The two charcoal males returned to the aviary without issue (although the smaller of the two seemed to have poor flying stamina initially).  I returned the pair of fawn Zebra Finches (nothing eventuated from the seven eggs they laid) and it turns out the male is quite the bully.  He insisted on occupying a large perch and would let no other bird near it.  The male grey Zebra Finch in the aviary is a solid little bird and even he was being pushed around.  I was hearing squawks I never thought finches could produce.  This was not good.

So late today I removed the pair of fawn Zebra Finches and placed them in a small cage indoors.  The atmosphere in the aviary changed instantly.  When I have cleaned out their cage they can go back in there.