Possibly due to a large number being outside of their home region. I've currently got 13 migrated from CC into HI, and 4 migrated from SI into CC, but I have had the game crash now twice in a row when migrating a fifth into CC
#Game crashing when migrating vultures
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might be pushing the game to its limits here
oh there's also uhh 17~ ish scavengers migrated from HI to SH, which might also be having an effect
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it appears that 2 vultures were straight up erased from the file also (not in SI or CC/HI). The other two that were supposed to be in CC migrated to HI without incident