This question was initially asking for two separate works, was trimmed down to one and reopened. As OP never came back but still provided a decent enough summary of the second question, I'm reposting it so that it doesn't get lost:
The second story is also set in a future Earth, where people live in a huge enclosed megalopolis. The outside is uninhabitable (nuclear war? pollution?) and the people who live inside are only concerned with their artificial world. However, one eccentric old man (who I think remembers how it used to be) has a hobby of sending artificial, simulacra pigeons outside. One day a real bird comes back with the simulacra, showing that the outside is habitable once more.
OP originally indicated that they read it "many years ago as a teenager", for what it's worth (i.e. not much).
