Hello hoodat,
Perhaps these mammoths can adapt to the climate change and form new adaptations? Only time will tell if scientists are going to enable this species to roam once again on earth. As for the passanger pigeon, I'm pretty sure there must be a good reason to revive these birds, like you mentioned they are capable of blocking out the sun and create havoc amongst the environment, but we as humanity are no different from these pigeons. Anyway, there is a close distant cousin for the passenger pigeon that scientist can genetically engineer a new species that resembles these extinct birds.
Here's a link that provides a criteria checklist for the
Passenger Pigeon.
Blessed be,
Lepus
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