We had some blackbirds build a nest in our backyard not long back. There were soon 4 eggs and they all hatched out within a few weeks giving us these little birds. When they got older we found one was missing, but we do not know what happened to it.
When they first left the nest they couldn't fly properly, so they hid as best they could while waiting for the parents to bring them food. I found this one waiting on a bike for meals on wheels maybe?