Animated post-apocalyptic film

My memories of this one aren’t too clear. I remember getting it on VHS from the library as a young kid, probably around 18-20 years ago. I’m guessing it’s early 90s, or more likely an 80s animated film. Though possibly as old as 70s, can’t remember animation too well. All I can remember is that it seems that it was post-apocalyptic, that people (I think the main character was a child) were wandering around, I believe underground, possibly in a subway. I think they were looking for a bird of something to get to the surface. Tried numerous times to figure out what this film is to no avail, though I’ve met others who remember a similar movie.

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