Regarding birds, I think modelling them could be simple. They can be placed in an area on the map, and have a small radius in which they fly in. When A/C are flying, the birds will take flight, but when all is calm they just settl on a fixed location in that area. In this way, the game won't be focusing on their moments, and they will only be disturbed when in visual distance of the player
I think it is a possible workaround. I mean, in Bob2 there are birds, but you will never see them really because their size is realistic and their numbers are limited IIRC.