Air warden doesn't follow ground troops unless there are no air in his aura. However as for ground GW I can't say. I've never had him not follow the seige...granted I usually have a mass of bowlers following the seige....
If the mass of loons is outside of his aura he won't leave some air troops to go to that mass. He only switches when they are in his aura...well he tries to stay in the center. Hastes/rages often will take that mass out of his aura before he "realizes" and moves with them...particularly if he is helping destroy a building. It's one reason I like to use his GW ability somewhat early...to prevent the chance of this from happening.
Frankly the one logic I hate the most is the healers switching to him from the AQ and never switching back because he stays just far enough behind her so that she never gets back in the healer vision. To resolve this it would be nice if the healers had a 2nd level aura...for when the unit they are healing is at 100% health....so they can "search" for someone knew to heal.
This could be similar for the GW...say once the number of troops in his aura is under 5 (units...aka 1 loon or 5 pups)....he expands his search for a larger group.
Both of these ideas would solve a lot of the problems we have...but I think would be a nightmare to code.