I agree those birds aren't Passenger Pigeons, but am not convinced they would not adapt to breeding in smaller groups or single pairs.Passenger Pigeons needed very large breeding colonies, without which individual birds would not survive. If flocks of this size still existed, WE WOULD KNOW ABOUT THEM!!!!!!!!!! Most of those sightings came from heavily birded states (Wisconsin, Missouri, ect.). I doubt a Passenger Pigeon could survive there undetected.
Besides, those photos are clearly juvenile Mourning Doves in the early mourning (or late evening) light.