I really think a Tornadus might fit well on this team, so then Archeops and Ho-Oh don't have to worry about being revenge-killed and can sweep easily thanks to the boost to speed from Tailwind (but its a spirit so idk if you could call it a bird (but you called Gira-O a bird... then again it has wings... but tornadus-I's wings are hidden behind clouds) also slightly eases problem against Kyogre and is more reliable against Deo-s then Skymin, but your going to have rely on a physical set using Flying Gem Acrobatics + BU + Tailwind + Superpower instead or Hurricane + Focus Blast. The problem is I don't know what it could substitute really, so its just a suggestion.
This team clearly demonstrates mastery of the metagame, I wish you luck with it trickroom :P.
I don't really see Shaymin S as a bird though due to the fact that he is a mammal with no beak, talons, or feathers who flies with Dumbo elephant ears, but he does check Kyogre, Groudon, and in some cases Palkia very well. Although, he kinda didn't do his job in flinch haxing very well in my experience.
Skymin is not a dumbo, its just so lightweight and its petals allow it to glide and the air that can be pushed under it outweighs the force / accel that is gravity due to the lack of mass.