The finches survived Monday night okay, although it gave all of us caring neighbors a start when I saw they weren't in the their artificial nest anymore - until I heard baby-cheeping and saw Ma and Pa finch hovering over the bush right next door, and sure enough, there they were. Because the cold rain was already starting, a neighbor and I were about to bring them inside for the night, so I caught them and put them in a big box first so their parents could see they were still present and alive. But before I knew what happened, the better-developed one flew right out of the box and landed in the tree across the street! Dad followed on his/her tail. The younger one stayed in the box.
Quite surprised, I changed my mind and left the younger one in the artificial nest for the evening. This morning he was gone again, and I haven't looked for him/her, but maybe s/he flew away too. Because I will likely have no proof of any outcome to this story, that's what I'll choose to believe.