It was mentioned before but I believe Prehistoric Kingdom figured out aviaries perfectly - customizable aviaries of different sizes, materials, and environments, with semi-looped animations for the flighted creatures.
(It isn't necessary a loop - there are various set locations in the aviaries the birds fly to, then perform a randomized activity - perching, preening, calling, etc - and then fly off to another spot to avoid endless loops)
And if Frontier is determined to have the birds be fully blown habitat animals, perhaps similar to the brachiation for the gibbons, a new enrichment item could act as designated flight zone - a modular perching system of sort.
And hey, let's get creative! Here in the USA, almost every single raptor in zoos is either born in captivity or more commonly, a rescue deemed non-releasable by the US Department of Fish and Wildlife. Similarly, many parrots and other tropical birds are survivors of the abusive pet trade. I could see PZ implementing a rescue market dedicated for birds - you'd bypass the flight issues, as many of these birds can't fly.
I simply refuse to give up on birds. Remember, prior to the Conservation Pack, gibbons were thought to be a pipe dream, but Frontier thought out of the box and provided an innovative approach to include their iconic movement.
I have no doubt they can implement that same ingenuity to bring birds into the game.