty strong one (STR 18) would be limited to about 100lbs total weight carried to actually be able to take off.