We kept watch over the baby for a little while and when we realized that it wasn't going to return to the nest on its own and that the mother was still nowhere to be seen, we went next door and while Griffin blocked its path, I picked up the baby and we returned it back to the nest and its sibling. I have to confess that in spite of returning the baby safely to the nest, it was pretty freaked out by its sudden unplanned trip, standing bolt upright on top of the other baby with a wild look in its eyes for quite some time. Eventually the second bird grew weary of the little traveler crushing it, and pulled itself up and out of the nest. The two waited for mommy to return and when she didn't, they both left the nest which has now sat sadly empty for several days.
I was terrified that I had caused the mother to abandon her babies because I had touched one of them, but she was gone before my lapse in judgment. And given the size of her children, I believe it was time for her to leave them so they would become hungry and thus be compelled to leave the nest and face their new lives out in the world. My only regret is that I gave one of them a really nasty start to its first day of freedom. Sorry little bird and the neurotic life you will now probably lead...