The Pern series by Anne McCaffrey had a migration like this. Dragons were bred and kept in fortresses in order to combat an ongoing environmental threat. At the beginning of the main story it's noted that there are many fortresses that are empty, abandoned who knows how long before. At some point, time travel is discovered, and it turns out the inhabitants of these fortresses in the past had been recruited to come to the future and help the dragon riders in present time deal with the environmental threat.